MaxClient Directive in apache

Question: we have php web application on two db linux web server connected with RAC load balancy with two Oracle db server..
we have a high traffic .. about 500 client concurrency on the same time conenct the web application ..
our linux web server is 2 giga RAM .. and 2 processors.
we want to make apache more performance ..
i have installed zend core for oracle on linux web server with prefork module.
how can i calculate exaclty the value of MaxClient directive in prefork module ..
or make any thing to improve apache performance .
or make any configration on oracle server.
Answer: Find the average memory consumption of an Apache Process. Divide the amount of "spare" memory you have by this number. This is your MaxClients number.
When I say "Spare" memory - I don't mean "memory not in use" - I mean "Memory not in use when apache isn't running, minus about 20% for system stuff"
Answer: This will depend on how many children you are forking on startup and the MaxRequestsPerChild value you want to use, the ThreadsPerChild values, etc. It's not a simple calc.
What other values are you using?

MaxClient Directive in apache


How do you send an Ethernet frame with a corrupt FCS?

Question: I'm not sure if this is even possible since this might be handled in hardware, but I need to send some Ethernet frames with errors in them. I'd like to be able to create runts, jabber, misalignment, and bad FCS errors. I'm working in Python.
Answer: It can be handled in hardware, but isn't always -- and even if it is, you can turn that off; see the ethtool offload parameters.
With regard to getting full control over the frames you create -- look into PF_PACKET (for one approach) or the tap driver (for another).
Here's an article on using PF_PACKET to send hand-crafted frames from Python.
Answer: First, you disable your ethernet card's checksumming:
sudo ethtool -K eth1 tx off

Then, you send the corrupt frames from python:
#!/usr/bin/env python

from socket import *

#
# Ethernet Frame:
# [
#   [ Destination address, 6 bytes ]
#   [ Source address, 6 bytes      ]
#   [ Ethertype, 2 bytes           ]
#   [ Payload, 40 to 1500 bytes    ]
#   [ 32 bit CRC chcksum, 4 bytes  ]
# ]
#

s = socket(AF_PACKET, SOCK_RAW)
s.bind(("eth1", 0))
src_addr = "\x01\x02\x03\x04\x05\x06"
dst_addr = "\x01\x02\x03\x04\x05\x06"
payload = ("["*30)+"PAYLOAD"+("]"*30)
checksum = "\x00\x00\x00\x00"
ethertype = "\x08\x01"
s.send(dst_addr+src_addr+ethertype+payload+checksum)

See A similar question
Comment: Hi @brice I am a beginner to network things. Can you please add one liners/example for each variables you mentioned. Do IP address need to be in decimal or hexadecimal format? What all values need to be changed?
Answer: try using scapy for python, there are examples to generate jumbo frames, a runt frames too. http://www.dirk-loss.de/scapy-doc/usage.html
Comment: Thanks for the pointer, but please always post examples in the answer, because the content at links change, and we can't always find what we're looking for.  For example, I can't find an example of a jumbo or a runt here.  (Jumbos are easy, just use a big payload.)
Answer: The program did not work as intended for me to generate FCS errors.
The network driver added the correct checksum at the end of the generated frame again. Of course it's quite possible that the solution is working for some cards, but I'm sure not with any from Intel. (It's also working without any ethtool changes for me.)
With at least an Intel e1000e network card you need a small change to the code above:
Add the following line after "s = socket(AF_PACKET, SOCK_RAW)":
s.setsockopt(SOL_SOCKET,43,1)

This tell the NIC driver to use the "SO_NOFCS" option defined in socket.h and send the frame out without calculating and adding the FCS.
You may also be interested in the following C-programm, which did show me how to do it: http://markmail.org/thread/eoquixklsjgvvaom
But be aware that the program will not work on recent kernels without a small change.
The SOL_SOCKET seems to have changed the numeric ID from 42 to 43 at some time in the past.
According to the original author the feature should be available for at least the following drivers: e100, e1000, and e1000e. A quick grep in the kernel sources of 3.16.0 is indicating that ixgbe igb and i40e should also work. If you are not using any of these cards this socket option will probably not be available.


How do you keep up to date and find the most relevant papers?

Question: What tools or methods you use to keep up with the frontier of the field? I follow some specific papers and  authors. I was wondering what's the best way to keep up to date with relevant things with so much information there's available.
Answer: Seminars and conferences are the best ways to get exposure to working papers, in addition to following specific authors. (Some of my colleagues also follow economists on Twitter, where they sometimes post new working papers.)
In addition, subscribe to the email alerts from the journals in your field/interest. This will get you up to date with what comes out in the latest issues. The AEA journals (as well as Econometrica) will also send you the titles and abstracts of the articles; the Elsevier journals will only send you the TOC with the article titles only.
Comment: I'm not in academics but, to even have any hope, I think you have to narrow down the field considerably to some specific sub-field or even sub- sub field.. I've been looking at RE for ~3 years and I feel like there's still  more to learn about that . So catching up to the latest and greatest state of the art ( which I guess would be the topic of "learning" ) isn't even possible, atleast right now. Time in the day is also an issue. If you aren't in academics, I also highly recommend JSTOR.


NOx emissions in diesel engines

Question: What are the specific challenges and potential solutions for reducing NOx emissions in diesel engines without sacrificing fuel efficiency, considering the trade-offs between various aftertreatment technologies and in-cylinder combustion strategies?
Comment: Your question is  "how to design a modern diesel engine."  I suggest this is overly broad and not well suited to this format.  Maybe a graduate level engineering course.
Comment: There is SO much available to read about this topic. Chapman, Judge, Ricardo should be on your "read and understood" list...
Answer: NOx is formed at high combustion temperatures, in the presence of abundant nitrogen. Diesels get high combustion temperatures by virtue of their high compression ratios, which are necessary for autoignition. Reducing the temperature at which heat is added in the Otto cycle will reduce NOx formation but also reduces the Carnot efficiency of the cycle. N2 concentration in the combustion chamber can be reduced by diluting it with exhaust gas but this decreases the concentration of oxygen available to support combustion and thereby reduces the power output of the engine.
The best way we currently have for reducing diesel emissions is by aftertreatment of the exhaust or, for the ethically-challenged, Volkswagen-style cheating.
For those of you who did not know about this, there's an exhaustive (pun intended) article about the VW defeat device cheat on wikipedia.
Comment: All VW really did was partially automate what nearly every mechanic does to a car before conducting an emissions test. Sabine Hossenfelder described it best as a typical example of German efficiency.  Now they are back to doing it all manually, and the price is fixed by regulations. So there is flourishing business of beating the tests that is a pure drain on the economy with absolutely zero environmental benefit. But that doesn't make a good headline.
Comment: What VW did was build that defeat into the system interrogation hardware so that the EC system wasn't just defeated when a mechanic was testing the system, but *all the time* the system was *in actual use*. So when a VW diesel was on the road, its EC system was *always defeated*
Comment: @phil sweet, ...that is, *never operating*, and of course VW did not inform the EPA that there were a quarter of a million VWs driving around the USA with their EC systems *locked out* and that this had been going on for *nine years* and that the lockout was implemented on the firmware level of the engine management computer and that VW's own regulatory compliance division wasn't told about it.
Comment: @nielsnielsen but the US government have been dodging environmental issues for many more years than VW... Having a Petroleum Stooge making "science" decisions about what is and is not driving global warming likely did more harm.
Comment: @solar mike, that is a different issue. Defeat devices attached to mandated emissions control systems are specifically prohibited by law. VW was caught red-handed in flagrant violation of that law. prosecution was a slam dunk.

NOx emissions in diesel engines


Plastic or metal pipes? what do you recommend?

Question: I need to install piping for hot and cold water, do you suggest plastic or metal pipes?
Comment: This question is subjective, and very broad.  You might want to think about narrowing it down a bit.
Comment: Eg: What is it for - Domestic potable water or something else? Where is it being installed? Are the pipes exposed or in walls (as this can help make the choice of rigid vs flexible)? Are you looking for plumbing in residential fixtures in an entire house? Outside buried piping? There is really not one good answer to this, it largely depends on what the situation calls for. It also depends on your experience/skills and what tools you have (or are willing to learn/buy).
Comment: @ppumkin That's not always true.  For the pipe itself, this might be true. But when you factor in tools and fittings, things may change a bit.
Comment: Add more details to your question or it will get closed.
Comment: Plastic = cheap / copper = expensive
Comment: For the pipe it self - plastic was always cheaper than copper. For complex fittings and junctions... i suppose its debatable.
Comment: I give 24 hours before NARQing a skimpy question.  This is a prime candidate so far.
Answer: TLDR: copper pipe with compression fittings.
In the UK, your choices, in approximate order of decreasing compactness and decreasing skill required, include

Copper pipe with end-soldered connections.
Copper pipe with solder-ring connectors.
Copper pipe with compression fittings.
Copper pipe with push-fit connections.
Plastic pipe with push-fit speedfit connectors.
Plastic pipe with push-fit Hep2o connectors
Plastic pipe with push-fit Polyplumb connectors
Plastic pipe with push-fit Floplast connectors
Plastic pipe with other push-fit connectors.

However, if you already have copper pipe, I would use compression fittings and copper pipe (if you have space) to maintain earth continuity. You probably need to add equipotential bonding where the pipes join the taps.
Plastic pipe is easier but, in my experience, the couplings are more bulky than even compression fittings and this can make a significant difference to the end result (e.g. being able to fit joints within existing space).
Plastic pipe connectors usually have to match to plastic pipe type. Some plastic push-fit connectors (but not all) can be used to join copper pipe to plastic.
Plastic pipes can be bent, but I've had nearby push-fit joints leak, so it may be best not to rely on the flexibility too much.
The disadvantage of soldering is that you have to make sure the pipework is empty and dry, even a small amount of water several inches away can make it impossible to raise the temperature of the joint high enough for solder to flow.
So really it depends on how handy you are with a blowtorch.
Comment: Your answer would be better if you talked about different types of "plastic" and "metal" piping (eg, PEX, PVC, poly, copper, galvanized) and didn't just list off a bunch of brand names for push-fit connectors -- there is way more out there for "plastic" pipes than push-fit.
Answer: I like copper.  I disagree with RedGrittyBrick partially on the compression fittings.
Soldering copper pipe is pretty easy. For a complete beginner, there are even joint pieces (elbows and collars) that come pre-loaded with solder:

You simply heat the pipe (not the joint) until the solder melts.
However, soldering brass fixtures to copper is something I've never been able to master.
My solution is solder the piping, and use compression or shark-bites for the termination.
As far as drainage goes, stick with ABS (Rigid black plastic).
Comment: For what it's worth, I would also use solder-ring connections when making up long runs of new fixed copper pipe. I use compression fittings where soldering is impractical (e.g. to avoid setting fire to surroundings when joining existing pipes in cramped places)
Comment: @BradMace Nice edit comment.


Is Verizon Still Injecting Perma-Cookies into Customer Traffic?

Question: This article was published in 2014. Is Verizon still doing this to customer data? It makes sense to assume they are, but I'm hoping for some kind of official news article or something to validate its continued existence.
Comment: I'm sure you could test it yourself pretty easily if you have Verizon.
Answer: Despite a 2016 FTC settlement against an advertiser abusing the X-UIDH header and a 2016 FCC settlement (PDF) with Verizon, Verizon's legal pages indicate that it is still in use in some circumstances.
There have been changes to FCC rules within the past couple of years, impact on this issue unknown.
There are some user controls in the form of opt-outs and opt-ins to various Verizon advertising preferences that affect the X-UIDH header:

Verizon Wireless stops inserting the UIDH after you opt out of the
  Relevant Mobile Advertising program, unless you have also opted in to
  the Selects advertising program. The UIDH will continue to appear for
  a short period of time while we are updating our systems. If you
  choose to participate in Verizon Selects, the UIDH will be present
  even if you have also opted-out of the Relevant Mobile Advertising
  program.

It's only possible for Verizon to inject the header on HTTP traffic. HTTPS (try EFF's HTTPS Everywhere browser add-on to use as much HTTPS as possible), or Tor, or a VPN, will block this behavior.
You can check for the X-UIDH header on any unencrypted (HTTP; not HTTPS) website that displays all of your HTTP request headers.
Comment: True, though probably a remote possibility. In that case, you'd need to host your own.
Comment: What if Verizon or other internet access providers use whitelists (websites of partners) or blacklists (websites known to echo request headers)? Using one of those websites, you can get a yes-or-maybe but not a yes-or-no answer.


How would i go about making a melee character that doesn't wear armor?

Question: I've been brainstorming an idea about a character that doesn't wear any armor at all using only their mobility to avoid getting hit.
I already know about Monk, Bracers of Armor, Mage armor and the like. I was hoping I could get some other ideas on how to go about it.
Answer: From X to Y, a Google Doc, includes all (as far as I know, as of its last revision) ways to gain alternate stats to various things. For AC:
Note: Per this statement, you can only add an ability score to a statistic once, so if you already have Int/Wis/Cha to AC, other effects that tie that AC to that ability score would not increase it further. Note that this is based on the bonus type, so for example Canny Defense (an untyped bonus) would stack with Spell Shield (a shield bonus).

Canny Defense* (Magus, ACF Kensai 1) Intelligence to Ac while unarmored addition to dex, max 1 pr class level
Spell Shield (Magus 3; Magus Arcana) Spend 1 arcane point to gain Int as a Shield bonus to AC until end of next turn, doesn't stack with shield
Canny Defense* (Duelist 1) Intelligence to AC while unarmored addition to dex, max 1 pr class level
Whispering Spirits (Investigator ACF Spiritualist 4) Use a commune with  spirits attempt to gain Wis as insight bonus to AC and Saves for 1 min.
AC bonus (monk 1) add wisdom to AC and CMD when unarmored and unencumbered
AC Bonus (Warpriest ACF Sacred Fist 1) add wisdom to AC and CMD when unarmored and unencumbered
Smite (Warpriest ACF Champion of The Faith 4, 8, 12, 16, 20) Gain Cha to hit if target has chosen opposed alignment (Evil/Good/Chaos/Law), and Cha to Ac vs target.
Smite evil (paladin 1,4,7,10,13,16,19, class feature) add charisma to hit/AC against evil smite target
Bastion of Good (oathbound paladin ACF Sacred Shield 1) Cha to Ac against smite target, replaces smite evil
Smite Good (Antipaladin 1,4,7,10,13,16,19) Charisma to hit and AC vs target good creatures
Nature's Whisper (Oracle, nature mystery; revelation) use Charisma to AC and CMD instead of dex
Sidestep Secret (oracle, lore mystery, revelation) Use Charisma to Ac and reflex saves instead of dex
Prophetic Armor (Oracle 7, Lunar Revalation) Use Cha instead of dex for AC and reflex saves.
Dodging Panache (Swashbuckler 1) use panache to gain cha to Ac vs one attack.
Osyluth Guile (combat feat) add charisma to Ac against a single target, when using fighting defensively/total defense


What happens if high via current flows in PCB?

Question: If I used a via of 8 mils diameter with 20 um thickness. As per sierra tool calculator it can carry current of 1 A at 10 degree rise in temperature. What happens if the current is more than 1 A? When will the via fail? How the failure will be in PCB?
Comment: Depends on lots of factors such as via length, board material, via's top and bottom surface areas (i.e. if the via is connected to a trace or a copper pour), duration of high current flow, environmental conditions, frequency of the flowing current (i.e. if the current is DC or high frequency AC), adjacent heat sources (if any), etc.
Comment: I have seen like when temperature cross more than Tg of PCB, then board may become instable, Whether it is true to via, if via temperature crosses more than Tg, it will fail?
Answer: 10 °C rise isn't much for a PCB trace. Even 20 °C or 30 °C is usually fine. In some cases, voltage drop across the trace, or power dissipated in the trace, are too high for system design reasons when the rise gets high, say 30 °C or 50 °C.
From a strictly thermal standpoint, there are maximum temperatures for the dielectric being used. For common FR4, this is often 110 °C to 130 °C and is determined by the rated Tg (glass transition temperature). Above this temperature, the board becomes soft, and mechanical and electrical properties are degraded.
In the absence of detailed information, I'd keep thermal rise to 10 °C to 30 °C absolute worst case, and if pushing 30 °C, check that voltage drops aren't an issue.
Comment: Thanks, it means I can allow the via current at the max temp rise of 30C. More than that it is not preferable. Am i right?
Comment: @Selva97 Bear in mind that this is above ambient, so add the maximum expected operating ambient temperature to the temperature rise
Answer: Higher temperatures are also undesirable due to thermal stress and cycling.  The laminate expands much more than the copper plated onto it, causing the copper to stretch, fatigue, and eventually crack.
Reduce heat by using less current in the first place, using more vias in parallel, wider traces, more layers, etc.
Comment: I think this is at least as important, and possibly more important than the other answer. The Tg will only really matter at high ambient temperatures like 65C+ (unless you're putting like 5A through one trace), but the thermal expansion and contraction will matter at all temperatures


How and when are commerce licenses created, and when does the state change to "active"?

Question: I'm working on a module that attempts to tie commerce_license in with the ACL (Access Control Lists) module.
I am attempting to sell access to an individual node. I have a product type, product, and product display which does this. I have a node reference field which allows an admin to select which node to license on product creation, and this reference field is being correctly referenced in my license class (the one in plugins/license_type/ which extends CommerceLicenseBase).
In my save() function, I'm checking to see if the status is COMMERCE_LICENSE_ACTIVE before I add the license owner to the acl, but it appears that the license never gets in this state. I have gone through the checkout process, and even manually completed the order, but I don't ever see the license get to the "active" state.
I've also noticed that the license seems to be created (the save() function is called) when a product that is associated with a license is added to the cart, and NOT on checkout, as is stated in the documentation.
I think the real issue is in my lack of understanding of how the commerce license module works (and possibly how the ACL module works, but that's a different discussion). I've read through the documentation, watched a few tutorials, and looked through the example code, the commerce_license_role module, as well as the commerce_license_og and commerce_file modules, and I'm still not sure what I'm doing wrong - I'm beginning to suspect that my error is not in the code itself, but rather in configuration.
Here is my code:
<code><?php

  public function save() {

    if ($this->uid && $this->product_id) {

      // Get the entity referenced
      // (this works fine - field is defined in the .module file)
      $node = $this->wrapper->product->commerce_license_acl_node->value();
      $acl_name = 'ACL_' . $this->product_id . '_' . $node->nid;

      // Get the acl associated with this node and product ID
      $acl_id = acl_get_id_by_name('commerce_license_acl', $acl_name);

      // If the ACL doesn't already exist, create it
      if(empty($acl_id)){

        // Create ACL if one is not already associated with this license
        $acl_id = acl_create_acl('commerce_license_acl', $acl_name);

        // Grant ACL users read access to this node
        $read   = TRUE;
        $update = FALSE;
        $delete = FALSE;
        acl_node_add_acl($nid, $acl_id, $read, $update, $delete, 0);

      }

      $owner = $this->wrapper->owner->value();

      // I realize a lot of this is redundant. I'm covering all the cases here for my benefit
      // in trying to understand what is going on.
      switch($this->status){
        case COMMERCE_LICENSE_ACTIVE:
          // *** This case is never true
          acl_add_user($acl_id, $owner->uid);
          drupal_set_message('license_acl set to active');
          break;
        case COMMERCE_LICENSE_CREATED:
          // **** This is the state that it always seems to be in
          drupal_set_message('license_acl created');
          break;
        case COMMERCE_LICENSE_PENDING:
           drupal_set_message('license_acl set to pending');
         break;
        case COMMERCE_LICENSE_EXPIRED:
          acl_remove_user($acl_id, $owner->uid);
          drupal_set_message('license_acl set to expired');
          break;
        case COMMERCE_LICENSE_SUSPENDED:
          acl_remove_user($acl_id, $owner->uid);
          drupal_set_message('license_acl set to suspended');
          break;
        case COMMERCE_LICENSE_REVOKED:
          acl_remove_user($acl_id, $owner->uid);
          drupal_set_message('license_acl set to revoked');
          break;

    }

    parent::save();

  }
  ?>
</code>
Is there any documentation out there which goes into depth explaining the workflow of commerce license creation and statuses? Thanks in advance.
Answer: It looks the license is being activated when the "order paid in full" trigger fires, and I didn't have a payment system in place. I thought I had it configured to bypass this, but enabling the example payment system and the payment ui solved the problem.
However, the user still doesn't have access to the node associated with the ACL, so I need to figure that one out, as well.


How to change the title of the page?

Question: Can anyone tell that how to change the title of the page? Is it possible in SharePoint Designer? The below image with  yellow marked.
Comment: You can do that by going to "SitePages" library and editing the properties of this page. However please note that the url will also get updated when you change the title. The other solution would be to add the javascript mentioned in the answers below to the page using a content editor webpart (Chrome type none and visibility hidden),
Answer: Yes, Please try below code :
<code>document.title = "Your Title Goes Here";
</code>
Answer: That is not the title of the site, but instead - it's a title of a page. Go to the library where your pages are stored (in your case "Site Pages"), edit properties of that page and change the title to whatever to like.
Answer: You can do so by clicking on the Edit button on the ribbon under Page tab to enter the edit mode or by clicking the Edit Properties button.
Comment: Should note that the pictures here are using 2010, so it may look different for the OP.

How to change the title of the page?


Will new bottom plates in basement allow radon penetration?

Question: I am getting ready to finish my basement.  I can use Tapcons or a Ramset to attach the bottom plates of the walls to the concrete slab with drain tile underneath.  Will these holes from the screws or nails allow radon in?
Comment: I've sealed on the cracks in the slab and the sump pump, and then installed my own radon system a year ago.  My fear is that these holes will nullify it.  maybe if I drill the holes, then put sealant in them, then screw...
Answer: If you have radon, it's already likely getting in--concrete is permeable. So adding a small nail hole isn't going to change anything. Also note that tapcons don't actually penetrate all the way through the slab.
If you have radon, you need to get a radon remediation system installed. This will put a hole in your slab so the radon has a place to go (and then it's vented out the roof).


The outline of my object is really hard to see

Question: I have been trying to follow Blender Guru's guide on how to model a chair, and it involves using reference images. When I apply a subdivision modifier to my model and try to edit it in wireframe mode, I cannot see the outline of where the subdivision is happening, because its light gray and blends in with the reference image. I can see it against the dark workspace background though. In the tutorial video, his outline is completely black. I have no clue on what is causing this, because I don't remember fiddling with such a setting. Really want to continue but this is very frustrating and i can't seem to find an awnser to this.
Answer: These bright lines are additional geometry generated by modifier. It is bright (or invisible on white) to give you a feedback of current silhouette, but mainly keep user focused on real topology ...

If you want to see them all disable Optimal Display in modifier properties ...

Alternatively ... If you want to keep Optimal Display ON you can enhanced your view by switching to Solid View mode, but with X-Ray enabled (icon next to Wire View mode).
Comment: Yes, the load factory settings didn't change anything. I tried downloading 3.1 and it works perfectly on that, but whatever I do I can't seem to get it to work on 3.2. It worked before, but after the power cutoff It seems it changed the setting on its own and is bugged now. I guess i will use 3.1 for now, but its still quite annoying that something like this can randomly happen. I will report this issue.
Comment: I have optimal display off and it does show the outline when I look at it with object mode, but when I want to edit it in wireframe mode the outline becomes invisible against the background. The solid view mode with x-ray is also very bright and hard to see against the background.
Comment: Sorry now I noticed it ... Have you tried another blender version? Please share your file via https://blend-exchange.com ... it is weird, lines you are mentioning are those I described ... so it doesn't make a sense. Thanks you
Comment: It was working before when i tried to do the same thing, but I forgot to save and my power went out so I lost all the progress. When I tried to redo it the lines just don't appear the same. I tried to reset to default setting, but that didn't have an effect either. I might just have to restart my PC or something.
Comment: What happen in new blend file with default Cube and SubSurf modifier ... can you reproduce the issue? Or in your issued file delete modifier and add again. BTW under Viewport Overlays > Geometry you can change Opacity, but it hides real geometry and keeps only light grey outline.
Comment: I really don't know anymore. It still happens in new file with default cube. The opacity is 100% and I haven't changed it. Might have to try and reinstall blender I guess.
Comment: Okay nevermind its still broken after reinstall with completely default settings.
Comment: By "Reset to Default" do you mean "Load Factory Settings", right? Have you tried a different version of blender as I asked in my first comment? Anyway you can search in bug reports if already there, if not try to report (go to Blender menu > Help > ... just to be clear Optional Display on/off does nothing, right?


Understanding OLS regression slope formula

Question: I understand the intuition behind the OLS model: to minimize the squared residuals.  Is there a way, however, to interpret the formula for the slope of the regression line intuitively?  That is $m = r(sd_y/sd_x)$.  I know the formula gives me the slope, but how?  Put in another way, what is the most intuitive way to visualize or think about the slope formula for the regression line?
Answer: The correlation coefficient $r$ gives you a measurement between $-1$ to $+1$. This gives you information about the strength of the linear relationship that can be interpreted independently of the scale of the two variables. Again, when $sd_y=sd_x$, then $m=r$.
So, $r$ is the slope of the regression line when both $X$ and $Y$ are expressed as z-scores (i.e. standardized). Remember that $r$ is the average of cross products, that is,
$r=\frac{\sum Z_xZ_y}{N}$
So, it turns out that $r$ is the slope of $Y$ on $X$ in z-score form. This correlation coefficient tells us how many standard deviations that $Y$ changes when $X$ changes $1$ standard deviation. When there is no correlation ($r = 0$), $Y$ changes zero standard deviations when $X$ changes $1$ standard deviation. When $r$ is $1$, then $Y$ changes $1$ standard deviation when $X$ changes $1$ standard deviation.
The regression $m$ weight is expressed in raw score units rather than in z-score units. To move from the correlation coefficient to the regression coefficient, we can simply transform the units:
$m=r(sd_y/sd_x)$
This says that the regression weight is equal to the correlation times the standard deviation of $Y$ divided by the standard deviation of $X$. Note that $r$ shows the slope in z-score form, that is, when both standard deviations are $1.0$, so their ratio is $1.0$. But we want to know the number of raw score units that $Y$ changes and the number that $X$ changes. So to get new ratio, we multiply by the standard deviation of $Y$ and divide by the standard deviation of $X$, that is, multiply $r$ by the raw score ratio of standard deviations.
Comment: Does the same intuition work for m = cov(xy)/var(x)?
Comment: I would say, yes, but actually in variance and covariance term it is difficult to explain, because these measures are in squared units. That is why the formula you mentioned previously is more suitable to interpret how a regression coefficient can mean a conditional per unit change.


Write data to json file?

Question: I am developing an app using jqm+phonegap. I want to store the selected options from two selection menus in a json file locally. I know how to read data (using getJSON) from json file but i dot know how to write?
Answer: PhoneGap exposes a file api: http://docs.phonegap.com/en/1.2.0/phonegap_file_file.md.html#File
I haven't tried to use PhoneGap's file api it for several months but Lawnchair is quite a bit easier to use (just my opinion).
You could also check-out Lawnchair, it makes storing data on a device easy.
Comment: i am already using json to store data and i am accessing that data in my app. Actually my real problem is: I have two selectmenus (with 5 options each and both are linked for example, for option A in first selection there are 3 options in second menu and for option B there are 2 options and so on..) on a single page.

I want to create another selectmenu on the same page whose options should be the (last 10) recent combinations of options selected from the other two selectmenus.
Comment: @G.S JavaScript natively does not have any methods for writing to a filesystem. So you'll need to rely on HTML5 web-storage or PhoneGap's API. There are examples in the links I posted for both of those methods.
Comment: $("#selectShade").change(checkShade);

  function checkShade(){
  localStorage.setItem("title","My best  App");
         localStorage.setItem("author", "john");}
   This is not working when the checkShade() is fired.When i try to select some option from my menu #selectShade, selectmen hangs and i am not able to select the option. If i remove these two lines (localStorage) from checkShade() and write alert("hi"); then it works.help?
Comment: sorry. it's working. Actually i was not running it in browser.

Write data to json file?


When is the best to add control variables in multilevel modelling?

Question: Is it better to add control variables before or after the main predictor variable while conducting a step-wise multilevel model?
Answer: If you want to see what the control variables do to the main independent variables, then you need to do two runs: One with just the IVs and one with the IVs plus the control variables.
This is often a good idea, as it helps get away from automatic methods of variable selection. It may give you a reason to keep control variables that are very far from significant in the model, as they may affect the parameter estimates and standard errors of the IVs.
This is true regardless of whether it's a regular OLS regression, a MLM, some other kind of regression (e.g. logistic) or whatnot.
It encourages thought on the part of the data analyst.  That is a good thing.


Can we characterize the submultiplicative matrix norm such that $\| A\| \le \rho(A) + \varepsilon$?

Question: We know for $A \in \mathcal M_n$ and every $\epsilon > 0$, there exists a submultiplicative matrix norm $\| \cdot\|_m$ such that $\|A\|_m \le \rho(A) + \varepsilon$. Is this norm vector norm induced? In other words, could we define some norm on $\mathbb C^n$ such that $\|A\|_m = \sup_{\|x\| = 1} \|Ax\|$ for $x \in \mathbb C^n$? Thanks.
Answer: Indeed! Let $S$ be the matrix for which $A=SJS^{-1},$ where $J$ is the Jordan canonical form of $A.$ Let $E=\mathrm{diag}(1,\varepsilon,\varepsilon^{2},\ldots,\varepsilon^{n-1}).$ Then $A=SEJ'E^{-1}S^{-1},$ where $J'$ is like the Jordan canonical form of $A$, but wherever $J$ had a one on its first superdiagonal, $J'$ has a $\varepsilon.$ Let $V=SE.$ Then if we let $\|x\|:=\|V^{-1}x\|_{\infty},$ we see that
\begin{align*}
\|A\|&=\sup_{\|x\|=1}\|Ax\|\\
&=\sup_{\|V^{-1}x\|_{\infty}=1}\|V^{-1}Ax\|_{\infty}\\
&=\sup_{\|y\|_{\infty}=1}\|V^{-1}AVy\|_{\infty}\\
&=\sup_{\|y\|_{\infty}=1}\|J'y\|_{\infty}=\rho(A)+\varepsilon.
\end{align*}
I leave it to you to show that $\|V^{-1}\cdot\|_{\infty}$ defines a valid vector norm on $\mathbb{C}^{n}$, and the last equality.
Comment: May I ask your reference for this construction? I want to reference it. I am using Matrix Analysis by Horn & Johnson, but it seems like they are not concerned with the vector norm. Thanks.
Comment: I'm 99% sure all of the supporting material is in H&J, but some of it might be in exercises. I put the material together to make this argument myself, but I'm sure that it's not new. I think citing H&J would be sufficient.
Comment: For me, the vector norm is more of interest. But in H&J (as least in the main body), it does not been mentioned.
Comment: Okay, I don't think I was understanding before. In the chapter on norms, there should be an exercise asking you to prove that for a nonsingular matrix $S$ and vector norm $\|\cdot\|$, $\|S^{-1}\cdot\|$ defines a vector norm.


How do I emulate MPC5xxx embedded microcontroller on QEMU?

Question: I've seen that QEMU can emulate PowerPC architectures, but in the documentation in the supported machines part I cannot find the MPC5xxx target, however it's mentioned as CPU type in the source code:
https://github.com/qemu/qemu/blob/7fe7fae8b48e3f9c647fd685e5155ebc8e6fb84d/target/ppc/cpu-models.c#L220
My question would be which command line arguments do I need to give to the qemu-system-ppc binary, or else how do I compile QEMU specifically to emulate MPC5xxx target? (on Linux host)
Do I need additional configuration? If yes, where do I find reference for it?
Answer: I pulled the latest source:
<code>$ git clone https://git.qemu.org/git/qemu.git
</code>
Then configured:
<code>$ mkdir qemu-build
$ cd qemu-build
$ ../qemu/configure --target=ppc-softmmu
</code>
Then built:
<code>$ make
</code>
I see these "mpc5xxx" CPU types listed:
<code>$ ./qemu-system-ppc -cpu help | grep mpc5
PowerPC mpc5200_v10      PVR 80822011
PowerPC mpc5200_v12      PVR 80822011
PowerPC mpc52xx          (alias for mpc5200_v12)
PowerPC mpc5200          (alias for mpc5200_v12)
PowerPC mpc5200_v11      PVR 80822011
PowerPC mpc5200b_v21     PVR 80822011
PowerPC mpc5200b         (alias for mpc5200b_v21)
PowerPC mpc5200b_v20     PVR 80822011
</code>
I've never used these, but you'd use the command-line option "-cpu mpc52xx", for example.


Company name translation with LTD

Question: I know that Company name should not be translated, but LTD or Ltd. should be translated?
Just for example, Apple Ltd.
Comment: "Ltd.", "Inc", "LLC", "S.A." etc. are part of a company's official business name, and therefore should not be translated.
Answer: This seems like an odd question for ELL. "Ltd." is already English ("Limited"), so it doesn't need translation.
However, in a more general case, if the company is incorporated in a particular country, then you should use the incorporation designation from that country, untranslated.  For example, there is a large international bank headquartered in Spain that is referred to as Banco Santander, S.A. in English-language articles, even though "S.A." stands for the Spanish phrase "Sociedad Anónima."
This is, however, confused by the fact that multinational companies often have wholly-owned subsidiaries that are locally incorporated.  There is an entity headquartered in Boston that is called Santander Bank, N.A. (where "N.A." stands for "National Association").  It is the wholly-owned subsidiary of Banco Santander, S.A. that conducts the parent company's business within the United States.
Comment: Agreed.  I've dealt with a lot of international business, and in legal and formal documents, you always retain the original company type.  A "limited" company in Europe will not be accountable to the same laws and regulations as a "limited liability company" in the US, even though they might have a very similar structure and be treated similarly in their respective countries.

Company name translation with LTD


How to disable Style Cop visual studio extension?

Question: How to disable the Style Cop visual studio extension? It's not listed in Tools / Extensions or in Tools / Add-in manager
Comment: Fixed it by running `devenv.exe /setup` but BEWARE I don't know what this does it may have had unintended side effects
Comment: /setup forces Visual Studio to merge the resource metadata that describes menus, toolbars, and command groups, from all available VSPackages. (http://msdn.microsoft.com/en-us/library/ex6a2fad.aspx)

All of my extensions are running properly so far.
Comment: Did you restart VS after you were done? That should fix it
Comment: Yes, but it didn't fix it. There was an error message about extensions.  Just restarted my whole computer, but Style Cop remains, bleeding. This is Visual Studio 2012
Answer: I had the same issue: I uninstalled it using Add/Remove programs and got the expected error, when I opened Visual Studio next time. Still after hiding this error message, the StyleCop entries remained in my context menus.
After some research, I found a solution to remove all remains by running devenv.exe /setup on the command prompt. You must run the console elevated and you might have to switch to the C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE folder (or similar; depending on your system) in order to run the command.
Remember to remove the assotiated Code Cleanup profile if you are using ReSharper.
Source: http://stylecop.codeplex.com/discussions/257772
Comment: Thanks.  I thought I was doomed to having these orphaned context menu items forever.
Answer: You can't, StyleCop deliberately hides itself from the uninstall menu in Visual Studio.
You can uninstall it in Add/Remove programs. You might get an error the next time you open Visual Studio but it will be gone after that.
Answer: why it happens is : mostly because you uninstalled the style-cop while visual studio was open
how to fix it

remove the registry keys

HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\11.0_Config\InstalledProducts\StyleCop
  HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\11.0_Config\Packages{629EB7CC-69C2-43AC-9BC9-482B0F810C4E}

OR

reinstall & then uninstall (make sure your visual studio is closed during this process)

if you are using resharper
you would need to reset the Code Cleanup Profile to factory defualt to get rid of style-cop completely.
Answer: I had the same issue. I have already uninstalled it from Visual Studio by ReSharper Extension Manager and it there was under the right click menu still when I click any projects on Solution Explorer.
This way will solve the problem:
Steps:

Remove the StyleCop by Add/Remove Programs on control panel
If you have already uninstalled the StyleCop, install once again by ReSharper extension manager.(There will be many error at the first opening Visual Studio after the styleCop uninstall process. don't worry about it, press enter all)
Then Remove it by Resharper extension manager again.
Answer: I had the VS15 StyleCop plug-in installed and later added the ReSharper extension on top.
I removed the extension from ReSharper, then uninstalled StyleCop via Control Panel Add/Remove Programs and then let CCleaner run with registry clean-up.
The StyleCop entries have disappeared from the menus and the billions of squiggly lines have as well.
Answer: The easiest way is to install "StyleCop" extension from Visual Studio Marketplace. Which also can be found from the link :
https://marketplace.visualstudio.com/items?itemName=ChrisDahlberg.StyleCop
Now right click your project and click on "StyleCop Settings", there you you can disable or customize the rules.


Django CMS: Custom plugin preview in editor escaping some characters

Question: I made a custom plugin for Django CMS, and as is the default behavior, the preview icon of the text field showed the returned value of the model's __unicode__(self) as the icon in the WYSIWYG view of the editor:

(where "Email address" is the name of the custom plugin.) However, if the __unicode__(self) method of the plugin returns anything with characters such as ', &, or < >, they get escaped in the preview:

I'm pretty sure the preview uses the alt text property of the plugin instance. The closest thing to people having the same issue I can find is this report, as well as this suggested fix, but neither solution causes any change in the alt text behavior, as far as I can tell.
Basically, even though the plugin's output on the actual page isn't affected, I'd like to be able to display characters like ', &, <, and >in the editor preview. Is this possible?
Answer: Try to use mark output as safe using django helper method.
Example:
from django.utils.safestring import mark_safe

...

def __unicode__(self):
     ... do stuff ...
     return mark_safe(output)


Reconstructing vectors from inner products

Question: Suppose that I have two unknown vectors $v_1,v_2 \in \mathbb{R}^3$, and a known vector $v_{3} \in \mathbb{R}^3$. I know all inner products $\langle v_i, v_j \rangle$, $1 \leq i,j \leq 3$. Is there a closed-form (or least painful iterative) way to reconstruct $v_1$ and $v_2$? intuitively it looks like there might be two solutions since any $v_1$ and $v_2$ can be reflected through the known $v_3$ to get the same inner products. Nevertheless getting them both would be helpful. Note that the problem is not generally rotation invariant as $v_3$ is known.
Comment: They can be rotated about $v_3$ so there is some rotation invariance. I would suggest rotating the whole system to point $v_3$ along the $z$-axis.
Comment: You can use Cholesky decomposition to get some vectors matching the inner products, but then you need to rotate to get $v_3$
Comment: You are right, there is a full degree of freedom. rotating $v_3$ to be the z axis would give me the $z$ value of both vectors, which makes it a $2D$ unconstrained "square root" problem.
Answer: Let $v_i = [a_{i1},a_{i2},a_{i3}]^T$.
As we know $v_3$, we have 6 unknowns thus we need 6 equations.
Since you have all inner product, we have
\begin{equation}
\begin{bmatrix}
v_1^T & v_2^T & v_3^T
\end{bmatrix}
\begin{bmatrix}
v_1 & v_2 & v_3
\end{bmatrix} =
\begin{bmatrix}
v_1^Tv_1 & v_1^Tv_2 & v_1^Tv_3 \\
v_2^Tv_1 & v_2^Tv_2 & v_2^Tv_3 \\
v_3^Tv_1 & v_3^Tv_2 & v_3^Tv_3
\end{bmatrix}
\end{equation}
and the right hand side is known.
Since you know $v_3$, we obtain following 5 equations which involves 6 unknowns. Note that $v_3^Tv_3$ does not give any information.
Thus we have 6 unknowns and 5 equations.
For example. let $v_3 = [0, 0, 1]^T$, $v_1=[1,0,0]^T$ and $v_2=[0,1,0]^T$.
Even though we know all inner products,
$v_1' = [\sin \theta, \cos \theta, 0]^T$ and $v_2' = [\cos \theta, -\sin \theta, 0]^T$ will satisfy the inner product conditions.
Comment: @MartinArgerami Since $v_1' = [\sin \theta, \cos \theta, 0]^T$, $\langle v_1',v_1' \rangle = \sin^2 \theta + \cos^2 \theta = 1$.
Comment: Yes, my bad. Thanks!
Answer: Let
\begin{align}
v_1&=x_1e_1+x_2e_2+x_3e_3\\
v_2&=y_1e_1+y_2e_2+y_3e_3\\
v_3&=ae_1+be_2+ce_3
\end{align}
and $\langle v_1,v_2\rangle=A$, $\langle v_1,v_3\rangle=B$, $\langle v_2,v_3\rangle=C$, $\langle v_1,v_1\rangle=P$, $\langle v_2,v_2\rangle=Q$. The conditions give a nonlinear system
$$
\begin{cases}
x_1y_1+x_2y_2+x_3y_3=A \\
x_1^2+x_2^2+x_3^2=P \\
y_1^2+y_2^2+y_3^2=Q \\
ax_1+bx_2+cx_3=B\\
ay_1+by_2+cy_3=C
\end{cases}
$$
of five equations in six unknowns. If the system is solvable, it will generally have infinitely many solutions.
You can first solve the last equation (parametrically). Then the first and fourth equations become a linear system. Finally, you have to normalize the solutions to satisfy the conditions on the norms.
Comment: @MartinArgerami Yes, I noticed, thanks
Answer: Assuming the vectors are in general position, i.e. no two are parallel:
Select an arbitrary unit vector $t$ perpendicular to $v_3$ (the usual trick for doing so robustly is to take $t = (e\times v_3)/\|e\times v_3\|$, where $e$ is the vector with a 1 in the entry corresponding to the least-magnitude entry of $v_3$, and a 0 elsewhere.)
We can assume
$$v_1 = \alpha_1 t + \beta_1 v_3$$
$$v_2 = \alpha_2 t + \beta_2 v_3 + \gamma_2 t\times v_3$$
Now
$$\beta_1 = v_1\cdot v_3 / \|v_3\|^2$$
$$\alpha_1 = \sqrt{\|v_1\|^2 -\beta_1^2\|v_3\|^2}$$
$$\beta_2 = v_2\cdot v_3 / \|v_3\|^2$$
$$\alpha_2 = \frac{v_1\cdot v_2 - \beta_1 v_2\cdot v_3}{\alpha_1}$$
$$\gamma_2 = \pm \sqrt{\frac{\|v_2\|^2 -\alpha_2^2 - \beta_2^2\|v_3\|^2}{\|v_3\|^2}}.$$
The answer is of course only unique up to (1) selection of $t$ and (2) selection of the sign of $\gamma_2$, i.e. the handedness of the resulting $(v_1,v_2,v_3)$ coordinate frame. There are two smooth families of solutions.


Does a statement exist which is its own proof?

Question: In fake/pseudo mathematical notation: does there exist a statement S such that S = Proof(S)?
Comment: If you tried feeding S = Proof(S) into a computer, you would have just created an infinite recursive loop, and you'd kill your app with a stackoverflow within seconds. Perhaps we can take the computer's lead here...let's not create stackoverflows in our own minds. I tend to avoid circular logic. A natural property of circles is that you can keep going forever but you don't really get anywhere...
Comment: "This sentence is true"
Comment: Cogito ergo sum?  or how about the [Weak Anthropic Principle](http://www.merriam-webster.com/dictionary/anthropic%20principle)?  isn't this about what is a tautology?
Comment: Just to point out that the question being asked is "does there exist a statement `S` such that...", rather than "please present me with a statement `S` which is such that..." . Would a non-constructive existence proof satisfy what you're looking for, whitman (which Mauro has the canonical demonstration of below), or do you actually want to see examples for what `S` might be?
Answer: I don't know much (or any) formal logic, but I believe the sentence "It is possible to construct an English sentence with thirteen words in it" serves as its own proof (since to prove it's possible to do something, you only have to produce an example of it).
Comment: It actually hinges on the difference between statements and propositions, but roughly speaking what you get is a statement that expresses a true proposition and serves as evidence of the truth of the proposition.
Answer: Exploiting the technique of arithmetization of syntax of Gödel's incompleteness theorem we can formalize in a system T whose language contains a "certain amount of elementary arithmetic", like (first-order) Peano arithmetic PA or Robinson arithmetic Q, a provability predicate Prf(x, y).
With it we can "manufacture" a formula Prov(x) [i.e. ∃yPrf(x, y)] that holds of the number g iff the formula with Gödel number g is provable in the system T.
Now we apply the so-called The Diagonalization Lemma :

Let T be a theory containing Q. Then for any formula B(y) there is a sentence G such that G ↔ B(g), where g is the Gödel number of the formula G.

Now we have only to apply the Diagonalization Lemma to the formula Prov(x) to get :

S ↔ Prov(s)

where s is the  Gödel number of the "diagonalized" formula S.

This is a statement that "express" its own proof ...
Answer: The answer is yes. In a formal system a formula "S" is provable/derivable/deducible from a set of formulas "Gamma" if there is a finite sequence of formulas A1, A2, ..., An such that An=S and each Ai (1<= i <= n) is either an axiom, or a member of Gamma or derived by one of the inference rules of the formal system (Ex. Modus Ponens) using previous elements of the sequence.
Therefore for any formula S, we can define proof(S) to be the finite sequence defined above. So for any axiom "A", we have proof(A)=A (because the sequence of proof of axioms has only one member which is the axiom itself).
Comment: I agree in spirit, but the sequence (A) of length 1 is not really the same thing as A.
Comment: @WillO Good point! But I think it is not a big problem. It's just the representation. I think We can slightly change the representational part in definitions (of WFF or Proof) to resolve this issue.
Comment: Yes, I agree with you.
Answer: In classical logic is no possible because a valid statement is a formula, which is a sequence of primitive symbols (and, or, implies, etc.). By other hand, a proof is a sequence of formulas, where one is a deduction of previous formula.
Although, exists something called intuicionist logic, where a statement is a "construct", i.e, a statement which is a "construction" from other statement; in short, this statement is in self a consctruction. So, if one see this construction like a proof, you can assert "S = proof(S)", in the sense of intuitionism.
Comment: If you state this argument, you must take into account that a proof is a (finite) sequence of formulae where each formula in the sequnce is *(i)* an axiom, or *(ii)* derived with a rule of inference (e.g. *modus ponens*) from previous formulae of the sequnce. Thus, a one-line proof with only an axiom as conclusion is a formally valid proof. Having sais that, I think that the OP's question means something more "vague": a statement expressing its own proof, that sound different ...


Will a Schengen visa allow an Indian citizen to Mexico?

Question: I am an Indian citizen having a valid multiple entry Schengen visa and want to travel from Delhi to Mexico via Amsterdam. I will stay in Mexico for 3 days & will head towards Paris. Do I need a Mexican visa or only a Schengen visa?
Answer: following the info in Mexican Consulate in SFO, the Schengen visa allows you to enter Mexico.

With the purpose of facilitating and promoting travel to Mexico,
  effective May 2016 all those foreign nationals, regardless of their
  nationality, visiting Mexico for tourism, business or transit are NOT
  required to obtain a Mexican visa IF they hold a valid (non-expired)
  Visa or Permanent Residence of any of this countries: United States of
  America, Canada, Japan, United Kingdom or Schengen area (European
  Union).

Source: Department of the Consulate General of México in San Francisco
In spanish:

b)  Visa válida y vigente de Canadá, de los Estados Unidos de América,
  Japón, el Reino Unido de la Gran Bretaña e Irlanda del Norte o
  cualquiera de los países que integran el Espacio Schengen.

Source: Países y regiones que requieren visa para viajar a México
Answer: Per Timatic, the database used by Airlines:

Passengers with a valid visa issued by Canada, Japan, USA,
    United Kingdom or a Schengen Member State for
    a maximum stay of 180 days¨

So no, you do not need a visa


Kernel Read/Write Userspace memory

Question: First, malloc a buffer from userspace and fill the buffer with all 'A'
Then, pass the pointer of the buffer to kernel ,using netlink socket,
Finally, I can read and write the buffer, using the raw pointer directly passed from userspace.
Why ?
Why directly access to user space memory from kernel is allowed?
Linux Device Driver, Third Edition, Page 415, said that The kernel cannot directly manipulate memory that is not mapped into the kernel's address space.
Comment: @BaliC Linux Device Driver,Third Edition, Chapter 15, Memory Mapping
and DMA, High and Low Memory Part said The kernel cannot directly manipulate memory that is not mapped into the kernel's address space.
Comment: Sorry my mistake, it looked like homework! :)
Answer: The point is that accessing user addresses directly in kernel only sometimes work.
As long as you try to access the user address in the context of the same process that allocated it and that the process has already faulted it in and you are using a kernel with a 3:1 memory mapping (as opposed to 4:4 mapping that is sometimes used) and that the kernel did not swap out the page the allocation is in - the access will work.
The problem is that all these conditions are not always true and they can change even from run time of the program to another. Therefore the kernel driver writers needs to not count on being able to access user addresses.
The worst thing that can happen is for you to assume it works, have it always work in the lab, and have it crash at a customer site every so often. This is the reason for the book statement.
Answer: In this book - words 'The kernel cannot directly manipulate memory that is not mapped into the kernel's address space' is about physical memory. Other words - kernel has only 800-900 MB (on x86) that can be mapped to physical memory at one time. To access whole physical memory kernel need constantly remap this region.
Netlink not dealing with physical memory at all - it is designed for bidirectional communication between userspace<->userspace or userspace<->kernelspace.

Kernel Read/Write Userspace memory


Visiting Assistant Professor before a postdoc position?

Question: I am considering a 1 year visiting assistant professor position so as to have employment upon my defense in the next few months. However, after that time I would seriously consider other options including postdoc positions, but I wonder how difficult this 1 year "gap" would make obtaining a postdoc position. I would still be able to collaborate with the research group I am currently in and would also look to develop some theoretical/computational aspects of my research abilities during this time. I also have a paper to be published likely after my defense. Looking for any insight or if anyone else has been through this same situation.
Comment: Don't take a teaching job unless you really want to teach.  It's not good for students to have unmotivated teachers.  The pay is rarely good anyway.
Comment: What is your actual question? "Looking for insight" is [not on topic](https://academia.stackexchange.com/help/on-topic) for this site.
Comment: In mathematics in the US, VAP positions are often considered to be a kind of postdoc, most useful for people who want to go into college teaching at regional comprehensive universities or community colleges.  It is possible to move on from one of these positions to a research postdoc, but don't expect that you'll be doing things during your year as a VAP that will improve your chances of getting a research postdoc- you'll probably have no time for research.
Comment: What field roughly?
Comment: _I am considering a 1 year visiting assistant professor position...after that time I would seriously consider other options including postdoc positions_ A visiting assistant professor position is surely higher ranked than a postdoc position: Why take a demotion? _I wonder how difficult this 1 year "gap" would make obtaining a postdoc_ The visiting position doesn't seem like a gap, especially when you _would...collaborate with the research group I am currently in and would also look to develop some theoretical/computational aspects of my research abilities during this time_
Answer: If the alternative is a gap year then it is a good idea to take it or, at least, consider it.
Presumably a visiting assistant professor, if you don't hold an academic position elsewhere, is just about the same as a post-doc but under a different name, since it isn't tenure-track. It would, of course, depend on what is expected of you.
If the duties are very heavy on teaching, then it is more like a better paid adjunct. You probably want to avoid that. If it is similar to an ordinary faculty position, with some teaching and some research then it can have long term advantages for an academic career. But consider, also, what courses you would have to teach and how much support you get in doing that. And, some post-docs do teach.
If you are doing research (successfully) during this year as a non-student, you have the opportunity to change the relationship you have with the faculty, building up a circle of collaboration that can be advantageous for your career.
Overall, though, I'd think that it gives you more, not fewer, options.  And certainly more options than being unemployed for a year. But if this is the first option that pops up, don't neglect to look for others before you commit.
Comment: And who knows what the world (and job market) will look like in a year. More graduates chasing fewer (?) positions overall is not great. But a (likely) multi-year post-doc might be better if it can be found now giving more time to ride out Covid-related weirdness.


Use of 우리 for "my"

Question: From what I've understood, when I talk about things that are not entirely mine I have to use the term 우리. In formal situations in which I'd have to use the ㅂ니다/습니다 form do I have to use 저희 instead?
Answer: Yes. "우리," be it "our" or "my," always becomes "저희" in formal situations.
Answer: '우리' and '저희' means totally same.
But the difference is coming from whether other party is higher than our.
If you're saying to senior/older people or formal situation you should use '저희'.
ex) 저희는 한국사람입니다. - We are Korean.
You are same qualification(like friends) or older than other party, you should use '우리'.
ex) 우리(나라)는 미국과 동맹국가입니다. - Our Country is an ally with the United States.
This usage is one of the many things that is wrong in Korea. When you confuse in grammar, You can simply use '저희' in many situations.
Comment: I'm not sure if my question was clear, I'm going to ask it in another way since I think that you didn't really answer my question. I already knew that 저희 was used in more formal situations than 우리 but my question asked if this applied when 우리 has the meaning of "my" as well. For example in an informal situation I would say "우리 선생님은 교실 안에 있어" meaning "my teacher is in the classroom". But if I had to say this same sentence in a formal situation would I hace to say "저희 선생님은 교실 안에 있습니다"? (Ignore the fact that I didn't put proper honirifics in the two sentences, I don't know them well)
Comment: @EmaTavox Yes. And you made perfect sentences!

Use of 우리 for "my"


Mapping KVM guest VMs to subdomains

Question: I've just installed KVM on a Ubuntu host system and intend to install two guest VMs on it. I have a domain, e.g. example.com. How can I configure my system so that VM(a) is accessible by example.com and VM(b) is accessible by internal.example.com?
Answer: Assuming the VM's have publically accessible ip addresses (that's a different question) you will want to contact whoever manages the example.com dns server and provide those ip addresses so that they can add these two systems to it.

Mapping KVM guest VMs to subdomains


How is Crystal Oscillator implemented in digital circuit to generate clock frequency. What part of crystal oscillator makes the clock signal?

Question: How is Crystal Oscillator implemented in digital circuit to generate clock frequency?
What part of crystal oscillator makes the clock signal?
I know the fact that crystals has its resonating frequency, can someone explain what happens to the voltage at one end and the other end of crystal?
Answer: First, consider what crystal component really is: a capacitor with a fancy material between its plates. This should give you an insight at what happens to voltage/current across a crystal component.
The insulation material selected for this "crystal" capacitor exhibits piezoelectric properties (as other answer already expanded): it will mechanically deform a little when electrical field is applied; it can also produce some electrical field when elastic deformation is relieved. This particular property makes it behave similarly to an inductor. This observation allows us to come with an equivalent circuit of a crystal (a rather simple LC bandpass filter):

Then, to the bit of oscillator theory: to create oscillation you, commonly, need a filter and a high gain amplifier, feeding each other. During the oscillator start up, amplifier will produce some wide bandwidth thermal noise, filter will pass through only the desired frequency, this particular frequency will be further amplified by the amplifier until it's much higher power than noise, and the loop will close producing a sustained oscillation. Clearly, filter can be of any variety, crystals being the most convenient and stable ones (but you can easily substitute the crystal with a "wound" LC circuit or electromechanical tuning fork or any other filtering contraption of sort).
A bit of update on similarities between inductors and piezoelectric materials
As stated above, electric field applies a force to the piezoelectric material. The force exists, because matter inside the piezoelectric crystal is arranged in the form of tiny electric dipoles (for a bit of reference, the most common "dipole" molecule around is water, which acquires a lot of interesting properties because of this).
However, if there were only external electrostatic forces present, piezoelectric crystal will fall apart - evidently, some force is needed to counterbalance the electrostatic force and return the crystal to "normal" state when it is removed (thus, enabling its use in oscillators). This is a so called "elastic force" - force arising from (quantum) electrodynamic interactions between electrons and nuclei of the matter. That's the force responsible for preventing us from falling through the floor.
Not surprisingly, there exist 2 other types of oscillators which rely on elastic forces to provide the "push back" against the externally applied force:

Mechanical spring pendulum - external mechanical force is balanced by the elastic forces in the spring material.
Inductors - Lorentz force acting upon the conductor is counterbalanced (yet again) by the elastic forces of the wire. After all, if not for the elastic forces the electrons will simply fly away from the conductor, and no inductance will be observed.

That last effect is not readily observable in small signal electronics (copper is a fairly strong material and the currents are tiny) but becomes the dominant factor when currents start climbing into the 100A ranges and above - wires would jump all around the floor and solenoids will explode violently when Lorentz force suddenly overcomes the elastic forces.
To summarize:
In piezoelectric crystal oscillator:
Step 1: external field applies a force to dipole nano-sites inside the crystal affecting the electron configuration inside the crystalline structure, creating elastic deformation (and thus elastic counter force).
Step 2: when external field is removed, elastic forces restore the electron configuration to its "normal" state. As charge redistribution is involved, we observe a voltage across a crystal of a nature, rather similar to the "back emf" of inductors.
For comparison, in inductor based oscillator:
Step 1: external field invokes current in the inductor (because, unlike the piezoelectric crystal, electrons in conductors have a very high mobility and won't stick to specific sites inside the crystal). Current results in Lorentz force which deforms the electron configuration inside the material and creates elastic deformation (and thus elastic counter force, just like in the piezoelectric case).
Step 2: when external field is removed, we have exactly the same situation we had in the piezoelectric case - electron configuration is restored to normal, charge is being redistributed and "back emf" created. However, as we are dealing with highly mobile charge carriers in this case, it will be manifested in form of current, rather than voltage spike.
With the right choice of parameters, the math will look rather similar in both cases.
Comment: "It will mechanically deform a little when electrical field is applied; it can also produce some electrical field when elastic deformation is relieved. This particular property makes it behave similarly to an inductor."  - Can you explain how does the mechanical strain and stress vs electric field generation process makes the crystall look like an Inductor?
Comment: I will ask you an opposite question: when we say that magnetic field energy is stored in the inductor, what exactly do we mean? What forces are at work to create the potential energy? What will happen if inductor is mechanically weak, but the current is strong (hint: Lorentz force)?
Comment: megnetic energy is store in inductor when current is increased in the inductor, the more current strengthens the megnetic field around it but changing the current induces emf by lenz law. As emf is generated in opposite direction it opposes the current to get back to its original current. plz correct me if i am wrong. I don't know what happens if the inductor is mechanically weak. help!
Comment: Ok, edited the answer to add more info.
Comment: Refering to your oscillator theory: How do you get like 3-10 Ghz frequency generation. The maxium crystal oscillator I have seen is 480 Mhz. Are they any osicllators exist that can be used to filter frequency ranging from 3-10Ghz after the amplification stage? Usually I have heard the PLLs are capable of generating this high frequencies using inbuilt VCXO to filter high frequencies (GHz range) but what are those VCXOs made of? Crystals?
Comment: I believe, this is a Q/A site, not a discussion forum for all related topics. You can always ask more questions on the main page (or do some self study with the abundant literature available on the subject). Also, this: http://en.wikipedia.org/wiki/Surface_acoustic_wave
Answer:
can someone explain what happens to the voltage at one end and the other end of crystal?

Yes, the author(s) of the relevant Wikipedia articles made an attempt at explaining what happens.

A piezoelectric disk generates a voltage when deformed

A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency.

http://en.wikipedia.org/wiki/Crystal_oscillator

Piezoelectricity is the electric charge that accumulates in certain solid materials (such as crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress

...

The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in crystalline materials with no inversion symmetry.[4] The piezoelectric effect is a reversible process in that materials exhibiting the direct piezoelectric effect (the internal generation of electrical charge resulting from an applied mechanical force) also exhibit the reverse piezoelectric effect (the internal generation of a mechanical strain resulting from an applied electrical field).

http://en.wikipedia.org/wiki/Piezoelectricity


Summary of recent results presentation slide: proper citation

Question: In some conference presentations a slide with a summary of recent progress in the field is included. One way to do it is to present a graph, where the recent results are compared in terms of some parameter. I was wondering what are the good ways to properly reference all the publications. Needless to say, a full citation of every result (let's say there are 20 results from several different groups compared) would definitely take up too much space in the slide. So, how much information is enough for the publication to be properly referenced (i.e. name of the first author + publication venue + date)?
Comment: Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
Answer: First author plus year.
However, if you present 20 results from 20 groups, you are not doing an introduction, you are starting a review paper.
On the other hand, if you are presenting 20 results, but they are actually only 4 parameters from 5 groups ... you are still doing a review!
Focus on the 2-3 most recent results relevant to your work. Show that you are either on par with the best ones, or that you have something very different with respect to the most recent (and you have good reasons for that).


How to drop missing dbf in recovering instance

Question: Colleague (really!) attempted to create a dbf on primary prod DG instance. This worked (though file subsequently dropped), but db_file_convert not set up for file location secondary so creation failed and "Missing" dbf created in $OH/dbs.
Errors in alert log:
<code>ORA-01119: error in creating database file '/<path>/<file>.dbf'
ORA-27054: NFS file system where the file is created or resides is not mounted with correct options
Linux-x86_64 Error: 13: Permission denied
File #1124 added to control file as 'UNNAMED01124'.
Originally created as:
'/<path>/<file>.dbf'
Recovery was unable to create the file as:
'/<path>/<file>.dbf'
MRP0: Background Media Recovery terminated with error 1274
Errors in file /<path>/trace/<instance>_pr00_33046.trc:
ORA-01274: cannot add datafile '/<path>/<file>.dbf' - file could not be
created
</code>
This then causes DG on HA instance to fail with:

ORA-01111: name for data file 1124 is unknown - rename to correct file
ORA-01110: data file 1124: '/$OH/dbs/UNNAMED'
ORA-01157: cannot identify/lock data file 1124 - see DBWR trace file
ORA-01111: name for data file 1124 is unknown - rename to correct file
ORA-01110: data file 1124: '/$OH/dbs/UNNAMED'

File does not exist in prod and reading regarding non-recovering DG databases states to create file or drop it. Neither of these options are available in the secondary database. What are my options? Recreate control file on HA without ref to missing file then restart? Doesn't help that my DG knowledge is a tad rusty - by tad I mean solid iron oxide.
Answer: Copied control file from source where missing file did not exist


how to send mail to developers about build status

Question: I would like to send build status mail after maven build. I configured in pom.xml.
<developers>
<developer>
<id>phani</id>
<name>phanikumar</>
<email>email@example.com</email>
<url>http://phani.net</url>
<organization>codeauroara</organization>
<organizationurl>http:/mojocodeaurora.org</organizationurl>
<roles>
<role>Lead</role>
<role>developer</role>
</roles>
</developer>
</developers>

Presently, I am using jenkins for automate build it is sending mail suppose build fails.
I want to do jenkins should read in pom.xml file those who are in developers list of pom.xml file.
for those members only send mail through jenkins about build status.
Please tell me if any plugin required for this or if I need to configure any mailing lists in pom.xml file.
Answer: Maybe you could first look at the Jenkins's Mail Server Configuration and at Email Notification in the job's configuration.
You could also use a Jenkins plugin : email-ext to perform more advanced tasks.
See also this SO answer : how to add developers list in pom.xml file.whenever the build fails it will send notifications
Comment: Your answer doesn't help. None of your suggestions utilizes the developers information from the pom. The SO answer you refer to doesn't help either.


Multiple SNMP servers on single agent

Question: I am trying to test a new SNMP server (Nagios Core) along side the current one (Check MK) which has already been running for quite some time now. Will this cause any problems on an agent device (Cisco Catalyst 2960X), if a second SNMP server is added on with the first one?
Comment: Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
Answer: Short answer: It may.
Network devices usually don't have powerful CPUs and I've seen devices running into timeouts when more then one system was polling data via SNMP. It all depends on how much data you poll, how often you do this and if the device CPU is currently busy with something else.  I'll guess the only thing for you to do is try it and keep a close eye on your systems while doing it.
Answer: No, I've seen up to 4 different NMS polling the same devices with 1 minute intervals for about 10 or more sensors per device.

Multiple SNMP servers on single agent


Are Ph.D. dissertations always required to include new discoveries?

Question: Are Ph.D. dissertations always required to include new discoveries? I was wondering if a dissertation could, instead, consist of a reorganization or reformulation of known material. Inventing new things is not my strength. But I think I'm pretty good at organizing information and making it clear and easy to understand. In some cases, I think that this activity might even be more valuable than discovering new things, and I find it to be just as satisfying. I can see that it might lead to a nice book, but I don't know if it could ever get me a doctorate degree. The field is mathematics, if that matters.
Comment: http://matt.might.net/articles/phd-school-in-pictures/
Comment: @bobbrown Exactly what I thought while reading the question. That is *the* best visual explanation I've ever seen to what is a phd.
Answer:
Are Ph.D. dissertations always required to include new discoveries?

What distinguishes a PhD from all other academic degrees is the requirement of a novel, sufficiently substantial intellectual contribution.  In practice, what "novel" and "sufficiently substantial" mean are handled on a case-by-case basis.
The word "discovery" does not appear in the above description and would not be appropriate in certain academic fields, but in mathematics a PhD thesis is  required to contain new theorems or new proofs.  It is often contrasted (e.g. in various programmatic descriptions and graduate handbooks) with a master's thesis, which may contain only novelties of exposition.
So if you go around mathematics departments asking "Can I write a PhD thesis in which the novelties are all expository?" I would expect the answer to be a quite consistent no.
However:
1) "Inventing new things is not my strength." I too was once extremely daunted by the prospect of inventing/discovering significant new mathematics...so much so that I spent five years as a PhD student learning how to do it.  (I got it now.)  My point being: if you are otherwise talented and interested in mathematics, there is no good reason to think that you will not become good at this.
2) If you are otherwise talented and interested in mathematics, you will very likely become good enough at discovering new mathematics to write and defend a PhD thesis.  The requirements of a PhD thesis are partly aspirational: in reality, we settle for a bit less much of the time.  I have seen some PhD theses where I was confident that there was nothing really new in them.  To be a bit specific, I know a thesis in which a student attained the main result through five pages of calculation, whereas a more conceptual framework would yield it in about a paragraph.  The result was of the form "For all n, there is a [mathematical object of type X] for which the [parameter value associated to any mathematical object of type X] is n."  It had not been stated before in the literature, but when I searched further I found that the theorem could have been proven simply by citing extant examples in the literature.  And by the way, this was not the worst PhD thesis I've seen -- on the contrary, the theorem was natural and interesting, and I rather liked it.  It resulted in a paper published in a reputable journal.
3) You write

In some cases, I think that this activity might even be more valuable than discovering new things,

Yes, I agree most heartily.  I suspect that the number of theorems proved in 2016 lies in the interval [10^5,10^6].  Without smart people taking time to reorganize, simplify and expose these results, we're all going to get carried away in the flood.

and I find it be just as satisfying.

I do a lot of expository work, and I find it more satisfying in some ways and less satisfying in others than discovery-oriented research.  It is an unfortunate professional reality that its perceived value by the overall mathematical community is lower.
However, really good exposition is rare enough so that people who are good enough at it can make academic careers out of it.  Michael Spivak is a legendary figure for his lifetime of mathematical exposition.  Keith Conrad, a mathematician of my generation (and a friendly colleague of mine) is well on his way to the same achievement.  Both of these people can do discovery-oriented research and have the papers to show for it.  However it seems to be the case that they are more interested in exposition and have spent more of their time on that.  To become the Spivak or Conrad of your generation would be a worthy goal.
4) There are doctoral degrees that are not so heavily focused on discovery-oriented research.  In comparison to the PhD they are much fewer and they are not very widely known, but they exist.  Let me quote from my UGA colleague Michael Klipper's website:

Before being hired by UGA, I received a Doctorate of Arts (D.A.) degree from Carnegie Mellon University in 2011. Instead of doing research, I wrote a textbook draft for an analysis course.

I know nothing about this degree program other than what I just said, but I know Dr. Klipper and the value he brings to my department.  Based on that alone I recommend that you give it serious consideration and search for similar programs if you feel that this is where your interests lie.
Good luck.
Comment: Euclid comes to mind. Reorganization, if really well done, is valuable.
Comment: +1; this answer nicely reflects the practice rather than repeating slogans. I see you gallantly omitted yourself from the expositors' hall of fame :)
Comment: Let me add to this the remark that in mathematics, there is no 100% sharp distinction between novel exposition and novel research. Novel exposition often slightly improves the results it exposes (i.e., cleaning up a proof allows you to get rid of redundant assumptions or generalize it) and sometimes connects them with different results. There is no specific point where a generalization becomes a novel result; it's a spectrum.
Comment: Wrt the "textbook thesis": it is possible to do original research on education aspects of a field, which directly leads to an interdisciplinary thesis - and whether that would be more at home in the field in question or in didactics would probably depend a lot on details. E.g. how much original work is done to find new/better ways of explaining topics? How much work is done to find which explanation works best with which students? and so on
Answer: Generally: yes, you have got to do something "new" in mathematics.
It is open to debate what constitutes "new", however. It doesn't need to be the proof of the Riemann hypothesis.
Keep in mind that new discoveries and inventions don't appear out of nothing, but usually build upon existing work. That might mean that the basic ideas are already floating in the air at the beginning of the PhD. You just fill in the details of the possibilities that other people have conjectured.
Summarizing and organizing, however, can be a valuable contribution to your research. You will typically find holes in the theory as you organize it, and so you can fill those in. If you find among those data a good abstract principle that has been overlooked previously, all the better.
Answer: The thesis is supposed to be publishable ... So if there are, in your field, published papers that do not include new discoveries, then (theoretically) there could also be Ph.D. theses that do not include new discoveries.
But in general what is acceptable for a thesis is something you decide with your advisor, not with us here.


Mosaico and 'View in your browser' not working

Question: I am running Mosaico beta 1.0 beta1 with 4.6.22 and 'View in your browser' does nothing.  Of course, this is a beta so not surprising.  But... is there a way round this? Otherwise quite impressed with Mosaico after a shaky start, but it would be good to have a greater variety of block shapes to add.  Anyway, for now the above is the problem that I would love to find a workaround for.
Answer: In general, bugs with extensions should be reported on the extensions' github page.
In this case, you may be interested to know that there is a new CiviMail/Mosaico extension nearing completion which will supercede the one you are trying out.


How to Download the video files on internet?

Question: Like Internet Download Manager for windows,Is there any software to download all files like videos,audios,softwares and all other files in Ubuntu 11.10?
Comment: Umm... click on the file in your web browser and choose save?
Comment: Possible duplicate of: http://askubuntu.com/q/54837/32037 And why ubuntu-one and jdownlader tag?
Answer: Here is a quick solution. You do not even need a software to download videos from web.
All you have to do is run following commands after the video has been completely loaded on browser.
$lsof | grep Flash
$cat > /proc/PID/fd/FD > filename.flv
Replace PID and FD with their corresponding value from the output obtained from the first command.
Consider following example.
lsof | grep Flash
chrome     3151     abnormal   23u      REG       8,23  1274744  1058929 /tmp/FlashXXmkVBxq (deleted)
cat /proc/3151/fd/23 > "video.flv"
Comment: I disagree (and probably lots of other people will disagree too)  @tachyons It is always good to have alternatives (and command line works across lots of systems and sometimes across browsers too).
Comment: It is better to post gui tools
Comment: An experianced user always prefer commandline tools ,Because of it strength, But a normal user(most of the users) prefer gui way ,personally I dont like to suggest CLI methods
Comment: @I'mnotthisperson JDownloader is a good GUI tool
Comment: And what is `(deleted)` at the end of the output?
Answer: If you just need to download videos (as the question title suggests), Video DownloadHelper it's a nice add-on for Firefox that works fine.
If you need a download manager (as the question text suggests), I'd recommend steadyflow. Also, you can use FlashGot for firefox integration.
Answer: Videodownload helper extention for Firefox works fine for videos.
DownloadthemAll extention for other files (download acceleration, filter rules to download multiple files at one go, etc.)
Answer: Even in IDM (or Orbit or any other downloader), you need a link or the URL, which locates the resource and can be used to download the file from the remote server.
If you have a link, you need a download manager (like how you used IDM).
If your query is limited to downloading videos from the Web, you might want to get their links and use a download manager still. If not, you might want to go over the extensions of your browser.
Download Manager for Ubuntu
I personally recommend SteadyFlow, but you're recommended to go through all of them and choose for youeself. For doing that, open Software Centre, and search for "downloader". SteadyFlow is the most rated as of now.
Videos Only?
Most popular browsers have extensions that facilitate downloading videos from online streaming websites. Search for them. But in the end, they too just grab the link and download via your browser's download manager (which is pretty slower than a standalone download manager).
For example, if you have Firefox, you can press Shift+Ctrl+A to open the "Add-Ons". Here, you can search for extensions with queries related to "download videos". I personally use FlashGot, which is used to get the link of any media: video, image, music, etc.
Alternatively, there exist many online tools for that purpose. You can run a Google search with "download videos online" for the latest results. You can use those services to download videos from particular websites.
Answer: Try JDownloader, follow the link.
Comment: (Downvote reason). Link-only answers are usually considered spam.
Comment: @Abhimanyu - Well the question clearly asks for a GUI tool for download, which i mentioned in my answer. If the link dies there is still name to search for. If there is better way to present this answer please be my guest.
Comment: I recommend you read hexafraction's comment. I just put an extended comment on that, because it's quite rude to just downvote without a reason. Please read his comment.


IC which can drive an RGB input to two outputs

Question: Is there a way to duplicate a wire from an RGB source without degrading the signal. I wish to take a wire and split it into two wires without the current halving. (edit) 75 ohms and one RGB signal wire i.e just the blue line.
Comment: Please define RGB source, and "degrading the signal". What is your bandwidth? What is the RGB source current?
Comment: Is the RGB source a DC source or a variable digital one?
Comment: This question is almost useless without a better definition of what "RGB source" means. It could have many definitions, each with different answers.
Answer: VGA means you have a 75 Ohm source on each RGB signal that is ~1Vpp when loaded by a 75 Ohm load (monitor)
Loading with 1/2 * 75 Ohms implies you need the same source impedance to get the same voltage. for black and white levels.
It will work but only only at 50%luminance. (peak voltage =100% bright)
Then you can compensate on monitor or leave as is.
When you use long coax cables, if you have 75/2 Ohm load and source now any mismatch may result in ghosted edges with cable being 75 Ohms.
A video splitter amp uses separate 75 Ohm video buffers for each output to drive monitors directly near DC to xxx MHz .... unlike RF splitters which use hybrid transformers.
Here's a collection of suitable designs from LTI
http://cds.linear.com/docs/en/application-note/an57fa.pdf
Answer: It depends on what you want to do with the signals once you separate them. It also depends on the specific type of "RGB" interface you are dealing with: 3-wire, 4-wire, or 5-wire.
The 3-wire interface has only the R,G & B signals and carries the sync information (usually) on the G wire. Aka: sync-on-green. The 4-wire interface has an additional wire which carries both H-Sync (horizontal sync) and V-Sync (vertical sync) OR'ed together into one electrical signal. The 5-wire interface has two additional wires - one for H-Sync and one for V-Sync.
The R,G, & B signals are almost always "75 Ohm signals", meaning they have a source impedance of 75 Ohms and they must be terminated with 75 Ohms at the receiving end (generally a video monitor, but possibly a repeater input, splitter input, and similar). The connecting cable must have a characteristic impedance of 75 Ohms as well, and is usually co-axial cable.
On the other hand the sync signals in a 4 or 5-wire hook-up are typically NOT 75 Ohms, they are essentially TTL logic levels, most typically 0-5 volts. That's typical, there are systems where these signals are also "75 Ohm signals" and are therefore carried by coax cable. If you are dealing with a PC, laptop, TV-set with auxiliary "VGA" or "RGB" input, these are almost universally TTL signals. Sometimes these signals are terminated at the receiving end, sometimes not. If terminated you will find anywhere from 470 to 2,000 Ohms as terminators.
Now, to split the RGB signal passively (not using a powered splitter, but a simple "Y" connection) you have several problems to overcome. The first is not losing signal amplitude when you connect the receiving ends to two 75 Ohm loads simultaneously. This will halve the signal amplitude and cause various problems, primarily loss of contrast in the image. In a 3-wire system the sync-on-green signal may be so adversely affected that the connected monitor(s) will not be able to acquire sync. Depends on the actual monitors you are connecting and how much internal gain they use to recover the RGB signals. There is one cheap trick fix I describe below for this situation which will work in some instances.
The sync signals present less of a problem if they are the TTL type (very typical in PC and Laptop environments). These you can usually split with no ill effects because of the way they are usually lightly terminated at the receiving end. If the syncs are 75-Ohm, you have more of a problem. The cheap trick described next might even work for these.
Cheap Trick: Remove the 75-Ohm termination resistors from one of the monitors. If the connecting cables are "short enough" and the signals you are sending are low-resolution (e.g. 800x600 @ 60 Hz), you might get a suitable image. There are lots of variants possible with this trick. If you have two monitors, leave the termination resistors in the far monitor and remove them from the close monitor. Daisy chain the monitors together, remove the termination from the first monitor and run the signals to the second monitor right from the connector of the first monitor, but leave the terminators in the second monitor. Don't worry about the sync termination resistors unless all else fails. Leave them intact in both monitors.
Your best bet is to try both monitors first with their original 75-Ohm termination resistors intact, and adjust the contrast on each monitor to see if you can get a decent enough picture on the screen. The "Contrast" control is actually the video gain control. Depending on the specific monitor this gain might have enough high-end margin to squeak you by. (The "Brightness" control is actually video offset and will have much less of an effect on weak signals.)
If all else fails, buy a splitter. They are actually pretty cheap these days.


Are USB 3.0 ports yellow?

Question: My laptop has a yellow USB port. Is it USB 3.0?
Comment: Did you consult your Notebook user guide?, edit your post to include your Make and specific model if you wish a correct answer instead of an educated guess.
Comment: @DaveRook maybe in 2010, when I asked this question, there were no answers on Google?
Comment: No. The standard color for USB 3.0 ports is blue. A yellow port is likely to be a sleep-and-charge port.
Answer: A yellow USB port usually denotes an "always on" port. It supplies power to the device plugged into it even when the laptop is off.
Comment: This tends to be the standard but it is not always the case.  That said, I have yet to see a USB3 port that isn't blue.
Comment: @Shinrai Apple MacBooks have white USB3 ports
Comment: I've seen a USB 3.0 port in WHITE as a keystone jack - no, it's just the casing that is white.  The inner bar of the port is still blue.
Answer: If it's a Lenovo ThinkPad chances are it's the "always on USB", which is a special port that remains powered when the computer is asleep, or when the computer is fully powered off and the AC adapter plugged. This allows you to, for example, keep charging your phone or wireless gamepad without having to keep your computer running.
http://www.asprangers.com/post/2012/10/02/What%E2%80%99s-in-color-Yellow-Blue-Gray-color-USB-Ports-on-Lenovo.aspx
Answer: It can be whatever colour that your manufacturer wants it to be - I do not think there is a standard.
That being said, every product I have seen that supports USB 3 seems to have a blue socket, so there may be one!
If it is yellow, my guess is that it may be a way to indicate enhanced/increased/special power output.
What model do you have? If you write the model number, I can take a look to be sure.
Comment: Actually, section 18.104.22.168 of the USB 3.0 spec says, "As an aid to users, USB 3.0 mandates standard coloring for plastic portions
of USB 3.0 plugs and receptacles." However, the spec later only "recommends" Pantone 300C as the color for USB 3 connectors.
Answer: I Have tested with CrystalDiskMark and  128GB JetFlash USB3 Transcend JetFlash®810
In my Thinkpad E530c the right side single yellow port is USB2

but with one of 3 left side blue USB ports the below result returned showing the blue ones are USB3 in this Thinkpad Edge

Are USB 3.0 ports yellow?


Wallace effect on human populations

Question: Could sympatric speciation occur in humans, for instance, separating blue collar workers from white collar workers?
Answer: No, it won't happen
Even geographical isolation failed to allow speciation in humans (see Does it make sense to classify all humans in a single species?). So if there is some small reproductive isolation between working groups, it is definitely not going to be strong enough to cause speciation.
In an imaginary world
If there was strong reproductive isolation
In an imaginary world where there is a more important reproductive isolation that lasts for long enough between blue and white collar workers, sympatric speciation might be possible. Such reproductive isolation could be caused by very important social stigmas.
Wallace effect
Wallace effect is a type of speciation that is mediated by a reproductive isolation that has been selected for as fitness of offspring is higher when mating is assortative.
There is probably relatively little genetic predisposition to being blue collar or white collar worker. But in an imaginary world where there is very strong genetic predisposition to being a good blue collar or white collar worker, and where worker quality will affect income which will affect fitness, then yes, wallace effect could be a thing.
But really, we do not live in one of this imaginary world. In our world, speciation won't occur between blue collar and white collar workers.

Wallace effect on human populations


Can I use Time Machine to Mirror two Macs?

Question: Sooo.  I have a guy retiring soon, and it looks like I get to take over his mac.  I made a critical error in updating to Catalina, and lost use of alot of the apps we use here at work.  Could I restore Mac 1 (My mac) using the Time Machine set up for Mac 2 (The retiree's mac). Thinking we could use it to get Mac 1 prepared for another trainee in a week or so.
Comment: I have not tried this, but I am guessing that it would work best to reinstall the OS on that Catalina PC and then "restore" it using the desired TM backup. And be sure you have a backup of your data too, you'll likely need that as well
Answer: Migration Assistant can 'copy' one machine to another.  I would ask your IT department first as this can really muck up management and other things.
https://support.apple.com/en-us/HT204350


Is it possible to only allow people in a particular SharePoint group to be selected in a workflow?

Question: Is it possible to set up the Page Approval workflow so that only people in the Approvers group can be selected in the "Assign To" field?
Comment: Please **tag by feature or topic** and not by version or product. This helps to attract more attention to your question and keep it relevant. See [How do I use tags](http://meta.sharepoint.stackexchange.com/questions/264/) for general guidelines. Thanks
Answer: You can do this by first creating a copy of the Approval - SharePoint 2010 workflow and then open ReviewApproval_1033.xsn Association/Initiation form in InfoPath.
Then right click on the Approvers e-mail adressess -field and select Person/Group Picker Properties. In the SharePoint Server tab write down the address of your site collection and then open the General-tab.
You can select a group from the Choose from dropdown field that you want to limit the workflow to.
Comment: I do not have InfoPath so that I cannot test it out. Is there any alternative?
Answer: Yes, Sure you can set the approval group in the workflow configuration
Comment: Cheers but I can't find the option anywhere in the workflow settings. I hope I'm not overlooking something real simple haha! Basically I've set up a "Approval - Sharepoint 2010" workflow for all of the pages in a subsite. When I open the settings for that workflow (Site Actions -> View All Site Content -> Pages -> Library -> Workflow Settings), I cannot see an option that would let me limit the list of people who can be selected in the **Approvers** section. Thanks anyway Paddy!
Comment: You can set the approval group in configuration but the workflow initiator can always change it to someone else.