Why does steam in Chernobyl reactors speed up nuclear reaction?

Question: After watching the TV series Chernobyl I am struggling to understand why steam in a reactor core would increase the rate of the nuclear reaction. My first guess would be that liquid water would accelerate the reaction as it is used as a moderator in other reactors.
Answer: The reactor type installed in Chernobyl (Wikipedia: RBMK) uses graphite as moderator. The water also moderates, but its effect is proportionally much less important than in a water-moderated reactor. This means a steam bubble forming results in only a small reduction in moderator efficiency but a large reduction in neutron absorption, thereby increasing the fission rate. This behaviour is referred to when the reactor is described as having a high positive void coefficient (the link goes to the subsection of the above Wikipedia article discussing this).
Answer: Liquid water both moderates and captures neutrons. Steam (gaseous water) does neither to any effect. At the initiation of the "pump run-down test" there was very little steam production in the fuel channels (thermal power <7%). An expected consequence of the "test" was a drop in Steam Separator (and reactor coolant) pressure as the #8 Turbine-Generator valves opened to power Reactor Coolant Pumps during a (simulated) loss of plant power. This caused the now super-saturated water in the core to boil as the pressure decayed.
Because the coolant was very near saturation temperature throughout the entire system, voids formed throughout the entire core almost simultaneously. Neutrons that would have been absorbed by water milliseconds earlier were moderated by the graphite and causing more fissions... causing more heat, more steam, more neutrons, and so on.
liquid water is more efficient at absorbing neutrons than moderating them compared to graphite. As nearly ALL of the control rods were fully withdrawn, only Xenon and liquid water were limiting the reactor (by absorbing neutrons). Steam forming in the coolant channels stopped absorbing neutrons, but the graphite continued to moderate them so the fission efficiency actually improved as voids formed. The term 'Positive Void Coefficient' is somewhat misleading... In an absolute sense, gaseous steam is 'Less Negative' than liquid water in a graphite moderated reactor.
The core configuration and particularly the coolant thermal conditions were such that it all makes sense, predictable with hindsight (or foreknowledge). the HBO mini-series short changes the accident sequence-of-events (my opinion - and I would think that). The "Test" preparation, delay, and finally, the foolish execution; set the stage and triggered the unimaginable.
How the void coefficient, normally a small influence in the reactor's operation and power calculations, came to trigger an order-of-magnitude runaway power excursion was not portrayed very clearly in the mini-series. The "test" was the motivator for virtually everything leading to the event, and technically kind of interesting in its own right. I wish there was a scene in the pump room as the additional circulating pumps spooled up with appropriate sound and pipe motion.
Answer: Water is a good shielding material for neutrons. Moderating the neutrons will slow the reaction rates. When steam forms, it forms from the water used for moderating hence decreasing the amount of water.  This decrease in water will speed up the reaction rates.
Comment: Don't the neutrons need to be moderated in order for the reaction to happen in the first place because the fast neutrons from fission must be slowed down to thermal neutrons by the moderator (Graphite or water) in order to be captured by a uranium nucleus


Where is the one-eighth in 1/8-27 NPT?

Question: Where is the "one-eighth" in 1/8-27 NPT? I know it has 27 threads per inch. I see references that 1/8" is the nominal pipe size, does that mean the hole inside the thread is 1/8" ID? Or something else?
Answer: Threaded pipe sizes seem weird if you think they are like threaded fastener sizes, because they are not. They use a tapered thread, and the outside diameter of the pipe is not the "size" of the pipe.
They also are weird, because the fixed size of pipe is the outside diameter of the pipe, so the fittings will all work with any pipe of that size, but the nominal pipe size is the inside diameter and the actual inside diameter varies with the wall thickness or "schedule" of the pipe. So a 1/8" schedule 5 pipe has a considerably larger hole (0.405") than a 1/8" schedule 80 pipe (0.215".) And neither of those holes are 0.125" - just put it down to "some things are weird," and move on because logical consistency is hard to come by, other than "generally the hole in the pipe is somewhat larger than the nominal size" with a side order of "except when it's not."
1-1/2" Schedule 80 is the weirdo with an actual ID of 1.500" - I presume all the other pipes avoid them at parties.
Comment: it's a weird world where the weirdo is not weird
Answer: 1/8" was the nominal inside diameter of the pipe, back when they first started making standardized pipe.  That is a matter of the limits of the metallurgy and fabrication methods at the time.
As metallurgy improved, they were able to achieve the same strength with thinner walls. Now what?
They still need to fit legacy fittings.  But those attach to the outside diameter.  To avoid having 100 different sizes of fitting, they decided to keep the outside diameter the same and modify the inside diameter to suit the material thickness.
So, much like modern railroad track width being defined by Roman horse carts, we owe it all to history!
Comment: @crip659 because booster segments are shipped by rail, because they had to be built in Utah for political reasons.
Comment: Indeed at https://en.wikipedia.org/wiki/Nominal_Pipe_Size, and plausible, but haven't found a primary reference for this. You?
Comment: Roman horse/mule carts were also the reason for the size of the shuttle's booster rockets(might be an urban legend)

Where is the one-eighth in 1/8-27 NPT?


Windows Media Player pause control suddenly gone from task bar

Question: Up until last week, I could hover my mouse over the Windows Media Player icon on my task bar, and after a second or two, a play/pause button appeared (and I think a skip forward and skip back button too). Now it's gone, so I need to fully open the WMP window then click the pause button within that.
How can I bring back those buttons when hovering over the task bar icon?
It should look like this (when paused, that is. When playing, there's a pause button instead of the play button):

The installed version of WMP is 12.0.7601.17514
Answer: Though I did close and re-open WMP when I first noticed this problem, and the problem remained, today closing and re-opening it again seemed to fix the issue.


Online training login still active after redundancy

Question: My previous company had a subscription to an online training website for all employees.  I was made redundant a few months ago but I recently discovered that my login still works.  Should I login and do a few courses while it's still active or could there be some repercussions?
Comment: Is it possible that your former employer is intentionally allowing you to use it for a limited time as part of your severance?  It may be worth asking.
Answer: No, you should not login and do any courses.  You should assume the subscription is paid for by your former company and that it is only for current (not former) employees.
Your safest bet is to assume there could be repercussions.
Answer:
Should I login and do a few courses while it's still active or could there be some repercussions?

No, you should not.
It does not cost you very much to buy these online courses on your own because most online training websites often have lots of discounts, which can be 90% off all the time. It is safer that way as you don't want to have to deal with any possible "unauthorized access" and other headaches later on. Note that unauthorized access to a computer system is an actual crime in some countries (see e.g. the Computer Fraud and Abuse Act in the USA, or StGB §202a in Germany).
However, you still can email your former manager or call HR to ask if it is OK for you to logon and take the courses. There are 2 possible scenarios:

They may say that the courses are granted to ONLY current employees because the company only buys a limited number of licenses for those courses.
Or, they may say that it is OK for you to take the courses because they already paid for all employees for life. Even if you no longer work there, the courses are still friendly gifts for you to improve your skills and knowledge.
Comment: In the event of #2, it's critical to get it in writing from an official source with the authority to make the claim.
Answer: Different firms offer different kinds of subscriptions. It's possible your company bought 1-year subscriptions (for less a month than monthly ones) and can't get a refund if someone leaves with time on their subscription. It's also possible that they could get a refund, but consider the amount too small to spend time getting back. In these cases, you using the subscription would probably be ok, though it would be good to check.
In other cases, the company buys "seats" and can shuffle them around either using some sort of dashboard or by contacting a sales rep. Your subscription might be destined for someone who hasn't started working there yet. It could disappear at any moment, or you using it might cause them to be charged a fee since you used it this month.
In still other cases, they are being charged a monthly fee for you and will until they remember to turn it off. It doesn't matter whether you use it or not, but if you are nice enough to remind them about it, they can save a few dozen dollars a month by turning it off.
In all of these cases the good and nice thing to do is ask. "Sure am going to miss my [training company] subscription... does that stick around for a bit? Could I use it without costing you anything?" If you asked me that, I'd give you the honest answer. And whether you get to do the training or not, you'll have a clear conscience.
Answer: The fact that your login still works only means that someone forgot to disable it, not that you have the right to use it. So in the worst case, you could be accused of the legal equivalent of hacking into their computers. Especially if it costs the company money, or if someone doesn't like you, or both.
On the other hand, it may be that they don't mind at all; they might have a contract where you accessing the courses doesn't cost them anything. For example, if they pay for "up to 20 users" and you are user #17, they might be fine, but block you when they have four more users and you would stop an employee from using the course. So the solution is to call them and to ask for permission.
You might also be doing the company a favour: They might be paying for you and the person in control of these subscriptions hasn't been told that you left, and in ten years time their successor checks the list and asks "who is this Mr. Thyroid that we have been paying for for the last ten years?". So calling them is a nice thing to do as well.
Answer: I'm going to give two contradictory answers:
Answer number 1: The Sysadmin Answer - No you should not login, you should notify the company of the potential security breach and put in your communication that you are concerned that other logins you had may still be active - and that should there be an IT security incident you don't want to be blamed/held liable.
That's the 'correct' answer and the professional answer.
Answer number 2: They made you redundant, but couldn't be bothered to close off your access, so fill your boots and do any courses that you want and consider it part of your redundancy package.
This is the more morally grey answer - I don't recommend it or condone it - but...
Comment: That is dangerous advice. Yes, it is probable the company will either not notice, or not care about that type of access - however, they *might* notice. The danger is mainly that in some countries there is the crime of "unauthorized access to a computer system" (e.g. the [Computer Fraud and Abuse Act](https://en.wikipedia.org/wiki/Computer_Fraud_and_Abuse_Act) in the USA, or [StGB §202a](https://dejure.org/gesetze/StGB/202a.html) in Germany). So you may become criminally liable by doing this. Granted, this is not likely, but I don't think the risk is worth it.
Comment: I would also fill my boots.
Comment: +1 in Canada, did it a few times and never had any issue. They sometime notice later and close the account and if the account is not closed after an amount of time, I contact them to close it.


Are directors employees of the company?

Question: Directors tell the company what to do but most directors also get paid for their services. Does that make directors employees of the company?
Answer: In the U.S. a board member would not be an employee just by virtue of being a board member, even if they were compensated for attending board meetings. The CEO, an employee, might or might not also be a board member. Employees can be fired by their manager. A board member can't be fired from the board but, typically, only removed by a vote of shareholders. They are not assigned tasks by managers of the company.
This is 100% clear in CA under employment code 622

(a) "Employee" does not include a director of a corporation or
association performing services in his or her capacity as a director.

I was a CEO of a private company I co-founded I was a board member as a result of voting myself on the board via my stock ownership. The majority of the board could fire me as CEO but I would still be a board member.
If outside board members did get compensation, it would be as a consultant, paid with a 1099.
The CA code does say that a board member could take on actual work, like auditing financial information, that might fall under the activities of an employee.
Comment: Not an employee for tax purposes. But the term employee doesn't have the same definition for all purposes. For example, under worker's compensation law directors are usually presumptively employees but given an option to opt out.
Comment: For U.S. federal income tax purposes, pay to a member of a corporation's board of directors, in exchange for serving in that capacity, is classified as self-employment income.
Comment: And therefore not as an employee.
Answer: Maybe
australia

The role of a company director is to govern a company on behalf of the shareholders or members
of that company.

General duties of directors
That role can be fulfilled in three ways:

Volunteer without payment as is the case with many non-profit or charitable corporations,
Contractor for payment under a contract for service,
Employee for payment under a contract of service.

Only if the relationship between the company and the director is an employment relationship is the director an employee.
Whether a person is an employee or a contractor is a question of fact and generally what the company and the director think the relationship is, has very little bearing:

Indepdendent/Contractor vs Regular Employee
Independent contractor, feeling like an employee - Will California's AB5 change my classification?
What would go into proving you are an employee and not a contractor?

Being a director doesn't automatically make one an employee but depending on the totally of the relationship, they might be.

Are directors employees of the company?


vb.net and ContextMenuStrip?

Question: I'm using winforms and vb.net.
When I right click on a textbox I get the copy/paste/etc popup menu. This is ok. But How can I add something to this menu, and where/how would I intercept the event when one would click on the Item I add to it?
Answer: This context menu is baked into Windows, there is no documented way to alter it.  Nor is there a message that you could trap to hack the menu.  Replacing it is certainly an option, check my answer in this thread for sample code.
Be careful with this, stuff gets added to this menu in different Windows versions.  Like "Insert Unicode character" in Win7.  You definitely want to avoid this if it is likely that your product will run in an Eastern Asian country.
Answer: You'll have to add your own menu if you don't want to use the default.
You'll have to replace the existing default functions with your own code.

vb.net and ContextMenuStrip?


Product Listing Page Image Gallery Loading Big Images

Question: I am new at Magento and we have over 1500 products, most products has 7-10 color images. We show colors on product listing page. The problem that i am having is magento loading big images instead of thumbnails so page is over 10mb and slow.
Can you help me how can i change this or is there any way to load small thumbnails instead of big images ?
Page is http://www.tdnstores.com/rings/designer-wedding-rings
Thank You,
Answer: I checked your site.
In category page you display products with all images that why it slow.
And all images are displaying from the cache, so there is no issue of small images or big images.
You optimised images that is needed to optimised.
Answer: You're actually have big (not resized) images on page. These images are under small icons, but used to display main image of product on click.
You need to find theses lines in code - https://monosnap.com/file/iB2KzrLg6WkEqxaezGVsVRDo9y9zb5#
They are probably placed in app/design/frontend/smartwave/porto/template/catalog/product/list.phtml or somewhere under app/design/frontend/smartwave/porto/template/catalog/product/list/ folder
Your code will look like

<img data-product-id="<?php .... ?>" src="<?php echo Mage::helper('catalog/image')->init($_product, 'image', $image->getFile()) ?>" alt="...">

and you can change it to

<img data-product-id="<?php .... ?>" src="<?php echo Mage::helper('catalog/image')->init($_product, 'image', $image->getFile())->resize(258); ?>" alt="...">


Mistake in abstract

Question: I have submitted an abstract to a conference that has been accepted and will be published for the conference. There is a minor error that changes the data I have included in the abstract. What should I do?
Comment: The correct answer will depend on the usage of the field and the exact circumstances. I cannot tell whether only the abstract will be published or whether the complete paper is published or a short note made out of the paper such is done for poster sessions in some disciplines.
Answer: There is probably nothing to be done at the moment, though publishing a correction on your personal web site is appropriate. Even withdrawing the abstract is probably impossible now.
Make the correction in the final paper and, when you cite the abstract (as you should), note that it contains an error corrected here.
Lots of incorrect stuff gets published for various reasons. But we hope that statistically we wiggle toward the truth.

Mistake in abstract


Device detected by lsusb and logged in dmesg, but no /dev/ttyUSB0

Question: Previously my device could be communicated with via the device file /dev/ttyUSB0, which I could connect to via my terminal emulator.  I do not know why, but recently /dev/ttyUSB0 stopped being created when the device is detected, despite enumeration still being logged by dmesg and the device being detected by lsusb.  How do I get /dev/ttyUSB0 to be created again?
The device under question is the Freescale device enumerated below as usb-1-3, the exact same device is called "Google Inc. Nexus 7" under the lsusb output (Bus 001 Device 028).  The names differ because the hardware is running an Android port which presents itself as the "Nexus 7".  It is a NXP i.MX 6 SoM connected via micro-USB.
$ dmesg
[ 1074.879020] usb 1-3: new high-speed USB device number 24 using xhci_hcd
[ 1075.083989] usb 1-3: no configurations
[ 1075.083998] usb 1-3: can't read configurations, error -22
[ 1075.195166] usb 1-3: new high-speed USB device number 25 using xhci_hcd
[ 1075.387420] usb 1-3: no configurations
[ 1075.387428] usb 1-3: can't read configurations, error -22
[ 1075.499143] usb 1-3: new high-speed USB device number 26 using xhci_hcd
[ 1075.521617] usb 1-3: no configurations
[ 1075.521625] usb 1-3: can't read configurations, error -22
[ 1075.687121] usb 1-3: new high-speed USB device number 27 using xhci_hcd
[ 1075.704437] usb 1-3: no configurations
[ 1075.704446] usb 1-3: can't read configurations, error -22
[ 1075.704482] usb usb1-port3: unable to enumerate USB device
[ 1076.275060] usb 1-3: new high-speed USB device number 28 using xhci_hcd
[ 1076.413277] usb 1-3: New USB device found, idVendor=18d1, idProduct=4e42
[ 1076.413285] usb 1-3: New USB device strings: Mfr=2, Product=3, SerialNumber=4
[ 1076.413289] usb 1-3: Product: SABRESD-MX6DQ
[ 1076.413293] usb 1-3: Manufacturer: Freescale
[ 1076.413296] usb 1-3: SerialNumber: 0123456789ABCDEF

$ lsusb
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 004: ID 0bda:0153 Realtek Semiconductor Corp. Mass Storage Device
Bus 001 Device 003: ID 046d:c52b Logitech, Inc. Unifying Receiver
Bus 001 Device 028: ID 18d1:4e42 Google Inc. Nexus 7 (debug)
Bus 001 Device 005: ID 8087:0a2a Intel Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

$ ls /dev/ttyUSB*
ls: cannot access /dev/ttyUSB*: No such file or directory

I have tried different USB cables and different ports, resetting the device in the two different modes, using a hub and not using a hub.
In order to find which device file my USB device was using, I tried the command from https://unix.stackexchange.com/questions/144029/command-to-determine-ports-of-a-device-like-dev-ttyusb0?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa
Unfortunately, this did not yield the information I was looking for.
The result was:
/dev/input/event4 - Logitech_USB_Receiver
/dev/input/mouse0 - Logitech_USB_Receiver
/dev/hidraw1 - Logitech_USB_Receiver
/dev/input/event5 - Logitech_USB_Receiver
/dev/hidraw2 - Logitech_USB_Receiver
/dev/hidraw0 - Logitech_USB_Receiver
/dev/usb/hiddev0 - Logitech_USB_Receiver
/dev/bsg/4:0:0:0 - Generic-_SD_MMC_MS_PRO_20120926571200000-0:0
/dev/sdc - Generic-_SD_MMC_MS_PRO_20120926571200000-0:0
/dev/sg2 - Generic-_SD_MMC_MS_PRO_20120926571200000-0:0

At one point I had a script that piped echo output into /dev/ttyUSB0 using echo 'mytext' > /dev/ttyUSB0.  One day I realized the /dev/ttyUSB0 did not actually exist, so the script had simply created a text a file.  I deleted that text file because I was afraid it would interfere with creating the device file of the same name.  I don't know if this could be related.
I am on Ubuntu 14.04.
Thanks in advance!
Comment: There are plenty of error messages in your log, as you may have noticed. If you already tried different cables and ports, likely something in the device is broken, for whatever reason. If the USB configuration enumeration is incomplete/faulty, the correct kernel driver can't load (and wouldn't work even if loaded).  Which means no `/dev/tty/USB0`.
Answer: You can use the mknod command to create entries in /dev.
Try mknod /dev/ttyUSB0 c 188 n
See man mknod and Linux Allocated Devices


How To Change The Color On A Mouse Called HEMZONE USB Wired Optical Gamestart Gaming Mouse On A School Chromebook?

Question: If you look up this mouse you can get more information. This mouse is called  HEMZONE the company is GameStart. This mouse lights up and I want to know how to change the color and if it is possible?
Comment: There is an application and a library https://github.com/libratbag/piper it may work with your mouse on a typical linux desktop or laptop, but I doubt it will work on Chrome OS or inside a Crostini container on Chrome OS.
Answer: This usually requires a proprietary driver that is usually only designed for Windows or Mac OS X. If you have a PC with those operating systems I suggest you do it there & then maybe the mouse will remember those preferences when you reconnect it back to the Chromebook.
Amazon page for similar product of another brand has a user advising his was HEMZONE. Another user said to change colors on mouse you use a combination of buttons: press forward/back button + dpi toggle between auto color change and just red
This website indicates (under FAQ) that you cannot change the color of this particular mouse. This is echoed by some amazon users as well.


maximum number of randomly placed points in euclidean space which can fit inside of a circle of a given diameter

Question: my friend and I have a question regarding problem "Pesky Mosquitoes" ( https://open.kattis.com/problems/mosquitoes )
from Kattis OJ (online judge for competitive programming. This clearly is clearly a fairly simple geometry problem.
See how many points lie inside a circle of a given diameter. Our idea is pretty straightforward. We take every pair of points and find the midpoint
 between those two points. We call that the center of our new circle, which
will have the radius taken from dividing the previously given diameter by 2.
Then, we just check how many points lie inside of it. However, we get a veredict of WA on the dataset of Kattis. What is wrong with that idea?
Any help would be appreciated.
Answer: Consider an example where you have a bowl of diameter $2$
and three mosquitoes at the coordinates
$(0,-1)$, $(0.5, 0.866)$, and $(-0.5, 0.866)$.
If you put the center of the bowl exactly at $(0,0)$ you will just barely
trap all three mosquitoes.
But if you move the bowl more than a tiny distance in any direction—in
particular, if you move the center of the bowl to any of the midpoints
of the segments between any two mosquitoes—at least one mosquito will
escape.
So maybe the algorithm isn't quite as simple as it looks.


Bitlocker not auto-unlocking C: drive on server 2008 R2

Question: Here's the scenario.  We'got a Dell PE T110 (I know, I know), that we've enabled Bitlocker via TPM.  On reboots, it asks to re-enter the bitlocker key, and we have no idea why it's not auto-unlocking.  On the drive that's supposed to auto-unlock, we have the OEM parition, RECOVERY (with system,active,primary), and the OS (boot,pagefile,crash dump,primary partition) (Bitlocker encrypted).  I've been reading that the OS needs to be the active partition for the autounlock to function.  If not, any ideas where I should look next?
Answer:
I've been reading that the OS needs to be the active partition for the autounlock to function

That isn't accurate.
This can be due to the recovery key being regenerated but not saved to TPM. By  far the most likely cause is the C: drive not being first in the boot order. The next most likely cause is something in the hardware/firmware changed, which would basically mean it needs to be redone.
Here is a sample of what the manage-bde diskmgmt output should look like:
<code>C:\>manage-bde -status
BitLocker Drive Encryption: Configuration Tool version 6.1.7601
Copyright (C) Microsoft Corporation. All rights reserved.

Disk volumes that can be protected with
BitLocker Drive Encryption:
Volume C: [OSDisk]
[OS Volume]

    Size:                 232.56 GB
    BitLocker Version:    Windows 7
    Conversion Status:    Fully Encrypted
    Percentage Encrypted: 100%
    Encryption Method:    AES 128 with Diffuser
    Protection Status:    Protection On
    Lock Status:          Unlocked
    Identification Field: Some description
    Key Protectors:
        Numerical Password
        TPM
</code>
Suffice it to say if there is not a TPM protector listed, the drive was not encrypted correctly/as you expected.
Comment: Hey Greg, thanks for the quick reply.  Yup, checked the boot order and C: looks to be the first boot.  The -status was identical to yours.  Any further ideas prior to us redoing the encryption?
Comment: @user208160: No that sounds like what you need to do.


hackers - tracking my iphone

Question: I've received 7 emails from Apple that my phone was tracked using Find My iPhone, can I call apple and find out who the device belongs too? I've already changed my password.
Comment: see this http://apple.stackexchange.com/a/147128/46541
Answer:
Change your password (to make sure whoever is logged in can't change it).
Go to icloud.com.
Go to Settings.

Click "Sign out of all browsers currently logged in."
Change your password again (by now, they should be logged out).

As far as calling Apple to find out who did this, it is unlikely they will be able to tell you. You can try and call at 1-800-MY-APPLE.
Hope this helps.
Answer: Reset your "Find my phone"
iCloud: Remove your device from Find My iPhone
Then enable it again after 15 or more minutes.
Check for emails from Apple.
Also there might be a bug in iOS 8.0.1 so just turn it off for now (and do not loose your phone).
Turn off "Find My iPhone" on your phone if you're using the feature. You can do this under Settings > iCloud > Find My iPhone.
Answer: I was told by Apple they cannot find out but that you should change your password and make it something difficult with uppercase, numbers and symbols
I just logged into my own phone to find it and received an email 10 minutes later...I think it's something new they are doing and there are probably some bugs to kink out while they do it...hope this helps...I am not concerned on my end anymore and will just wait and see if it happens again.

hackers - tracking my iphone


trivial answer : shoud I use comment or answer?

Question: relating to that question .
How would you reply in that case : would you write an answer ? a comment ? or a comment then requesting the question to be close as it will likely not help others?
Comment: I would comment and vote to close (already closed). http://meta.stackoverflow.com/questions/272642/why-answering-in-comments-is-trending/272644#272644  That question will get deleted, and the non-presence of upvoted answers makes that easier.
Comment: The odds that this question is going to be helpful to anybody else are negligible.  There's a close reason for typos.
Comment: @HansPassant thank you...  so add a comment and choose close is the right options in those cases
Comment: ^ Ironic answer as comment
Answer: Will that question be helpful to other people. Obviously not! This is because that is a minor mistake on the OP's part and the probability of other people getting the same problem is very very low(Almost 0).
So just comment out the mistake(just as you did) so that the OP will get to know his mistake and then flag it as off-topic with this flag:

This question was caused by a problem that can no longer be reproduced or a simple typographical error. While similar questions may be on-topic here, this one was resolved in a manner unlikely to help future readers. This can often be avoided by identifying and closely inspecting the shortest program necessary to reproduce the problem before posting.


Downthemall addon safe

Question: I need to download set of files, it's easy to do in queue with downthemall addon (concurrent download setting = 1) . Is it safe to get the files with addon, will it still save via Tor? Will the file fingerprints still match?
I do not download large files that affect Tor network, just small ones because I already have a slow internet connection.
Answer: Short answer: Yes, it still uses Tor.
Long answer: I have tested this using a completely different method to the one outlined below. I just used DownThemAll! to download the Tor Project test page. First, I did a baseline test using curl with Tor (curl https://check.torproject.org/ -x socks5://127.0.0.1:9050 -o test_withtor.html) and without (curl https://check.torproject.org/ -o test_withouttor.html), just to verify that the test page works in downloaders. The respective pages showed that they were and weren't using Tor. So that part works.
Then, I added https://check.torproject.org/ as a DTA! download. Opened the downloaded file, and:

Success!

Downthemall addon safe


What is the purpose of the hats that I'm receiving?

Question: I've heard of Winter Bash before, but I have no idea what is the purpose of the hats that I'm receiving and what should I do with them. So I am just trying to understand here - what are the hats for, and why would I want to wear them?
Comment: For fun. That's it. Nothing more.
Comment: In 10years, this is the first time you notice it?
Comment: @10Rep possible that OP hasn't visited the site _during Winter Bash_, or maybe quickly found and read questions once or twice during this period, and din't pay attention to the top-right icon.
Comment: @Atiyar Usually you get a hat without even knowing that you did. Some hats are so easy to get. I'm surprised you didn't get one. For instance there's always a hat for visiting a question (unless you haven't visited a question in the past 10 years, which is highly unlikely).
Answer: Hats have no "real" functionality.
Winter Bash is intended to be a fun way to celebrate the year's end, encourage users to explore new ways to use SE they may have not tried yet, and participating in a joint effort to figure out the triggers for the secret hats.
If you're not interested, you could always opt-out of it by clicking the "no hats for me, please" button at the bottom of the Winter Bash menu.
Comment: Note the game as well.
Comment: @M-Chen-3 Don't reveal it! jk many people found out anyways
Comment: @atiyar The Hat Dash game, which you can access by wearing a mask hat. More info on it in my question about it. :)
Comment: Hat are you talking about? (just joking)
Comment: @10Rep, what game? Looks like I have a lot to "explore new ways to use SE" that I "may have not tried yet"  :)
Comment: Isn't a small part to encourage behaviour that lasts beyond the Winter Bash period?
Comment: @PeterMortensen The hats themselves will be removed when Winter Bash ends. One could presume, though, if users get into the habit of those behaviors, they would stick beyond Winter Bash.
Comment: TBH. you could argue the same logic exists for badges that can only be earned once - they promote behaviors that are considered positive, but after you get the badge, you have no "objective" reason to carry on with that positive behavior - unless it becomes a habit and/or you've internalized the value of this behavior
Answer: Another way to look at it is that they bring the community closer together, like:

"Hey, you're wearing fruits on your head... so am I!"

And they give the site more pop and color...
Comment: @Spectric What about it?
Comment: That is, until [this](https://meta.stackoverflow.com/questions/403735/how-can-i-see-a-milliners-activity) happens


What flaps position should be used for taking off from a wet runway?

Question: If I have to take off in a C152 or C172 from a runway that's 800-900 meters (2600-3000ft) long and wet but not contaminated, what flap setting should I use to get the shortest ground roll?
Comment: I learned how to fly in a 152 and a 172 on a field that was only 1900 ft long, or about 580 meters. I never had any issues with even a wet runway. I'd recommend using the exact same procedures as you normally would.
Comment: I would second this. In my experience with C152, there was no noticeable difference while taking off from a wet runway instead of a dry runway, although the instances of wet runway were few and far between for obvious reasons.
Comment: Flaps will shorten the takeoff roll at the expense of climb performance, see https://www.experimentalaircraft.info/flight-planning/aircraft-performance-5.php
Comment: Why would a wet runway be even s consideration?
Comment: "Why would a wet runway be even a consideration" -- steering?  Not in actual practice it seems.  The question would be more interesting if it asked about an ICY runway...
Answer: The POH for a 152 lists the flap settings for takeoff as between 0° and 10°. It makes no specific mention of take off from a wet runway, however you could follow the same procedures for a soft/rough runway if you are concerned about the surface conditions.

Soft or rough field take off are performed with 10° wing flaps by lifting the aircraft off the ground as soon as practical in a slightly tail low attitude. If no obstacles are ahead, the airplane should be leveled off immediately to accelerate to a higher climb speed

Source: Cessna 152 POH Section4 (Normal procedures)
I should note though that in roughly 180 hours on C150/152's and a short (650m) hard runway, I've never felt the need to even use flaps, let alone execute a soft/rough field takeoff as described above. It's not a particularly comfortable take off, and a wet hard-surface runway doesn't really present that much of a problem.
Answer: You don't give a lot of detail, e.g. altitude, temperature, obstacles. But the last time I flew a C172 there were no manufacturer operational restrictions for wet runway for this aircraft.
Bear in mind that a wet runway may nevertheless affect takeoff performance for various reasons e.g. harder to track aircraft, engine performance and so forth. But short answer is that flaps would be the same as for dry conditions, 10°.
Comment: Why would a wet runway make the aircraft harder for ATC to track, and how would that affect your takeoff performance?
Answer: Wet is such a varied expression. How much water are we talking about ? Just a little wet, some water puddles, or the whole runway has a few mm of water all the way through ?
Realize tires on such planes aren't that dissimilar to car tires. Can you drive at takeoff speeds without feeling the water is dragging the car ? If you can, you fly and take off the same. Try this out on a road near the airport (which received the same volume of precip and is just as flat as the runway).
I'd use first notch of flaps and would hold the aircraft until full power is achieved and then release so you don't waste any runway distance.
Doesn't matter if the runway is slippery. What matters is if the water produces significant extra tire drag that reduces acceleration. Probably obvious by might not be to others.
250 hours flying C150/152/172. Very few takeoff / landings on wet runways.
Look up the POH and make sure you have some extra runway length to spare just in case.
Comment: well wet runway means a runway with 3 mm of standing water or less in less than 25 % of the runway usable for take-off. In the POH there is no mention about a take-off on wet runway and which flap setting is recommended, for that reason I asked. Probably someone has tried to take-off with different flap setting and could give me some tips from their experience. I know that every take-off are different from each other for the too many variables included. It was just to share some experience with other pilots.
Comment: If the runway is very wet you could try a short field technique meaning you're trying to lift off as quickly as possible, accelerate on ground effect then get the climb going.


"PKGBUILD does not exist" when running "makepkg -cf"

Question: When I run makepkg -cf, I get an error:
==> ERROR: PKGBUILD does not exist.

But if I run makepkg -p PKGBUILD.asc it works.

Can someone explain to me someone why the first doesn't work?
Answer: How I solved it

The problem is that when downloading PKGBUILD, the browser added an unwanted extension to the file which turned out to be PKGBUILD becoming PKGBUILD.asc, the solution is to simply remove the file extension i.e. rename PKGBUILD.asc as PKGBUILD, you may have another extension that does not matter, the important thing is to remove it.
For More Information

makepkg is a script that automates the building of packages Arch. When you use it without telling it which file to build, you are essentially telling it do nothing, which leaves you with the error you see here.
This error explains it is trying to call PKGBUILD which is a package build description file that needs to be made before the package is built by makepkg. When you used the -p flag you are telling makepkg to instead build the package using buildscript in place of PKGBUILD
I highly suggest reading the man page for makepkg by running man makepkg so that you can learn more about this tool for future use. I will link it here for you as well https://www.archlinux.org/pacman/makepkg.8.html
Comment: thx, i was checking superficial the link that u send thx btw, i said that becouse the readme of the program that i want to install tell me to run `makepkg -cf` so if i want to run that command what i need to do if is something that need a path or something and not a short command line i ll read not superficial the link that u send. Thx
Comment: No problem, be sure to mark it as answered so it is easier for others to find that have the same issue.
Answer: August 2023 update, another alternative
If available, try a -bin package option. Whatever you are trying to compile might already be compiled. That is one option, or not...
This has been a problem in the past for various reasons. Consider the forum on Arch...

https://bbs.archlinux.org/viewtopic.php?id=52118
https://bbs.archlinux.org/viewtopic.php?id=154730
https://bbs.archlinux.org/viewtopic.php?id=201265
https://bbs.archlinux.org/viewtopic.php?id=143298

I was running make -si specifically for yay per the install instructions on GitHub. Searching this out led me to this question, so others might have a question also.
Maybe something worked in the past and maybe some dependency broke an update, et cetera.
I solved the problem by changing from the yay repository to the yay-bin repository, per the same instructions page. That solved it.


How do you say "immersion" as in "full immersion language program"?

Question: How do you say "immersion" as in "full immersion language program" or "I am moving to China so I can get some immersion in the language"?
Answer: Although in a 洗禮(baptism), a person may be immersed in water for a brief moment, but the purpose of baptism is mostly for symbolizing purification or regeneration. For example: 經過戰火的洗禮 (After the baptism of war). It is not a very good translation for "immersion" in this case.
沉浸 is the correct translation for "immersion"
"Full immersion language program" in Chinese should be 全浸式語言課程
Another suggestion :
單語環境學習課程 (single language environment study course)
Answer: I have only seen the direct translation of 'full immersion language program' once in an advertisement of a very specific English summer program, it is not a common word that you would go out and say to native speakers telling them that you want 語言洗禮 or 浸禮, because a lot of the time it would be interpreted as baptism. Tang Ho's answer '全浸式語言課程' is as close as it gets for a direct translation.
However I would suggest to use 'experience' 體驗 or 'train' 訓練 as an alternative for 'immerse'.

我想好好的體驗中國的文化和訓練我的中文,所以我會搬到中國去。
I would like to fully experience the Chinese culture and practice my language, therefore I am moving to China.


Java: Which of multiple resources on classpath JVM takes?

Question: If I have multiple files of the same name on classpath (e.g. I have multiple .jar with log4j.properties), what are the rules JVM follows to chose one?
Answer: It is specified by the order in which the resources (i.e. usually jar files) are specified using -classpath option. Resources 'earlier' on the classpath take precedence over resources that are specified after them. This can be also set in the manifest file of your application and then you don't need to provide -classpath option. You may want to check these articles on how to work with manifest files.
The exhaustive description of "how classes are found" can be found here, where the section on JAR-class-path Classes describes the logic of JAR-files searching.
Comment: Any official documentation that confirms that `Resources 'earlier' on the classpath take precedence over resources that are specified after them.`?
Comment: Since Maven 2.0.9, this can be handled, as described [here](http://stackoverflow.com/questions/793054/maven-classpath-order-issues).
Comment: If I use Maven, does that mean it's de-facto random? Setting classpath in MANIFEST.MF wouldn't work when testing (jars are taken from local repo)
Comment: The answer has been updated with a link to official documentation that describes the logic of class-path searching.
Comment: In addition to all that has been said, you can use `ClassLoader.getResources(String name)`. It returns all classpath resources with the given name, in the same order as described in the link provided in the answer. Useful if you need to load all of them.
Comment: How about `-jar`? for getting same name file
Answer: The ClassLoader determines where a resource will be located (taken from ClassLoader JavaDoc):

The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When requested to find a class or resource, a ClassLoader instance will delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself. The virtual machine's built-in class loader, called the "bootstrap class loader", does not itself have a parent but may serve as the parent of a ClassLoader instance.

So wherever in your code Class#getResource or Class#getResourceAsStream is called, this happens (taken from Class.java)
public java.net.URL getResource(String name) {
    name = resolveName(name);
    ClassLoader cl = getClassLoader0();
    if (cl==null) {
        // A system class.
        return ClassLoader.getSystemResource(name);
    }
    return cl.getResource(name);
}

ClassLoader.java:
public URL getResource(String name) {
    URL url;
    if (parent != null) {
        url = parent.getResource(name);
    } else {
        url = getBootstrapResource(name);
    }
    if (url == null) {
        url = findResource(name);
    }
    return url;
}

where ClassLoader#findResource is actually to be overwritten by the ClassLoader implementation. This implies that the behavior is different on an application server, a TomCat or if you are running from a jar file, it depends on the ClassLoader implementations of the environment you are currently in.
Here is an example that you may use to trace what's going under the hood in your particular case.
Comment: This should add what happens when the usual URLClassLoader is used, with multiple jars/directories.
Comment: True, it's classloader-dependent, but since I am using Java SE with standard classloaders, the other answer applies.
Answer: I am contributing a proven case that if classpath is, say, all jars in a folder, and you want to prioritize one (or some) of them, this does not work:
Windows:
bin/prioritized.jar;bin/*

Linux:
bin/prioritized.jar:bin/*

It appears that the first path bin/prioritized.jar is ignored just because the second one with a wildcard includes it in its own scope. This is what effectivelly breaks the specified order of classpaths.
Therefore, in order to have multiple resources prioritized (tested on Java 10.0.1), you need to put them in non-overlapping scopes and then they will work.


Is there a way to sort IPs by latency?

Question: Given a list of IPs, how can it be sorted so that the "nearest" or lowest latency is sorted to the top, "farthest" or longest latency, is at the end?
Answer: Here's my solution, specific to NIS, written up in bash:
yplist=( $( ypcat ypservers|xargs -n1 host|sed -n '/has.address/s/.*[[:space:]]//p'|sort -u ) )
nearest=( $( for pp in ${yplist[@]}; do ping -q -n -c 5 -i 0.3 $pp|awk -F/ '/^rtt/{printf "%d:",($5*1000)}'; echo $pp; done|sort -n -t: -k1|cut -d: -f2 ) )
printf "ypserver %s\n" ${nearest[@]} >/etc/yp.conf

That's a pipeline that grew in the telling, so here goes:

$yplist is an array of hosts, and works as my source list
$nearest takes $yplist and pings each in turn
the output of ping is narrowed by awk to match only the summary line
slight hack: using / as the delimiter, the avg roundtrip in ms is multiplied by 1000 to give an integer value in μs
the follow-up echo $pp just sticks the IP in question on the end of the line
the entire for loop is sorted numerically on field #1 - shorter response time averages will float to the top
the first field is snipped off leaving just the IPs
this list of IPs is then assigned into the $nearest array

This is the resulting list of IPs, with the shortest latency at the top.
(The final printf line just writes out a new /etc/yp.conf prefixing each IP by "ypserver" for the NIS config.)
I had a think about adding the minimum latency to a factor of the deviance (mdev in ping output), to expose potentially unreliable hosts. However, this statistic would only be useful if the ping per host ran over several hours or even days; and what would the factor be? It seems like somewhere between 10 and 100 would be about right, but this would have to be tuned for tolerance to unreliability. And that seems an awful lot of work to purify the results by maybe 5%.

Is there a way to sort IPs by latency?


Can a Non-Dvija wear the Tripundra Tilaka?

Question: Can a non-dvija wear the tripundra? (On the head only)
Comment: Non Smartha brahmins, like Vedic Shaivite Brahmins wear it. Non Strictly Vedic people like Lingayats, Veerashaivites, Yogis all wear Tripundra. Many Shudras and Dalits all over south India wear Tripundra.
Comment: everyone can wear tripundra, just the length of the lines change.
Comment: Yes. It would be better if you incorporate the references and answer this question. ShivaPurana details it with good clarity
Comment: @Athrey Ok that sounds good. I'll post an answer when I get time.
Comment: @Athrey, why you have deleted the answer? It's another view and valid too!
Comment: @Kanthri , Just wanted reconsider my answer, to avoid any inconsistency with Shiva, Skanda purana quotations provided by Apastamba_01.However it may be understood that (from DB 11.15.1), Dvijas abiding by the mantravath dhaarana of tripundra, on forehead and other parts of body , doesn't seem to forbid others from wearing tripundra, bhasma ,clarified in Devi Bhagavatham 11.13.(1-20)
Answer:
Only the twice born are to take this Tripundra on the forehead and the other parts of the body after carefully purifying the ashes by the mantra Agniriti Bhasma, etc. The Brâhmans, Ksattriyas, and Vais'yas are known as the twiceborn, (the Dvîjas). So the Dvîjas ought to take daily this Tripundra with great care.  Srimad Devi Bhagavatham 11.15

However it may be understood that (from DB 11.15.1),  Although Dvijas ought to have the mantravath dhaarana of tripundra, on forehead and other parts of body ,  it doesn't seem to forbid others from wearing tripundra, bhasma ,clarified in Devi Bhagavatham 11.13.(1-20)

ब्राह्मणैः क्षत्रियैर्वैश्यैः शूद्रेरपि च सङ्‌करैः ।
अपभ्रंशैर्धृतं भस्म त्रिपुण्ड्रोद्धूलनात्मना ॥ १३ ॥
This holding on of ashes is not prohibited to anybody; the Brâhmanas, Ksattriyas, Vais'yas, S'ûdras, mixed castes, and the vile classes all can use this Bhasma Tripundra(Devi Bhagavatham 11.13.13)

They that hold Tripundras with devotion can have Bholâ Nâtha under their control; no distinction is made here between the Brâhmanas and Chândâlas(Devi Bhagavatham 11.13.1-20)

PS- Alongside the other details , an anecdote in Srimad Devi Bhagavatham 11.15 says, when an iota of bhasma from Durvasa munis body fell on Kumbhipaka citizens, (when the sage happened to visit, look them downwards),they were freed from their sins, and had all pleasures. (Thus exalting the mahimaa of Bhasma)
Comment: Well Shiva Purana 1.24.9 states that all varnas can wear tripundra..
Comment: Even Skanda Purana 2.5.2 says that shudras can wear the tripundra but only on the head.
Answer: Sri Skanda Purana 22.214.171.124-22.:

O Brāhmaṇa, a Brāhmaṇa should always have twelve Puṇḍras.[3] O son, Kṣatriyas should have four. It is prescribed (in Smṛtis) that Vaiśyas should have two Puṇḍras. It is laid down that women and Śūdras should have only one Puṇḍra.
The following are the parts of the body where a Brāhmaṇa should have his twelve sacred marks: forehead, belly, chest, hump of the neck, arms, ears, back, right side and lower part of the spine, and head, O sinless one.
A Kṣatriya should have the sacred marks on his forehead, chest and the arms; a Vaiśya on his forehead and chest and a Śūdra and women should wear the sacred mark on the forehead.

Why one should wear Bhasma and Tripundras.:
Srimad Devi Bhagwatam 11.12.:

1-20. Śrī Nārāyaṇa said :-- O Devarṣi Nārada! Hear now the great secret and the fruits of besmearing one's body with ashes, yieding all desires. . .
.
The holding on of Bhaṣma (ashes) is according to the Vedas. Therefore one who does not apply it falls down. The Brāhmaṇas must use the Tripuṇḍras, repeating the mantra; and they are to besmear their whole body with ashes; if they don't do so, they are surely fallen. He can never expect to get liberation even after koṭi births who does not besmear his body with ashes devotedly and who does not hold the Tripuṇḍras. O Nārada! The vile man who does not hold Bhaṣma duly, know the birth of that man as futile as is the birth of a hog. Consider that body as a burning ground which does not bear the Tripuṇḍra marks. The virtuous man should not cast a glance at him even. Fie on that forehead which does not carry the Tripuṇḍra! Fie on that village which has not a single temple of Śiva! Fie on that birth which is void of the worship of Śiva! Fie on that knowledge which is void of the knowledge of Śiva. Know them to be the slanderers of Śiva who mock at Tripuṇḍra. Those that put on the Tripuṇḍras, bear Śiva in their forehead. The Brāhmin who is Niragnik (without the holy fire) is not nice in every way. So if the worship of Śiva be not done with any Tripuṇḍra is not praiseworthy, even it be attended with abundance of other offerings. Those who do not besmear their bodies with ashes or who do not use the Tripuṇḍras, get their previous good deeds converted into bad ones.
21-42. Unless the Tripuṇḍra mark is taken up according to the Śāstras, the Vaidik Karmas (works) or those performed according to the Smritis prove injurious; the good works whatsoever done by any man count for nothing; the holy words heard seem as if unheard and the study of the Vedas counted as if not studied.
The study of the Vedas, Sacrifices, Charities, asceticisms, vows and fastings of that man, who does not use the Tripuṇḍra, all become fruitless. Without using Bhaṣma (ashes) if one wants liberation, then that desire is equivalent to live after taking poison. There is no doubt in this. The Creator has not made the forehead vertically high nor round; but he has made it slightly slanting and curved fit to have the Tripuṇḍra. Making thus the forehead, the Creator wants, as it were, to inform everyone that every one ought to use Tripuṇḍra marks; the curved lines also are made visible for this purpose. Still the igorant illiterate man does not put up the Tripuṇḍra. Unless the Brāhmaṇas use the curved Tripuṇḍras, their meditation won't be successful; they will not have liberation, knowledge, nor their asceticism would bear any fruit. As the Śūdras have no right in the study of the Vedas, so the Brāhmaṇs have not any right to perform the worship of Śiva, etc., unless they use the Tripuṇḍras.

The merits of wearing it.:
Sri Skanda Purana 3.3.16.:

63b. He who applies Tripuṇḍraka redeems a thousand predecessors and a thousand yet to be born in his family.

In this world, he will enjoy all pleasures, a long life and freedom from illness and at the close of his life he will die peacefully.

65-69. He will then attain a splendid divine body possessing the eight valuable Aiśvaryas (such as Aṇimā etc.), will ride in a divine aerial chariot. He will be served by hundreds of celestial damsels. He will enjoy pleasures in different worlds in turns, in the regions of Vidyādharas, Siddhas, powerful and energetic Gandharvas, and the worlds of Indra and other Guardians of the world. He will enjoy in the cities of Prajāpatis and finally reach Brahmā's region where he rejoices for a hundred Kalpas. He will then sport about in the region of Viṣṇu for the duration of the life of three hundred Brahmās (viz till the life span of Viṣṇu is over). Thereafter he attains Śiva's (Rudra's) world and enjoys himself there for endless years (till Rudra's life span). Then he gets merged into Sadā Śiva. He is not born again.

The Life Span of Indra and the Tridevas.

I hope this clarifies all your queries. Prd .
Comment: Do you have a link to the chapter? Unable to locate it... https://www.wisdomlib.org/hinduism/book/the-skanda-purana/d/doc370678.html
Answer: While most sources are affirmative of the said issue, there are some sources who (rather regrettably) disagree.
For example Vṛddhahārītasmṛti states

रुद्रार्चनं त्रिपुण्ड्रस्य धारणं यत्र दृश्यते ।। तच्छूद्राणां विधिः प्रोक्तो न द्विजानां कदाचन । प्रतिलोमानुलोमानां दुर्गागणेशभैरवाः ।। पूजनीया यथार्हेण बिल्वचन्दनवारिणम् । यक्षराक्षसभूतानि विद्याधरगणस्तथा ।। चण्डालानामर्चनीया मद्यमांसनिषेवणाम् । स्ववर्णविहितं धर्ममेवं ज्ञात्वा समाचरेत् ।। रुद्रार्चनाद्ब्राह्मणस्तु शूद्रेण समतां व्रजेत् । यक्षभूतार्चनात् सद्यश्चण्डालत्वमवाप्नुयात् ।। न भस्म धारयेद्विप्रः परमापद्गतोऽपि वा । मोहाद्वै विधृताद्यस्तु ससुरापो भवेद्ध्रुवम् ।। तिर्यकपुण्ड्रधरं विप्रं पट्टाम्बरधरं तथा । श्वपाक इव वीक्षेत् न सम्भाषेत् कुत्रचित् । तस्मात् द्विजातिभिर्धार्यंमुर्द्धपुण्ड्रं विधानतः ।।

Translation: Where the worship of Rudra & wearing of the tripuṇḍra is seen, know it to be the rule for śūdras & never for the dvijas. Durgā, Gaṇeśa & Bhairava are to be worshipped by pratilomas (offspring of a marriage between a low-caste boy & a high-caste girl) & anulomas (offspring of a marriage between a high-caste boy & a low-caste girl) with (articles like) bilva-(leaves), sandalwood paste & water. Yakṣas, rākṣasas, ghosts & vidyādharas are to be worshipped by caṇḍālas by (articles like) wine & meat. Thus knowing the conduct according to own's own varṇa, one should act accordingly. A brāhmaṇa becomes on par with a śūdra on worshipping Rudra, on worshipping yakṣas & ghosts, he immediately becomes a caṇḍāla. Even in great distress
a brāhmaṇa shouldn't wear bhasma, he who wears out of delusion surely becomes an alcohol-addict. One should look down upon a brāhmaṇa wearing a curved sign (i.e Śākta tilaka) & silken clothes to be on par with a caṇḍāla & shouldn't talk with him. Thus brāhmaṇas should diligently wear the urddhvapuṇḍra according to the rules. [Vṛddhahārītasmṛti:2:43.02-49]


Does the nose-up attitude of a jet make it more difficult to land than a turboprop?

Question: Due to the more acute nose down attitude of a turboprop on approach, does this make it less difficult to control compared to the relatively high nose up attitude of a jet?
Comment: I find the heading and the question in the body a bit confusing. The heading seems to be about nose-up attitude of a jet, whereas the body seems to be about the nose-down attitude of a turboprop -- at least at first sight. So, I am going to edit the question make it less of a brain-twister. Hope you think it is ok.
Comment: If I am not mistaken, the nose-down landing approach on turboprops has more to do with the high wing (and high position of the engines) seen on most western turboprops, than the engine type itself. I remember landing as a passenger in a Lufthansa BAe 146/Avro RJ85 jet, and the approach was definitely nose-down.
Comment: You should watch a CRJ-200 land sometime...
Comment: @casey I may well do that - Iberia has a few. Nose-down, as you say.
Answer: You're asking, I think, two questions. The first in the title is very general; the second in the text relates to control as affected by aircraft pitch on the approach. The answers to both are going to be somewhat opinion based as well as depending on which turboprops and which jets you're talking about.
My thoughts are based on the following turboprops: King Air A90, Swearingen SA-226 and 227, Fokker F-27; and on the following jets: Cessna Citation 500, Boeing 727-100, and Boeing 747-100/200.
In my opinion, there is no general practical difference in the difficulty or ease of approaches and landings based on whether you're in a turboprop or a jet or what your pitch angle on the approach is. Some might say that a nose down attitude is going to give you better visibility. However, I never found that lack of visibility over the nose was a problem.
There are differences in the characteristics of turboprops and jets that make the one or the other the better approach and landing vehicle in certain circumstances. For example, if you need a quick descent, you can flatten out the pitch of a turboprop and get a lot of drag, more if you move into beta. For jets, the engines are always going to be producing some forward thrust (although I seem to remember the DC-8 allowed idle reverse in flight).
If you have a crosswind, the heavier the aircraft and the higher the landing speed, the easier it will be to handle the cross wind, and transport category jets in general are heavier than turboprops. Also, in a crosswind, a high wing is considered a liability, and there are a lot of high wing turboprops.
I guess what I'm saying is that individual aircraft and circumstances outweigh differences between turboprops and jets insofar as approach and landing is concerned.
The hardest and easiest aircraft I flew insofar as getting a good touchdown were both jets: the hardest being the 727-100 and easiest the 747.
The most flexible insofar as the approach was concerned were the SA-226/227. You could fly the approach clean at near-jet speeds and then slow down very quickly on short final. KSFO controllers back in the 1980s used to appreciate our ability to fit in. Or they could bring us in on a close-in downwind and then have us dump it for a well-timed turn to base and then a very short final.
---------- Edit for additional information
@JanHudec pointed out that turboprops have a faster response to power changes than jets. That's true, though in day to day flying, you plan for what you have. Obviously, though, in an emergency situation that required additional power, the turboprop would enjoy the advantage.
The difference in response to adding power is not as great as it used to be. As I remember, when the 727 first came out, there were a couple of incidences of 727s touching down slightly short of the runway because of the time required for the engines to spool up. The figure that sticks in my mind is that for those first engines, the time from full idle to full power was 7 seconds. Boeing fixed the problem by putting in a flight idle and a ground idle. The flight idle kept the engines spooled significantly higher than the ground idle. That involved no change to where you placed the thrust levers. You brought them all the way back. If you were still in the air, that wouldn't spool the engines all the way to full idle.
The 747 engines reacted very quickly, much more so than the 727 even with the flight idle setting. I remember two instances in the 747 in heavy gusting crosswinds where I used a lot of power at the last moment. In the 727 those touchdowns might have been a lot harder? In a turboprop maybe a little less hard?
Comment: Jet engines are known to respond to thrust setting changes rather slowly while turboprops respond practically instantaneously. Does it make make no difference in practice?
Comment: @JanHudec Good point! I'll edit that into my answer. Personally, I never had that make a difference, but I can certainly see where it could.
Comment: Turboprops have unswept wings of a higher aspect ratio than jets. This makes their lift curve slope steeper, so the pitch difference between cruise and touch-down is smaller for turboprops than for jets.


Does science blind itself by validating theories using just equations

Question: The theory of general relativity has a lot of math to say the least. It is one of the most successful theories that mankind has ever made. And yet we still don't know how gravity works. sure we have interpretations but that doesn't really help the problem because frankly most of the interpretations are pseudoscience, or barely even have enough people to actually look into the interpretations seriously. Another theory that has been constantly validated by equations is quantum mechanics. And it has it just as bad as general relativity. Is science taking shooting in the dark?
Comment: Special relativity and quantum mechanics were both developed because the contemporary theories couldn't explain *existing experimental findings*. How much do you actually know about the history of sciences?
Comment: From an instrumentalist point of view, it is enough that our theories make accurate enough predictions.  As others have often commented, our science tells us what things do, not what they are.  Personally, I don't find it surprising that the domain of quantum theory is unintelligible to our intuitions - and ditto for our intuitions about the cosmological.
Comment: @no name What else would they do? Scientists know that material terms have structural and aggregative properties/effects that, for better or worse, relate in terms of arithmetical operations, functional parameters, quantification, etc. The ability to infer the existence of something (e.g. black holes) from their role as solutions to equations even before any direct experimental/observational evidence appears, is surprising, but does corroborate the general method pretty well...
Comment: Gravity *is* what gravity *does*.
Comment: What does "how gravity works" mean here, are you asking for some kind of intuitive mechanical picture involving particles bouncing off each other or something? If so, why assume that the laws of nature would be explainable in terms of such an intuitive picture, instead of just being abstract mathematical rules governing how physical states change over time?
Answer: No. Theories (e.g. general relativity and quantum mechanics) are not validated by equations, there's no such thing as validating something with an equation.
Theories with equations are validated if you input facts about physical reality into equations, and what you get out of those equations are accurate predictions about other parts of physical reality.
Cat Agnosticism
Nobody knows what gravity, or a cat, is - but that's okay, because we can predict what gravity, or a cat, does.
A cat is a mammalian solo stealth predator with certain physical and social characteristics, for which we have good models. A mammal is a kind of animal with certain properties and we have a good model for animals. An animal is a kind of organism with certain properties and we have a good model for organisms. An organism is a kind of chemically powered machine with certain properties and we have a good model for chemistry. Chemistry is a kind of physical process in which atoms and molecules constrained in systems with certain properties seek their locally highest entropic state in various thermodynamic processes and we have a good model for thermodynamics. Thermodynamics is the gestalt behavior of countless interactions between atoms and molecules and we have a good model for atomic and molecular interactions. Atomic and molecular interactions are the gestalt behavior of interactions between fundamental particle interactions and we have a good model for fundamental particle interactions. Fundamental particle interactions just are the way they are. The universe probably has its reasons, but we haven't figured them out yet.
We can predict what a cat does - all the way up to its social group dynamics, and all the way down to its fundamental particles. But why do its fundamental particles do what they do? What do they look like under the hood? Nobody knows. So nobody knows what a cat really, truly is. Maybe someday we'll know more. We'll add one more layer. Then we'll know why the fundamental particle interactions that make up the furry gestalt process that is a cat do what they do. But we won't know why that layer works the way it does. It'll just be the way the universe is. We'll know even more than we currently thought possible about what a cat does, but we still won't know what a cat is.
Electrons Make Primates Angry
We evolved to be good at thinking about fruit, rocks, trees, running, jumping and climbing, prey, predators, parasitic insects, and the complex conditional logic of mating competitions. (The beautiful art of mathematics is, really, the discovery that we can apply complex conditional logic of mating competitions to quantities of fruit and get results consistent with the nature of the universe.) Anything that we can't model in terms of those things makes us uncomfortable. The universe just OWES a bunch of terrestrial primates with overdeveloped brains an explanation that we LIKE, and if it doesn't we will get angry and may start flinging things, jumping up and down, and hooting, unless we estimate that that will make us lose a mating competition and decide not to do it.
One important rule of the ancestral environment is that if something weird is going on and we don't know why, there's a really good chance that some unknown primate or primates are behind it, which is why a lot of spurious interpretations of scientific theories posit mysterious human agency, spiritual agency, or divine agency.
Answer: General relativity doesn't explain how space curves. There is no mechanism that says how this is achieved. It just states that spacetime is curved and relates mass, energy, and momentum to the curvature.
The same holds in quantum mechanics. A wavefunction is conjectured in relation to the probabilities in experiments. But it doesn't explain how these probabilities come about. The processes that determine the chances in throwing a die are known. Not so for the underlying structure for quantum probabilities.
The search for the mechanism of gravity might turn out to be connected with the search for the cause of quantum probability. If we have a new model of space and how particles cause it to curve, we might be close to solving the quantum question.
So to answer the question, it depends on how you view the models. GR and QM describe but don't explain. So if science considers them as explaing, then yes, it blinds itself. If not, it may open your eyes.


what exactly is the brute force algorithm

Question:
what exactly is the brute force algorithm? (besides just the approach only)
when a problem can use brute-force approach, and when not to?
What characteristics are there in an algorithm, when the algorithm uses brute force approach?
Comment: You can determine if it's brute force, if the approach would eventually hit every possible solution to a problem given any arbitrary situation.
Comment: From [wikipedia](http://en.wikipedia.org/wiki/Brute_Force): `Brute force may refer to any of several problem-solving methods involving the evaluation of multiple (or every) possible answer(s) for fitness`. There are no standard 'brute force algorithms' because each problem is different. If you wanted to guess a password, brute force is literally generating every single possible password until you find the right one. If you wanted to guess a person's age, you could brute force by just starting from 1 and increasing by 1 every time, etc.
Comment: example : "If we wanted to guess a person's age, we could brute force by just starting from 1 and increasing by 1 every time, etc"

by adding 1 if not found the solution, whether it is a brute-force measures?
Comment: It's brute force because you'd eventually reach the person's age, but you didn't do anything but try every possibility until one worked. An algorithm is not brute force if it exploits some advantage or approaches a problem such that you could arrive at a solution without having to try every possibility, ever. For example if a person was an adult, and you knew he was born in the 1970s, your 'guessing' algorithm would limit you to only 10 or so age possibilities, because you have some knowledge you can exploit to limit how many solutions you have to try.
Comment: so, whether it can be concluded if brute-force algorithm is as a human reasoning?
Answer: 1 and 3 : Brute force means that you will go through all possible solutions extensively. For example, in a chess game, if you know you can win in two moves, the brute force will go through all possible combination of moves, without taking anything in consideration. So the little pawn in the back that cannot influence the outcome will still be considered.
2 : As you consider everything, the problem quickly goes out of control. Brute force through 15 moves in chess is impossible because of combinatorial explosion (too many situations to consider). However, more clever algorithms that take into account "knowledge about the problem" can go much further (20-30 moves ahead)

Edit : To clarify, brute force is simplest (dumbest?) way to explore the space of solutions. If you have a problem is set in a countable space (chess moves are countable, passwords are countable, continuous stuff is uncountable) brute force will explore this space considering all solutions equally. In the chess example, you want to checkmate your opponent. This is done via a sequence of moves, which is countable. Brute force will go through all sequence of moves, however unlikely they may be. The word unlikely is important, because it means that if you have knowledge about your problem (you know what is unlikely to be the solution, like sacrificing your queen), you can do much better than brute force.
Comment: This is generally correct. The rule of thumb to know if it's possible to use brute force is to consider two things, the [branching factor](http://en.wikipedia.org/wiki/Branching_factor), and the depth at which you guess the solution is. Compute (branching factor) ^ (depth) and that's the number of situation brute force will consider. Most computers will not be able to consider 10^12 situations.
Comment: [Dynamic Programming](http://en.wikipedia.org/wiki/Dynamic_Programming) can sometimes help with exponentially complex problems.
Comment: @Alex : Of course, DP is really nice! But so are a lot of other classes of algorithms. Basically, everything is nice when compared to brute force ;-)
Comment: so that because of combinatorial explosion means brute-force can not be used in many problems?


Solve linear system with Cramer's formula

Question: How to solve this system of two equations with three unknowns with Cramer's formula:
$$\begin{cases} x-y+3z&=&0\\
  2x-2y+6z&=&0
\end{cases}$$
I tried to find $\Delta xy$, $\Delta xz$, $\Delta yz$ and all of them are equal to $0$, so I think the system doesn't have any solution.
Answer: Both equations are multiple of each other. Hence, all $(x,y,z)$ with $x=y-3z$ are valid solutions.
You could write down all solutions like
$$v = (y-3z,y,z)^T=y(1,1,0)^T+z(-3,0,1)^T$$
where $y,z \in R$ are arbitrary constants.


Taylor expansion of the square of the distance function on a Riemannian manifold

Question: I have recently read the problem named "Square of the distance function on a Riemannian manifold"(enter link description here) and I am interested in the formula
$ d^2(exp_{x_0}(tv),exp_{x_0}(tw))=|v-w|^2t^2-\frac{1}{3}R(v,w,w,v)t^4+O(t^5) $.
However I can not solve it by myself. Can one give me a complete solution?
Comment: See chapter 5 in Do-Carmo's Riemannian Geometry book. Specifically, this one is Proposition 2.7 there
Answer: This is just a remark about an alternative, more `low tech', derivation of this famous formula by using Taylor series.
It relies on this property of a Riemannian metric:  If $(M,g)$ is a Riemannian manifold and $y\in M$ is a given point, let $\delta_y(x)$ be the length of the shortest path from $y$ to $x$.  Then $\delta_y$ is not differentiable at $y$, but is smoothly differentiable on a punctured neighborhood of $y$, and satisfies $|\nabla \delta_y|=1$ there, i.e., its gradient has length $1$ and, in fact, $(\nabla \delta_y) (x)$ for $x$ in this punctured neighborhood is equal to the velocity at $x$ of the unit speed geodesic that starts at $y$ and passes through $x$.  Consequently, if $\sigma_y = (\delta_y)^2$ then one finds that this function is differentiable on a neighborhood of $y$ and it satisfies the smooth differential equation
$$
|\nabla \sigma_y|^2 = |\nabla d_y^2|^2 = 4d_y^2\,|\nabla d_y|^2 =  4\sigma_y.\tag1
$$
Thus, when $g$ is expressed in local coordinates $(x^1,\ldots, x^n)$ centered on $p\in U\subset M$, one can write $\sigma = d(x,y)^2$ on $U\times U$ (at least near the diagonal) as a smooth function of $(x,y)\in U\times U$ that satisfies $\sigma(x,x) = 0$, $\sigma(x,y) = \sigma(y,x)$, and the first order PDE
$$
g^{ij}(x)\frac{\partial\sigma}{\partial x^i}\frac{\partial\sigma}{\partial x^j} - 4\sigma = g^{ij}(y)\frac{\partial\sigma}{\partial y^i}\frac{\partial\sigma}{\partial y^j} - 4\sigma = 0.
$$
The function $\sigma$ is determined by these conditions plus the 'initial condition' $\sigma(x,0) = g_{ij}(0)\,x^ix^j+O(3)$.
Then, in local coordinates, expanding the above equation out in Taylor series and using the 'initial conditions' determines the Taylor series for $\sigma$.  In particular, in geodesic normal coordinates centered on $p$, where
$$
g_{ij}(x) = \delta_{ij} -\tfrac13\,R_{ikjl}\,x^kx^l + O(3),
$$
and $R_{ijkl}=-R_{jikl}=-R_{ijlk}=-R_{iklj}-R_{iljk}$,
examining the first three terms of the above Taylor series expansion of the PDE yields
$$
\sigma = \delta_{ij}\,\bigl(x^i-y^i\bigr)\bigl(x^j-y^j\bigr) -\tfrac1{12}\,R_{ikjl}\,\bigl(x^iy^k{-}x^ky^i\bigr)\bigl(x^jy^l{-}x^ly^j\bigr) + O(5),
$$
which is equivalent to the desired formula.
Remark:  To see another application of the formula (1), one might consult this answer of mine, where it is used to compute the explicit distance function for the complete metric of negative curvature on $\mathbb{R}^2$ given by $$g = (x^2+y^2+2)\bigl(\mathrm{d}x^2+\mathrm{d}y^2\bigr).$$
Answer: The "standard" proof using Jacobi fields can be found in section 1.3 here https://www2.math.upenn.edu/~wziller/math660/TopogonovTheorem-Myer.pdf
Comment: Morning Otis, thank you for guiding me through it. I got it. $\nabla{_{\cdot}}$ preserves the type of the tensor it is differentiating, and since $T|_{0,t}=0$, the covariant derivative i wrote and you pointed to is $0$ at $(0,t)$
Comment: hi Otis, I am studying this expansion provided by Myer's paper you suggested. There is an equality in display (24) on page 8 of the paper that I do not understand. It is $\nabla_{D_{\epsilon}} R(E,T)E |_{0,t} = R(E, \nabla_{D_{t}}E)E|_{0,t}$, where $\nabla_{D_{t}}E = \nabla_{D_{\epsilon}}T$. Do you know how this is derived? Thanks.
Comment: @Chee, can you be more specific about your issue with this? I think you should use that T vanishes at eps =0 so the given term is the only one that can appear.
Comment: hi Otis, thank you for your reply. I checked every thing in the proof but the identity was the only thing I did not understand. I do not see how $T$ vanishes at $\epsilon=0$ leads to the identity. The left-hand side is the covariant derivative of $R(E,T)E$ whereas the right-hand side is $R(E,\cdot)E$ with $\cdot$ as the covariant derivative of $T$. In a sense, for this specific case, $\nabla_{\epsilon}$ "commutes" with curvature endomorphism. Strange ...
Comment: Hi Otis, in the display (24) of Myer's paper, the first equality was obtained by the definition of curvature via 3th-order mixed directional derivatives, and to prove this identity, we should not expand the curvature via its definition and then covariant differentiate each term in the expansion, since this will lead us back to the first equality there in (24). i have tried $(\nabla_{D_{\epsilon}} R) (E,T)E$, i.e., the covariant derivative of the curvature tensor, which equivalently should be $0$ at $(0,t)$ if the identity holds. I was not able to prove that it is $0$ at $(0,t)$
Comment: The covariant derivative term you write vanishes since $T$ does. See the explanation before (13) in the paper.
Comment: Hi Otis, thank you. Did you mean that since $T|_{0,t}=0$, we must have $(\nabla_{D_{\epsilon}}R)(E,T)E|_{0,t}=0$?
Comment: Yes that's what I meant. Similarly if the covariant derivative hits E then there's a non differentiated T that vanishes so that term goes away


How to determine what type of encryption has been used?

Question: i play an android game which seems to send the player's high score in an encrypted format .
some thing like (f11cca35236eebbdc26a0ce45876d117)
a 32 character code
tried MD5 but no result found.
i wanna know is there any way i could decrypt it or at least find the encryption method?
thanks
Comment: Also, MD5 is not an encryption, it's a message digest one-way algo, so you can't "decrypt" it. All you can do is try and match its value against precomputed hash tables on known inputs. So it would be odd if the game you play uses MD5 or any other hash function to keep track of your score, since they're 1) digests and possibly result in a reduced output with respect to its input, and 2) the server would have to keep a lookup table of outputs to match against inputs to determine their meaning.
Answer: First of all, it sounds like you're trying to cheat at a game. Don't do that.
Now some background on what is going on. Lets first look at the difference between encryption vs hashing:

Encryption is the process on encoding information in such a way that it can be decoded again given the right information about the encoding process (such as an encryption key). This is used for secure communications such that the recipient understands a message (and is able to retrieve the original) but third parties are not able to read the message. The process goes something like this:
"Message" » {secret formula} » (scramble) » {reverse of secret formula} » "Message"
In other words what goes in can be gotten out if you know the secret. Obviously in this scenario what the secret is is pretty important. In computer cryptography is is usually not the formula itself that is secret, it is the values used in the formula that are the secret keys. More on this in a second.
Hashing is the process or reducing a blob of information down to a single value (usually of a fixed length much shorter than the original) that is unique to the given input. This is can be used to confirm the integrity of a message but does not actually contain the message. This looks something like this:
<code>[Value1] » 7a258c461b48c794c0b5110b35382d1e
[Value2] » 926e0eddb765d66502eac40a4ead9409
</code>
Hashing is often used to store passwords. Note that this is a one way operation. Given the same input you will always get the same hash1:
<code>[Value1] » 7a258c461b48c794c0b5110b35382d1e
[Value1] » 7a258c461b48c794c0b5110b35382d1e
</code>
But one cannot know just from the hash what the input was2.

That background aside, MD5 is a hash algorithm. It is not likely that your game sends hash data back to the server because the data would be uninterpretable. It wouldn't tell them what your score was! You can pretty much rule out anything that is a hashing algorithm.
Now there are two basic ways an encryption process like in the first bullet point above can work.

The same secret value can be used both to encrypt and decrypt the content.
This is the usual pattern for things like password protected documents.

User A uses {password} to encrypt a document.
The encrypted document is passed around.
Only users that know {password} can decrypt and read the contents.

A matched pair of values can be generated such that one half of the pair is used to encrypt content that only the other half of the pair can decrypt.

User A generates a key pair.
They then pass around the public half of the key pair but keep the private half to themselves.
Any user may encrypt documents or values using the User A's public key, but the resulting data may only be decrypted by User A using their private key.

If you game uses a basic password model as in #1, the password and algorithm used will be hard coded somewhere in the source code and burred somewhere in the actual app. Anybody that does some dissection work will be able to figure out what that value is and decrypt any further messages they intercept. Hopefully your application did NOT do this!
The proper way would be for to public key cryptography as in #2 above. The software developer would have generated a key pair for their software. The public half would be built into the software somewhere and all values can be encoded using that key before being sent over the wire. Once they reach the developer's servers, the data may be decoded using the private half of the key.
Conclusion
If the developer has used the proper technology for the job, you will not be able to decrypt the data being sent because you do not have the private key needed to decode it. The public key may be buried in the software somewhere, but that will not get you the data.
If they have NOT done their job properly then there might be trace clues in the software itself that give you enough information to decrypt the data yourself.
Actually identifying the method and specific algorithm used in this case is beyond the scope of this answer and is left for the reader to do further research on. Hopefully this overview of the possibilities will provide a better understanding of what sort of things you are looking for.
2 This is why using salted hashes is paramount to the security of password systems!
2  Note I said just from the hash because is is possible to guess or use pre-defined tables  to know what input generated a hash, but a secure hash itself will not actually contain the data used to generate it.
Answer: Okay. This is kinda cool. A great way to learn about encryption.
Few things, not really your answer.
Even if you know how it was encrypted, you wouldn't be able to do anything with it. The more important thing is the key used to encrypt it. What you have posted could be plain text hashed, encrypted bytes hashed or encoded.
If you just wanted to find out how its done, you can try running a dubugger on the android kernel and sift through the system calls to see what ssl lib they use if not builtin to their code. Im sure at that level, you can also try to run a debugger on their process and look at the system calls.
Look at this, and this and do some more google searches on that.
My other question for you is, how are you seeing that being sent? Wireshark, tcpdump etc...?
If so, you may be seeing the https packets and thats not telling the whole story.
Look into mitmProxy. They have a part on their site to do game hacking like you are saying. Something like this.
I'd recommend getting some experience with that.
All in all, if its being encrypted client side (your android) you can figure out how to undo it eventually. Might not be worth it for a game, but a great learning experience never the less.


How to add command line tools for math to Gazebo?

Question: I am trying to solve this issue on the gz-math repo. My initial approach was to make changes to CMakeFiles.txt inside gz-math and add a cpp code to convert a quaternion given in arguments to Euler Angles. However, this approach does not seem to work. The issue mentions the use of gz-tools or ign-tools and links a file as an example but I can't understand how to add a command line tool in that way. I would like the command usage to be like gz math --quaternion_to_euler "0 0 0 1". How should I proceed? I have the equations for converting quaternions to Euler angles, but how can I integrate the tool in the way required? The command gz does not currently have an option called 'math', so that has to be created too. And in the end, which directory do these changes happen in? gz-math or gz-tools?
Answer: The first step I'd take is to completely understand another gz command line tool.
https://github.com/gazebosim/gz-tools/

Each subcommand is implemented in a plugin that belongs to a specific
Gazebo project. For example, all the commands that start with gz topic
... are implemented by the Gazebo Transport library.

Trace out the code from gz-tools to an existing implementation and then find the plugin for that implementation and you should be in good shape to sketch one for math.
The Gazebo Transport command line tools seem to be implemented by the files in https://github.com/gazebosim/gz-transport/tree/gz-transport13/src/cmd, for example.


Statistical tool to test relationship between variables and perceptions with Likert-Scale

Question: I am currently doing my research about perceptions of residents toward community development. I am trying to find out if there's a relationship between the respondents' profile or their socioeconomic characteristics and their perceptions toward development. I used Likert-scale to measure their perceptions toward and the indicators for development. What statistical tool should I use? Is chi-square and T-test right? Or correlations like Pearson and Spearman's the better one?
Answer: You could use, for example, ordinal regression, with respondent characteristics as predictors and likert-scale perceptions as a response.


Proof by induction $\frac1{1 \cdot 2} + \frac1{2 \cdot 3} + \frac1{3 \cdot 4} + \cdots + \frac1{n \cdot (n+1)} = \frac{n}{n+1}$

Question: Need some help on following induction problem:

$$\dfrac1{1 \cdot 2} + \dfrac1{2 \cdot 3} + \dfrac1{3 \cdot 4} + \cdots + \dfrac1{n \cdot (n+1)} = \dfrac{n}{n+1}$$
Answer: Every question of the form: prove by induction that
$$\sum_{k=1}^n f(k)=g(n)$$
can be done by verifying two facts about the functions
$f$ and $g$:

$f(1)=g(1)$
and
$g(n+1)-g(n)=f(n+1)$.
Answer: Hint:
$$\frac{1}{k(k+1)}= \frac{1}{k} - \frac{1}{k+1}.$$
Hint2:
$$\frac{n}{n+1} = 1 - \frac{1}{n+1}.$$
Answer: $\textbf{HINT:}$ Actually this is the answer in itself.
$\frac{n}{n+1} + \frac{1}{(n+1)(n+2)} = \frac{n+1}{n+2}$.
This is all that you will need when you do induction.
Answer: Essentially you test the base case, p(1), which is true since 1/(1+1) = 1/2.
Then, you want to assume your p(n) is true, namely, that sum you wrote above:
1/2 + .... + 1/(n+1)

and then add the next number in the sequence.
1/[(n+1)+1]

Since you know your p(n) sum equals n/(n+1), you add the next number in the sequence above to your p(n):
1/2 + ... + 1/(n+1) + 1/[(n+1)+1] = p(n) + 1/[(n+1)+1] = n/(n+1) + 1/[(n+1)+1]
Given the useful hint above from user17762, they have done the algebra for you.
n/(n+1) + 1/[(n+1)+1] = (n+1)/(n+2)

You can rewrite as, (n+1)/(n+1+1), which is exactly p(n+1), so we assume it's true for all n because it's true for the next term in the sequence.
B.Y.U.


What is this large brown seed found in Barcelona?

Question: I found this seed on the ground in a park in Barcelona, Spain. I thought it might be a buckeye but it looks just different enough that it seemed possible that it might be something else. It's roughly 1.5 inches long at its widest point.
Comment: It looks like some species of *Aesculus* (chestnut) to me - but from the dark colour and generally battered looking condition it's in, I'm not sure it looked like that when it first fell off the tree so I can't guess the species. "Buckeyes" are American species so it is more likely to be a different European relative IMO.
Answer: This is very likely Aesculus hippocastanum alias European Horse Chestnut. The seed/fruit presented in the picture is the result of drying of the fresh seed into the object known as a "conker" in the UK. Once they get to that condition they are not much use as seeds to produce new plants. The wiki page has a picture of the fresh seeds and their bright warm brown colour. Spain is of course also famous for sweet chestnuts but the classic difference in seeds is that the sweet chestnut produces multiple seeds in the same spiky fruit so they have a flat side; this one was completely round when fresh. We would need a picture of the tree to be more sure of the ID.
A search on "aesculus barcelona" produces links that show A. hippocastanum is grown and used in the area, and that it has been crossed with A. glabra (the related Ohio Buckeye) and other species to produce interesting hybrids.
Comment: Thanks. I had thought of chestnuts too, especially with the light colored circle on one side, but the shape seemed wrong. But it makes sense that it's just dried out.


Do you know what spells are in a ring of spell storing?

Question: A character finds a ring of spell storing.  Does he automatically know what spells are stored in it? Does he need to attune to it? Does it matter if the character is a magic user or not?
Comment: The same way a character determines what's in any other storage, by querying it: `SELECT SPELL_NAME FROM RING_OF_SPELL_STORING`
Answer: The RAW from the item description does not explicitly say one way or the other, but it does seem to imply that the attuned user has knowledge of the spells in the ring.

This ring stores Spells cast into it, holding them until the attuned wearer uses them.

and

While wearing this ring, you can cast any spell stored in it.

The two rules above state that the attuned user can cast any spell stored in the ring.  As a prerequisite for being able to use something (a spell in this case) is knowing it exists.  As such, knowing which spells are present seems to be implied.

When found, it contains 1d6 - 1 levels of stored Spells chosen by the DM.

This rule would not be relevant if the attuned user could not know which spells were available.
A mechanic where the attuned user knows that there are spells stored, but not what they are breaks down when it comes to casting a spell.  Many spells require a target or targets to be cast.

A related post How does a Ring of Spell Storing function? about the mechanics of magic items that confer spell casting is an informative summary.
Answer: The spells stored become known during attunement

Does he automatically know what spells are stored in it?

No.  The ring specifies that it requires attunement.  (DMG. p. 192)

Does he need to attune to it?

Yes. (Or use the identify spell, see below).

Some magic items require a creature to form a bond with them before their magical properties can be used. {snip}
  ... at the end of the short rest, the creature gains an
  intuitive understanding of how to activate any magical properties of
  the item, including any necessary command words. (DMG p. 136-138,
  attunement)

A key magical property of this item is the spells in it that can be used.

Does it matter if the character is a magic user or not?

No.

While wearing this ring, you can cast any spell stored in it.

Any creature that is attuned to it can use it.  The magic item description does not limit the ring of spell storing to spell casters.
Compare the ring's description

rare, requires attunement (DMG, p. 192)

to Robe of the Archmagi, which has limitations on who can use it.

requires attunement by a sorcerer, warlock or wizard (DMG p. 193).

No class restriction on the ring of spell storing.
A spell caster is required to cast spells into the ring once the spells in the ring (when found) are used.  That makes this ring a potentially flexible party asset.

While wearing this ring, you can cast any spell stored in it.

You can also find out what spells are in it with the Identify spell
From the Identify spell description ...

If it is a magic item or some other magic imbued object, you learn its
  properties and how to use them; whether it requires attunement to use;
  and how many charges it has.  You learn whether any spells are
  affecting the item and what they are. (PHB p. 252)

Anecdotal account: in our first D&D 5e campaign, our party found a ring of spell storing which was first attuned by our 5th level bard.  Four different members of the party ended up using that ring, or casting spells into it: the Bard, the Wizard, the Cleric, and our Paladin.  It was a great party asset.
Answer: Just looking at the ring, no, you don't.  If you attune to the ring, so you can use it, you will know.  Also, casting an Identify spell on it could tell you before you attune.
Just looking at the ring, you have no way to find out what's in it. Looking at the Identify spell, it's clear that this would tell you (and bolsters the case that you can't tell otherwise, since the spell wouldn't be needed if you could tell without it). Knowing what spells are in the ring is necessary to the ability to use it, so rationally, attunement would bring that knowledge.