Calculating DV01 for Treasury Futures with CTD switch risk

Question: With rates rising, certain contracts, such as the USZ3, are prone to frequent CTD switches with sometimes large differences in the DV01 of an underlying CTD. Does anyone know of any resources for calculating the DVO1 for a treasury future that is prone to CTD change risk or alternatively any off the shelf solutions that can handle this? Bloomberg seems to have nothing. Thanks!
Comment: You can try this: https://rateslib.readthedocs.io/en/stable/api/rateslib.instruments.BondFuture.html#rateslib.instruments.BondFuture, needs some set up though. Interested in any feedback.
Comment: Thanks you! Will definitely let you know if I'm able to get enough into it to provide some worthwhile feedback
Answer: You are trying to calculate the so-called "option-adjusted DV01" (OA DV01). The nice thing about OA DV01 is that it's a smooth function of yield shifts. I'm going to be lazy here and simply post a figure from an ancient report titled "The Salomon Brothers Delivery Option Model" (Mark Koenigsberg, 1991):

As you can see, when yield level changes, the CTD's DV01 can of course jump around, but the model-based OA DV01 transitions smoothly.
If you don't want to calculate these numbers yourself, you can get them very easily from a lot of sell-side banks. At JP Morgan, for example, you'd search for "U.S. Treasury Future Basis Reference Sheet" and the numbers you are looking for are printed under "OA BPV." At Barclays, you'd search for "CBOT Futures Multi-factor Analysis Report" and the numbers are printed next to "$ PV01." Morgan Stanley also has "US Bond Futures Daily Report" and the label is simply "DV01." As a shameless plug, my current company also provides these numbers on the Augur Labs Infinity platform.
If you want to create a simple model to calculate this yourself, it's actually not that difficult. Here's a highly stylized example:

Imagine there are only two bonds that can be delivered into the contract.
One of the bonds is the current CTD. Start by creating 1000 yield scenarios as of the delivery date for it (more scenarios would be better, but 1000 is a good starting point). For simplicity, you can assume that yield changes are normally distributed; the yield on the delivery date may have a mean corresponding to the current forward yield and a volatility inferred from bond futures options. Based on these yields, you can easily calculate the converted forward price for the current CTD at each yield level.
For the other bond, let's assume that the yield curve can only move in a parallel fashion, so now you have 1000 forwards yields for the other bond too, and you can again compute the converted forward prices.
For each yield level, you'd compare the two bonds and see whose converted forward price is lower. That tells you who the CTD is for each yield level and you also know what the theoretical futures price should be at that yield level (simply the minimum converted forward price).
Now it should be straightforward to summarize the delivery probability of each bond from step 4, as well as the futures model price (the probability-weighted futures price).
To compute the OA DV01, you can simply perturb the initial yields by say 10 bps and see how much the probability-weighted futures price changes. Alternatively, you can calculate the probability-weighted DV01 (the former is preferred when we introduce more features into the model).

The simple model above ignores a lot of subtleties, which is only casually reviewed here:

In Step 2, you can either use the CTD as the reference bond or an on-the-run bond as reference. I personally prefer the latter approach.
In Step 2, you should also include a convexity adjustment to ensure that the probability-weighted forward price for each bond matches the current forward price, so that there's no arbitrage opportunity.
In Step 3, instead of assuming parallel shifts of the yield curve, you can shift the other bonds using their historical yield betas to the reference bond.
The model also ignores the a) timing option, b) the end of month option, and c) the wild card options. a) is not super valuable, but b) and c) can be meaningful. These are difficult to handle and I'd recommend consulting specialized readings. Instead of this kind of simplistic grids, a term structure model might be more suitable.
This model is a single-factor model. The advantage is simplicity – you can literally create this model in Excel with a 2-dimensional grid (deliverables on one axis and yield levels on the other). Ideally, you'd use at least a two-factor model to more accurately capture relative yield movements. For a real-world example that implements this kind of two-factor model using a grid, search for "The Lehman Brothers Multifactor Futures Model" (Phil Weissman & Ralph Axel, 1997).
Comment: @TimW I just saw a podcast from JPM regarding this: https://overcast.fm/+yzQHigNI4
Comment: I used this answer as the base to documenting some CTD analysis. I produced a chart similar to the above for the Dec 2023 30Y Treasury future. https://rateslib.readthedocs.io/en/latest/z_bondctd.html
Comment: Great post, thank you! Do you know whether the ancient report you cited is still available or accessible somewhere? If not, do you have an alternative source to read about OA DV01 calculation?
Comment: Great answer. Really interesting.
Comment: Amazing answer. Thank you, exactly what I was looking for and helped a ton.
Comment: The doc would really help me (for other reasons) if you have a copy of it by any chance. Can't seem to find this online.
Answer: Bloomberg has everything ! USZ3 Comdty DLV gives the delivery basket.  If you type CMS (go) it then shows how the ctd switches and the ensuing change in dv01 of the futures contract.  Today it shows that a 40bp sell off causes the ctd to switch from 2040 bonds to 2042 bonds, with a dv01 change from 12.6 to 14.0.
Comment: Thanks, yeah I've been using that function to back into some rough approximations but was hoping they would have a single, probability weighted number somewhere. It seems like a lot of the CTD analysis related functions are fairly stale which sort of makes sense given switching hasn't really been an issue for the past 15+ years. Appreciate the response!


Thoriated Tungsten filaments

Question: I have a question about doping a tungsten wire with thorium to improve the thermionic emission. I have found that the doping lowers the work function of the system, resulting in a thermionic current greater than the one produced from a pure W filament. The point is, WHY this happens? What are the mechanisms that lower the work function?
An idea that I have is that thorium (and most of its decay chain products) is an $\alpha$-emitter. The $^{4}$He nuclei then produce scattering sources for the electrons, enhancing the emission probability.
References are higly appreciated :)
Answer: This seems to be well described here, which, in turn contains a reference to L.W. Turner,(ed), Electronics Engineer's Reference Book, 4th ed. Newnes-Butterworth, London 1976.  I just might have a copy of that somewhere, but not to hand.
The mechanism agrees with what I remember being told it was: Thorium is added to the Tungsten filament, and then the whole thing is heated significantly hotter than its normal working temperature.  This causes (why?) the Thorium to migrate to the surface of the filament, resulting in essentially a Thorium-plated Tungsten filament.  Thorium has a lower work-function than Tungsten so the effective work-function of the filament is that of Thorium.
It should be fairly easy to convince yourself, based on the activity of Thorium compared with the number of electrons a hot cathode needs to emit, that its radioactivity can't be a factor.
I am not sure why Thorium is not just plated onto the filament, although I expect this is to make production simpler: if you can effectively plate the filament by heating it after production (which you can do electrically of course, since that's what you do anyway) then this makes it all cheaper and easier.  Probably the initial heating of the thing also helps weed out infant mortality: filaments which are going to fail early probably fail during the initial heating.
Answer: It's actually thorium oxide ("thoria"), by itself an insulator, that is added to the tungsten. Though it is, in fact, the metallic thorium film, a monolayer adsorbed at the surface, that is responsible for the reduction of the work function. Simply because thorium has a lower work function than tungsten does: 3.4 vs. 4.5 eV.
Or put another way… The Fermi level of thorium is higher than tungsten's, thus closer to the energy of free electrons in vacuum. As the electrons in the tungsten bulk and in the thorium surface layer are in equilibrium, their Fermi levels are… well, level. This means there is a contact potential across the interface, but, much more importantly, it makes it easier for the electrons to escape. The thorium monolayer thus lowers the energy barrier that the electrons need to overcome in order to make it to the free vacuum state.
Thermionic emission is thermally activated: Only the outliers among the conduction electrons, in terms of their distribution of kinetic energy, make it across the barrier. So the deceivingly small difference between 3.4 and 4.5 eV is actually a big deal, given that the average thermal energy is maybe 0.1 or 0.2 eV, depending on operating temperature.
The operating temperature is the issue, however. If it were just about the work function, we'd just use pure thorium, all the way. Or any old metal, really, as long as its work function is low. Of which there are many, compared to tungsten.
When it comes to thermionic emitters, though, there are always two competing effects. Increasing the temperature means more electrons are "evaporated", make it into the vacuum. But the surface atoms themselves also evaporate. They detach from the emitter and end up God/Newton knows where in the vacuum chamber.
At a temperature that would produce an appreciable electron current, thorium atoms evaporate all too easily. If it were bulk thorium, it wouldn't last long. But when dissolved into tungsten, only part of the surface is covered by thorium at any given time. And when it evaporates, which it does, the partial layer is replenished via diffusion from the bulk.
That's the trick to it. I doubt that there's a theoretical explanation based on "first principles" alone. After all, that would involve band-structure calculations, interface bonding, atomic evaporation, as well as computations of diffusion coefficients through the bulk, along grain boundaries, and across the surface. Irving Langmuir, back in 1934, sure tried to provide that theoretical base, in "Thoriated tungsten filaments", but ultimately it just works in practice.
The (weak) radioactivity of thorium is an unfortunate side effect. Though all it does is reduce the viability of thoriated tungsten in commercial applications.

Thoriated Tungsten filaments


Storing iBooks in iCloud

Question: Can iBooks be saved to iCloud then deleted on my iPad but still be available on iCloud? The reason for asking this is that my iPad is running out of space, so I want to store my iBooks elsewhere.
Answer: With the release of iOS 9.3 iBooks now supports storing PDF and ePub files in iCloud. You should be prompted to enable it when you launch iBooks after performing the update. If not then you can enable it via the Settings App under iCloud > iCloud Drive > iBooks.

Storing iBooks in iCloud


Is there a situation when a shift in supply and demand leads to a shortage?

Question: For my intro health economics class, my professor asked us to explain how certain scenarios impact the supply and demand of health care (using only supply/demand analysis). He raised an example of a pandemic resulting in widespread illness affecting both healthcare workers and the general public. To start off, I thought that there would be an increase in demand (more consumers entering the market) as well as a decrease in supply (fewer suppliers of health care). Theoretically, prices would increase and it'd be difficult to figure out how quantity changes. However, just logically and thinking about COVID, it would make sense that there is a shortage because quantity demanded would surpass quantity supplied given that suppliers of health care are now demanders. I'm struggling with how to represent this graphically where shifts in supply/demand lead to changes in equilibrium price and quantity, but simultaneously result in a shortage. Is my thought process correct, and is there a way to explain supply/demand shifts with a concurrent shortage?
Answer: This thinking is not completely correct. You seem to be confused about quantity demanded and demand and quantity supplied and supply.
Demand is the whole relationship between price and quantity demanded. An example of demand is not a number such as 100 units of hospital beds at day $t$ (even though media often incorrectly use word demand in such way), demand is a relationship between price an quantity. That is demand would be some $Q(p)$ for example $Q(p)= p-100$.
So if demand shifts that does not necessarily mean quantity demanded changed. Indeed demand might increase and supply might decrease but quantity demanded could stay exactly as it was before the shock if price adjusts.
However, in real life prices do not adjust instantaneously. Prices are sticky for various reasons. Walmart won't increase prices of toilet paper immediately  when shoppers start emptying the store. Similarly hospital won't just raise prices instantaneously when there are too many people in ER. Price adjustments usually take times as they cant be just done by nurses/doctors at ER who have the immediate information about demand, there are laws and regulation prohibiting it, issues like menu costs etc.
In principle even if supply is extremely limited and demand extremely high, there will be some large enough price that will make sure that quantity demanded is equal to the limited supply despite enormous demand. In principle there is always price at which quantity demanded drops to zero for some extremely high price. Prices do not have theoretical upper bound.  But in real life there are price rigidities that simply don't allow for instantaneous adjustment. Moreover, there is also 'anti-gouging' legislation in many countries so it might be even illegal to rise prices too much at a time. For example, in the US there is 'Price Gouging Prevention Act of 2022'. Especially when we talk about medical care the prices are tightly regulated almost everywhere in the world, no matter whether we talk about medicine or procedures. Even in countries where there isn't a strict price regulation (e.g. US) there are other laws such as the above mentioned act, and beside regulation there are natural reasons for price rigidities such as above mentioned menu costs and so on.
Hence the way how you model this is as a simultaneous shift of supply and demand assuming prices are fixed at below equilibrium level. You can visualise it as follows:


Importing from Excel to a Calendar

Question: I have a new calendar that currently has no events on it. There is a calendar on one of my other sharepoint sites that I'd like to copy all the events in it over to this new one. I've exported all the calendar data to an excel file (saved as a .iqy), but I don't see any way to import the data. I read you have to simply click the 'import' button, but I can't seem to find it anywhere.
Comment: Please specify your SharePoint tenants.
Comment: Sorry, I'm using Sharepoint 2016.
Comment: Did you get it working?
Answer: SharePoint doesn't offer a command or a tool that allows users to bulk add events into a calendar; but by using Office apps (Excel and Access) we can do that very easily.
Kindly check the below mentioned article for your reference. It will work same in SharePoint 2016.
SharePoint: How to Add Events into a Calendar Using Microsoft Excel and Access
Comment: This worked. Thank you! But I tried to color code them by adding new "calendars in view", but it couldn't resolve the web URL...
Comment: You can create it by your own, in new site where you have moved your calendar.
Comment: Create what on my own?
Comment: Color coded views of calendar
Comment: This link is dead. Please, in future, put the relevant info within the answer and not just a link.

Importing from Excel to a Calendar


When shifting to the 1st gear the chain shifts up to 3rd derailleur(rear)!

Question: I am using 21 speed bicycle and the problem is that I am unable to shift the chain to the largest derailleur(rear)....the shifters are thumb press type there's no problem in pressing the shifter...when shifting to the 1st gear the chain shifts up to 3rd derailleur(rear)!
Comment: Have a look at http://bicycles.stackexchange.com/q/26062/19705  sounds like your gear inner cable has stretched and needs minor adjustment.  Do be mindful of putting the cage into the spokes by mistake... that's not right.
Comment: Usually this just means that something (probably the cable's "barrel adjuster") needs adjustment.  But sometimes the problem is more elusive.
Answer: A stretched cable is one possibility. Has the cable been replaced recently? They stretch most when they're new.
A second possibility is that you removed the rear wheel and when you put it back on, didn't get it seated the same as it was before you took it off. That could change the position of the derailleur enough to make it impossible to shift onto the largest or smallest gear in the cluster.
A third possibility is that you need to adjust the high-low limit screws on the derailleur. Here's what Park Tool has to say about that... http://www.parktool.com/blog/repair-help/rear-derailleur-adjustment


AWS cloudwatch node.js metrics send performance concern

Question: I am new to using aws-sdk in a node.js project. I was going through the guide mentioned here about sending metrics to AWS cloudwatch. The example below in particular uses a promise based approach for sending metrics, if i am to use the below strategy for sending metrics, for e.g. whenever a particular endpoint is triggered, how performant will this usage be?
Node.js runtime is single threaded, so, for every request adding something on the call stack for a single threaded process seems like a performance degrading practice, in Java you could use daemon threads for sending metrics.
Should this be a concern? If yes, then what could be a better strategy for sending the metrics?
<code>export const run = async () => {
  try {
    const data = await cwClient.send(new PutMetricDataCommand(params));
    console.log("Success", data.$metadata.requestId);
    return data;
  } catch (err) {
    console.log("Error", err);
  }
};
</code>
Answer: This can be solved by batching metrics and making a single call periodically.
You can try the aws-embedded-metrics library for nodejs. I have not used this but looks like you can flush the metrics periodically.


Is there some means of expanding the range of a capped reed instrument?

Question: Given the pitch limits of capped reed instruments, I wondered whether some means of expanding the range of such existed. Perhaps with some sort of embouchure mimicking device surrounding the reed, but within the cap. The question assumes that the bore is not changed from the original conical/cylindrical bore native to the instrument(for instance, the crumhorn/rauschpfife, respectively).
Comment: I can't help with an answer, but a full two-octave rauschpfeife would be a thing of wonder! I suspect that if it was possible it would have been done already, but I hope someone can come up with a practical proposal ...
Comment: Well, it'd kind of be cheating :-)  to put mechanical lips next to the reed.  Perhaps you could place two reeds in parallel feeds inside the capped chamber, with different sizes and/or different bores?
Comment: Relative to one's audience, start moving really fast to get that sweet, sweet doppler-shifted high octave (posted as a comment to avoid getting -100000)
Answer: It's already been done. George Kelischek's rauschpfeiffen can be "overblown" into the second octave: https://www.susato.com/collections/rauschpfeiffen
Otherwise, no I don't believe it's possible to extend the range past nine notes without fundamentally altering the instrument. I've played some capped reeds with keys which extend the range slightly (a couple notes).
George's Kelhorn is a fine example of such alterations, and might prove to be the ultimate advancement of capped reed design: https://www.susato.com/collections/kelhorns


Devoid vs Bereft

Question: I am under the impression that few words have perfect synonyms that are interchangeable in all contexts considering their different connotations or literal meanings. Is there a difference between Devoid and Bereft? For example, in the famous Monty Python "Dead Parrot" sketch, the parrot is said to be "...bereft of life-it rests in peace!" Could devoid work in that sentence?
Comment: Suppose it turns out there's no life *currently* on Mars. It might be more appropriate to say Mars is ***bereft** of life* if there's evidence that it did exist there before (the planet is "deprived" of what it once had). To my mind, saying it's ***devoid** of life* carries more of the implication that it never had any life before either (equivalent to simply saying it's *lifeless*). An **"ex-parrot"** can (famously) be bereft of life, where a toy parrot might only be devoid of life.
Answer: The usual connotation of devoid is that a normally-expected, or even possibly-expected, thing or quality is missing:

I looked out over the hills, but they were completely devoid of motion: not so much as a blade of grass stirred.

The usual connotation of bereft is that a normally-expected thing or quality has been removed:

After his revelation, I sat there with my mouth agape, bereft of speech.

To me, then, a parrot that is devoid of life would be a manufactured, artificial animal that was never alive; a parrot that is bereft of life is one that was alive at one point but is now deceased, passed on, gone to meet its maker, shuffled off its mortal coil, crossed over the great divide, and so forth.
Comment: Haha - same distinction between "once-living" and "never alive" parrots I was making in my comment while you were composing this answer. (Obviously +1 :)
Comment: You seem to be suggesting *devoid* and *bereft* are mutually exclusive (per your opinion that one describes an artificial parrot and the other describes one which once lived), but your definitions suggest they aren't (e.g. a parrot is "normally-expected or possibly-expected" to be alive, so a dead parrot is devoid of life, and if it was killed then it is also bereft of life).
Answer: "Bereft" can be applied to people with roughly the same sense as "bereaved" (in this sense, it can stand alone and does not have to take an adjective complement); "devoid" can't be used this way.

(of a person) sad and lonely because you have lost something

He was utterly bereft when his wife died.
The shock of his departure had left her feeling alone and bereft.

from Oxford Learner's Dictionaries

Devoid vs Bereft


How do I get gcc-3.2.2 on RHEL 5?

Question: We just had our hosting provider build out a new RHEL 5 box for us to test some legacy stuff on:
uname -a: Linux myserver.foo.com 2.6.18-164.9.1.el5 #1 SMP Wed Dec 9 03:29:54 EST 2009 i686 i686 i386 GNU/Linux
cat /etc/redhat-release: Red Hat Enterprise Linux Server release 5.4 (Tikanga)
gcc -v: gcc version 4.1.2 20080704 (Red Hat 4.1.2-46)
I'm having a hell of a time figuring out how to get gcc-3.2.2 installed on this machine:

I can't seem to compile 3.2.2 with the gcc 4.1.2 compiler
I can't seem to find a 3.2.2 installation via yum
I'm afraid of what would happen if I rpm'd it manually

Any advice?
EDIT: thanks for the suggestions so far, but just to clarify - the "legacy stuff" I mentioned isn't actually my company's legacy stuff. It's a 3rd party package and I don't have access to the code (and wouldn't want to change it even if I did)
I guess I'll look into the chroot thing, because at this point going back to a different RHEL seems heartbreaking.
Answer: You could try to install a RHEL 4 machine, it has gcc 3.4 which might work to build your stuff. Then you could try running the binaries on your RHEL 5.
It may just be easier to debug the legacy code to work on gcc 4.
Answer: What I would suggest is installing the rpm manually within a chroot. You would have to google a lot for the method but it should in theory work well. Alternatively you could just rewrite/update your legacy code (yes, I know it is easier said than done).

How do I get gcc-3.2.2 on RHEL 5?


What do appleeventsd and AppleIDAuthAgent do?

Question: I looked in the activity monitor to clarify what are those processes and started with alphabetic order. I know different resources where many processes are described. But appleeventsd and AppleIDAuthAgent are not the case even for goole search - only issues results were found without explaining what does these processes stands for.
Answer: appleeventsd is the Apple Events daemon, which handles cross-application interaction. It is the underlying basis for AppleScript amongst many other things.
AppleIDAuthAgent is "used to check the validity of AppleID certificates".
Comment: Sorry, but I don't believe that. Even the name already implies that it has to do with "Apple IDs" and why would it connect to setup.icloud.com? There are no cert infos at that address. It may __also__ verify certs among many other things, but if take a look [at the traffic](http://klurl.nl/?u=UWogbM) it sends out, this doesn't even look close to anything checking validity of certs.
Comment: Your Apple ID private key is stored locally in a certificate in Keychain Access. The AppleIDAuthAgent process confirms the validity of the local certificate with the server. iCloud is an integral part of Apple ID and vice-versa, so it makes sense that an iCloud domain would be involved. The response includes things such as "VettingStatus" and "ValidForInterval" which would align perfectly with validating a certificate.
Comment: Private keys are not stored in certificates, public keys are. And where would this AppleID cert be in my keychain? I don't see any. Also there are common mechanism to validate certificates (OCSP for example), so why is Apple not using those (as they are in fact for all other certs). Also I see on keychain item that carries AppleIDAuthAgent in its access list (well, public certs have no access list of course, they are public, but I see no public AppleID cert)
Comment: http://i.imgur.com/4t6Gtoi.jpg


samsung s4 has suddenly started calling through Skype instead of the local network

Question: My samsung s4 has suddenly started calling through Skype instead of the local network. I have not set any defaults. I have also uninstalled skype, it still goes through skype, then stops then I have to call again then it goes thtough the network. How can resolve this? please help
Comment: When you say, "I have also uninstalled skype, it still goes through skype, then stops," could you explain precisely what you see happening?
Comment: it chooses skype to call. It shows the skype call view and then returns to keypad screen. when I call the second time it then goes through normal.
Comment: If a Skype window is coming up, Skype must still be installed.
Comment: Thats the thing..it is not! and thts wat is driving me mad! I have uninstalled skype
Answer: Looks like it just went in NSA mode. You can check in Settings->Apps, switch to the "All" tab, and slowly and carefully check the entire list whether Skype really has been removed (it should no longer be found here then).
If you find it there, make sure to really uninstall it. If it will not leave in peace, you can make it leave in pieces:
If you not already have one installed, get yourself a terminal emulator (e.g. Android Terminal Emulator). Open it, and enter:
pm uninstall com.skype.raider

This advices the "package manager" (pm) directly to remove this app.
If nothing of the above helps, please edit your question and add some details: Is your device rooted? Do you have an ADB setup available?

If your device is rooted, you can directly check for the app without the help of GUI elements. Via adb shell or above mentioned Terminal Emulator (lines starting with a # are comments and do not need to be typed):
# become root. This will popup a confirmation box, confirm:
su
# now we look for the data directory of the Skype app
ls /data/data | grep skype

If the prompt returns without any information, there is no Skype data directory. If it, however, returns something like com.skype.raider, the data directory is still there. Still being root, you can also check for Skype's .apk file:
ls /data/app | grep skype

If Skype is still there, this should yield something like com.skype.raider-1.apk. You can also list the packages known to the system, which does not even need root:
pm list packages | grep skype

And you could see if root powers help accomplishing what failed with the error message you described, and re-issue a
pm uninstall com.skype.raider

With ADB available, you could do all this in a much more convenient way from your computer:
adb shell
su

Then continue as described above. Note: If su was successfull, the command prompt should be prefixed by a #, otherwise it remains prefixed with a $.
Comment: Hi izzy
Checked all apps carefully. Skype not there.
Did the terminal emulator.  I get a message saying "failed to connect to dumpstate service. Killed"

Now wat?

I dnt know wat u mean by is ur device rooted? Do u have adb set up?
Comment: Please see the [rooting tag-wiki](http://android.stackexchange.com/tags/rooting/info) for clarification on the root part, the [adb tag-wiki](http://android.stackexchange.com/tags/adb/info) for ADB, and [Is there a minimal installation of ADB?](http://android.stackexchange.com/q/42474/16575) for how to get it (if you mind). On a rooted device, and ADB available, you have many additional options to find out and solve. One more question: Have you already restarted your device after uninstalling Skype?
Comment: Ok.let me c wat I can do with rooting and adb. Thnkx
Comment: Yes I did restart after uninstalling
Comment: OK. I've updated my answer with additional hints on what can be done with root/ADB. If nothing else helps, you still have one last ressort: doing a factory-reset. But after that, you would have to install and configure all your apps again. Well, most of them at least.
Comment: Thnkx soo much izzy. I dnt knw wat had happened. After 2 days it automatically started working fine. I jus hope the problem doesn't return! Thankx for your effort!!
Comment: Glad the issue is gone! So though it did so automatically, you might consider "accepting" the answer (click the checkmark left to it) if it at least proved helpful. To upvote, you need some more rep first for yourself ;)
Answer: There is no need to uninstall Skype. Go to Setting/Application Manager. You will get Skype as one one of tha Apps. Click on Skype. Launch by default, clear default.


How long before I can move a concrete lid?

Question: I cast a concrete lid to replace the broken one on my well pit. My lid is 30"x 36", and 2-1/2" thick and reinforced with rebar. I am keeping it covered with a tarp and misting it every few hours. How long before I can safely move the lid to my well pit?
Answer: As a teenager, I had a summer job pouring concrete to cast septic tanks and their lids.  The lid of a septic tank is exactly what you're creating - we made them in circles about 5 feet across and three or four inches thick with rebar inside and three rebar hooks for lifting.
Each morning, we would use a front-end loader (tractor with a bucket on front) to pick up the lids we had cast the previous day.  These would be set aside to cure for another few days before installation.  Then, by the afternoon, we would have the forms set up for pouring again.  The concrete arrives at 1pm, we pour by 2pm, and then mist the tops to keep them from cracking until sundown (Texas heat).  Like I said above, by morning, the lids are able to be moved and we start the process all over again.   Based on this, I would say that the lid can support its own weight after 24 hours, and will fully harden over the course of a few days.
To give yourself a safety factor, double those numbers, so go with this:
Wait 2 days to move it at all, and wait 5 to 7 days before putting it into place.
Comment: I was going to answer "by the time someone posts an answer, it is done", but good job answering with great specifics!


Refrigerator noise

Question: I just moved into a studio apartment -- unfortunately there's no door separating the kitchen and my bed. I've never noticed the sound of refrigerators before, but it's LOUD. It starts up at least once an hour, and wakes me up in the middle of the night. I heard that I can try to use acoustic foam to dampen some of the noise. Would it be a fire hazard?
Comment: How long have you lived there? If you give it some time, you  may just get used to it. I used to live near a busy train track and the late-night trains woke me up (I was close enough to the tracks that I could feel the house vibrate as well as the train horn at every intersection). After about a month, I stopped noticing the trains.
Comment: My guess is that there's something loud about this specific refrigerator, and stopping the sound at the source would be best. Is something rattling inside it? Is it leaning against a wall that is resonating?
Answer: A modern refrigerator can operate normally with three sides covered. If you block the grill at the bottom of the front it will not operate properly. It needs to exchange air to work right.
Normally they are manufactured with a couple inches of foam injected all around the inside compartment. I don't know where you could pack the foam that would quiet it down. If you cover the compressor with foam it may overheat and stop working until it cools down.
Might I suggest investigating a timer to shut it off at night while you sleep. If you are not opening it then it shouldn't need to run that often. this may affect how cold your milk is in the morning though.
Comment: I upvoted, but note that you should be really careful about shutting your fridge down for several hours a day.
Answer: Being an Apartment it's likely that the refrigerator has never been cleaned or regularly cleaned underneath & behind the bottom grill. Have the Landlord or Superintendent address this immediately for your enjoyment.
The refrigerator does all of it's breathing down there & if it's all clogged up it will struggle to operate. The inside of the refrigerator may be colding & freezing fine, but the components doing it may be cycling very frequently between overheating & recovering from overheating.
Unless you keep the Apartment's temperature very hot or open the refrigerator doors a lot it really shouldn't come on frequently at all.

Refrigerator noise


Etymology: Arabic falaha, German pflügen, English to plough

Question: Could there be some connection between Arabic falaha meaning to till the soil and German pflügen, Pflug or English plough, to plough?
Comment: Show us a dozen more examples of proposed cognates between Arabic initial /fal/ and English initial /pl/, and we might think there was, if they can be shown not to be borrowings. Otherwise, no.
Comment: @jlawler  I have the impression that connections between Semitic and European languages, especially Greek and possibliy Latin, are no matter of intensive study. But I think that Old Greek took over a certain amount of Semitic words just because of cultural nearness  and contact (Bible, sciences, arts etc.). Such borrowings must be single cases and I assume the words were assimilated in a way that the Semitic origin is hard to find. In any case is is not a matter where one can establish sound shifts as Grimm has found them out.
Comment: That's a lot of assumptions. They may all be true, and you may be completely right in this case. It's possible, like many such assumed things. But without other evidence to convince other linguists, it remains a private opinion.
Comment: *I have the impression that connections between Semitic and European languages, especially Greek and possibly Latin, are no matter of intensive study:* maybe not "intensive", but there's some work on the subject. You may be interested in Emilia Masson, *Recherches sur les plus anciens emprunts sémitiques en grec*, and in the more recent Rafał Rosół, *Frühe semitische Lehnwörter im Griechischen*.
Comment: If you are interested in this topic, which continues to attract cranks en masse as well as the odd serious linguist, I would encourage you to consult not only the work of Theo Vennemann (as Colin Fine recommends in a comment below), but also the work of Saul Levin, if only for the impressive academic rigour involved (review at https://www.academia.edu/2210922/Review_of_Saul_Levin_Semitic_and_Indo-European). I would disagree strongly with the suggestion that little work exists in this area.
Comment: @TKR and legatrix - Thanks for the useful links. I'll have a look at those sources.
Answer: Probably not, at least not provably so. Since Arabic and Germanic aren't related (certainly not at a time depth shallower than the development of agriculture), the words can't be cognate. Could one be a borrowing from the other? Well, English plough has no clear cognates outside of West Germanic, while Arabic falaḥa has cognates in other Semitic languages (e.g. Hebrew p-l-ḥ "to divide, cut asunder"). If there was borrowing, then, it would have to have been either from a Semitic language into West Germanic, or from a West Germanic language into Proto-Semitic. The latter is impossible because Proto-Semitic predates West Germanic by milennia. The former isn't strictly impossible, but these aren't language groups that are known ever to have been in contact, so it is unlikely. One could hypothesize that borrowing occurred through some intermediary language, but that would be highly conjectural and completely unprovable.
Comment: Theo Veenemann has proposed a number of borrowings from Semitic into Germanic, (including _'ard_ => _earth_), and gets published in reputable places; but I don't think he's convinced many people.
Answer: this may help, there are many connections(not sure about the german one though, could have been done at the time of the crusades?), as we have been traditionally taught, the words in the Quraan rooted from the Arabic/Semitic Fa-La-Ha, i dont know its dual-roots, but here is a site i found with many reference most seem legit and accepted by Arabic scholars as well as Muslims scholars http://www.memidex.com/fellahin, anyways the connection to success is a bit obvious if you understand Islamic or Arab philosophy  or even Sufism. Tilling the land for farming is how we use it, but farming takes steps stages, so example Allah has used it in Quraan Chapter 23 Verse 21 word No2 as aflaḥa, its a Verb in 3rd person masculine by gender singular form IV perfect(past) verb, know in Arabic the morphology
is in form 4/IV which tends to means it makes intransitive verbs transitive, and transitive verbs doubly so.
a verb is this form has the meaning of, he made himself do or perform an action. A reflexive causative, i.e. he made himself do something transformative to a place or a state. also see http://quran.com/23/1 tick all translations to compare and you'll see the basic idea
So i hope this clears up the connection of Success and Farming, basically Fa-la-Ha means to go through a process to achieve in a given field, traditionally farming. so its kind of a root used in context of success, prospering, but it is used when the success is based on a systematic process in the connect above its been used in the past/perfect sense it has been completed it is so. hope this helps
Comment: You haven't mentioned European languages which means you haven't answered the question.
Comment: as I said "this may help, there are many connections(not sure about the German one though", the info I provided helps in Etymology, now the person knows the Arabic meanings and context, it is up to them to do more research in between the runic, Italic, romantic and Semitic languages. It the nature of linguistics you can never really get a  definite answer with lost and forgotten languages (i.e. old English.firasian or high German etc...), others may be able to open up the English and German end of the theory


Paired t-test for 2 patients?

Question: For my doctoral thesis work, I conducted a small feasibility study on the use of a virtual reality therapy protocol in preventing aggressive recidivism in 2 treatment-resistant schizophrenic patients.
The study is prospective, observational, uncontrolled, and open.
I administered most of the scales before and after the sessions, with two of them being administered weekly for the primary outcome measure.
As a result, I have quantitative data resulting from the scores on these different scales.
I would like to calculate the before-and-after difference in scores on these scales for the 2 patients, and I tried to perform a paired t-test without success because I have too little data. I've tested this on 3 different softwares (R, SPSS, JASP) without success.
I'm a newbie and need help !
Do you have any suggestions on how to proceed?
Thanks in advance !
Comment: What do you mean "too little data"? A t-test requires (at least) 2 points to work, how much data do you have? How much per group? What are you comparing the groups to? Do you have a control group?
Comment: Can you be specific about what data you do have (eg. how many measurements for each of the two patients, how often, etc.) and what comparisons you want to perform?
Comment: Thanks for your responses. I have 14 measurements for each patient (14 measurements taken before the study and 14 after). I don't have a control group so I was hoping to just compare the before and after measurements.
Answer: You should probably resist the temptation (or the pressure) to conduct a hypothesis test on feasibility study data, particularly with N=2.
A feasibility study has met its objectives if it tests the feasibility of a treatment or a trial procedure.  You might also learn something about the outcome measures, or get a bound for the treatment effectiveness.  But a hypothesis test should only really be used in a trial designed to test that hypothesis.
Also, while it's possible to conduct a paired t-test on two observations, I don't think you are likely to learn very much from it.  If both of your participants happened to improve by the same amount over the treatment period you might get a small p-value, but would you be confident to attribute that to the treatment?  And if there's a large p-value, it would tell you nothing beyond just describing the raw data.
I realise I am making some assumptions about what you are trying to do, so please correct me if I've misunderstood anything.
Comment: Thank you so much, it makes a lot of sense. Can I then only use a descriptive analysis to comment on the outcome measures ?
Comment: Maybe.  The study aims should have been in your ethics approval / protocol, so do whatever analysis is needed to meet those.  From your comment you have 14 measures per patient per period, why did you take so many?  Perhaps there is a research question that motivates taking all of these measurements?
Comment: Indeed, the 14 data points were extracted from 6 questionnaires designed to assess various components of patients' social behavior (impulsivity, anger, aggressive behavior, social anxiety, emotional skills, etc.). We wanted to see if these various measures improved after the therapy protocol.
Answer: I am guessing the score you are dealing with is the BPRS which uses questionnaire data and produces scores for 24 separate domains, scaled from 1 (no symptom) to 7 (severe). So whereas your dataset features N=48 pre-BPRS scores and another N=48 post-BPRS scores, they are not mutually independent. In your case, I believe you can summarize the data entirely by showing the number of domains showing an improvement from baseline, comparing the treated individual to control. Statistical significance testing would have, of course, no generalizability.
Comment: Thanks ! That is exactly what I was planning to do. The only problem being that I don't have a control group ...

Paired t-test for 2 patients?


ACF-PACF could be the diagnostic model of seasonality?

Question: If seasonality is not shown in the acf-pacf plot, can I conclude that the seasonality does not exist? Or Can I say that the seasonality exists based on the monthly incidence data although there is no seasonality in the acf-pacf plot?
Answer: Did you plot the monthly incidence data versus month of year using side-by-side boxplots (presuming you have multiple years worth of monthly incidence data)? This would allow you to assess the presence of seasonality in a more intuitive fashion. See this link for details: itl.nist.gov/div898/handbook/pmc/section4/pmc443.htm.
This link is also useful, as it gives examples of what additive or multiplicative seasonality would look like if you examined a time series plot of your data: anomaly.io/seasonal-trend-decomposition-in-r. See the plots in Step 6 of the link for a nice illustration of the two types of seasonality.
If you detect seasonality in the side-by-side boxplots or in the time series plot of your data, then the acf-pacf plot will reflect that seasonality as well.
If you don't detect seasonality in the side-by-side boxplots or in the time series plot of your data, then you won't detect seasonality in the acf-pacf plot either.
You can't say seasonality is present just because you have monthly data - you need to have clear evidece from your plots that seasonality is present before you can make your claim. It is possible for monthly data to not exhibit any seasonality. It is also possible for monthly data to exhibit seasonality. Examining all plots carefully will reveal which situation you are dealing with.
Answer: I think you can go as far as saying that you didn't find evidence of seasonality after inspecting the ACF and PACF plots. Making the claim that seasonality does not exist is going another step beyond that.
You can't claim that seasonality exists just because you have monthly data. You can hypothesize that there is seasonality in the monthly data and have good reason for doing so, e.g. if you are dealing with ice cream sales. However, there are cases where this type of assumption wouldn't be reasonable, e.g. if you are dealing with monthly exchange rates.
Answer: Simple time series identification tools that assume that seasonality is auto-projective are often simply overmatched by the data. Check the assumptions underlying all tools.
Many time series are affected by monthly determinstic structure e.g. a Dec effect or a June effect .
In both cases the original acf/pacf seasonality suggestion/hint may be obfuscated by unusual values (pulses) or level/step shifts or local time trends or changes in model error variance or parameters over time.
Build your model iteratively following this paradigm https://autobox.com/pdfs/ARIMA%20FLOW%20CHART.pdf ultimately yielding a white-noise error process.


Why does Comparator declare equals?

Question: The Comparator interface has its own equals() method. Any class will get equals() by default through Object class. What is the need to have equals() method inside an interface?
Comment: Sometimes the explanation is quite simple - equals method in Comparator wasn't well thought out. There's no deeper reason to it. Any reasoning can put into question why thousands of other classes don't "redefine" the equals contract.
Answer: Comparator refines the contract of Object.equals: It has to satisfy the constraints set out by Object.equals and then some.

Additionally, this method can return true only if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus, comp1.equals(comp2) implies that sgn(comp1.compare(o1, o2))==sgn(comp2.compare(o1, o2)) for every object reference o1 and o2.

Declaring an equals inside Comparator allows you to document this in the form of javadoc.
Note that the documentation of the API also serves as the contract, so it's not just cosmetics here. It's explicit constraints that other code and your code can rely on.
In similar situations where you have less established methods, it may also serve as documenting an intent. I.e., Interface.method should be there, regardless of how its super interfaces evolves.
Comment: I disagree. The point of the method is to refine the contract of the equals method for Comparators: when can Comparator be considered equal and when can't they. You might think that no comparator can be equal to any other, but it's not the case. You might think that two comparators are equal if they return the same thing for any arguments, but it's not the case. The javadoc explicitely says that you don't need to override equals, because `Object.equals()` satisfies the contract.
Comment: I prefer your answer now, but I still don't see the point of the paragraph about the "decoupling" from the Object class. Comparator *doesn't* need this method.
Comment: That is a valid remark. In the end though, what is the contract of an API method? Its documentation. So in the end, that is roughly what I'm saying, albeit not as clear as you have expressed it ;-) Reformulating slightly.
Comment: @JBNizet, right, that's why I wrote "in similar situations". Still though, refining a contract, (or redefining it completely) in a sense "decouples" one interface from another. I agree that the paragraph is a bit misleading. :-/ You're comments and answers are always very insightful so (whenever you feel like doing so) feel free to edit my posts :-)
Answer: From the Java documentations, the reason why Comparator has it's own equals() method:
However, overriding this method may, in some cases, improve performance by allowing programs to determine that two distinct comparators impose the same order.
Comment: Yes, but it is still redundant to mention the method in the interface.
Comment: @aioobe well, your answer clarifies all the other scenarios. I just wanted to tell the OP that the reason it is there in the interface is to hint the programmer towards performance improvement by overriding the said method.
Answer: Read its javadoc. It's there only to explain what equals() must return if you choose to override it in a class implementing Comparator. You might think that no comparator can be equal to any other, but it's not the case. You might think that two comparators are equal if they return the same thing for any arguments, but it's not the case. The javadoc explains that two comparators are equal if they impose the same ordering, whatever the arguments given. The javadoc also says:

Note that it is always safe not to override Object.equals(Object)

Most of the time, you don't override equals() in comparators.
Answer: from the docs:
it is always safe not to override Object.equals(Object). However, overriding this method may, in some cases, improve performance by allowing programs to determine that two distinct comparators impose the same order.
Comment: Right, but I think the question was why this method is mentioned in the interface, as it is semantically speaking redundant.
Answer: Technically, the declaration of the method is redundant (the compiler does not care), but...
Declaring the equals method in this interface makes it part of the contract between caller and different Comparators and allows it to specify/extend its semantics.
It specifies that two Comparators are equal only if they impose the same ordering with their compare() method. This extends the semantics of Object.equals() and must therefore be documented in the interface.
Answer: Putting an Object method in an interface declaration allows Javadoc
declaration of the meaning equals is required to have in classes that
implement the interface.
Comment: As we have default implementation in Object, the compiler does not force programmers to implement equals even if it's presented in interface. But compare() is not defined in object, that's why programmers have to implement it.
Answer: Comparator interface have their own equals() method

Well,. first of all, it should be clear that whenever you implement  Comparable interface you should provide your program to decide when objects are equal, less or greater.
I am quite confuse about have equals() inside Comparator. Any class will get equals() by default through Object class.

The equals() method implementation u inherit from Object class only checks whether the two referances point to the same object or not. It dosn't apply any comparison. It's you who will provide in your class (or possibly in your Interface) the criteria for objects to be equal.
Then what is need to have equals() method inside an interface?

Obviously , whenever you implement when objects are less , greater than you must implement when they are equal. so rather than relying on default Object equals() method you should provide your logic to check equality
Answer: The equals() method in Comparator is provided to force a user implementing the Comparator interface to implement equals() with some additional rules and constraints in addition to the ones already applied on equals() from Object.
The additional rule being:

This method must obey the general contract of Object.equals(Object).
  Additionally, this method can return true only if the specified object
  is also a comparator and it imposes the same ordering as this
  comparator. Thus, comp1.equals(comp2) implies that
  sgn(comp1.compare(o1, o2))==sgn(comp2.compare(o1, o2)) for every
  object reference o1 and o2.

Why does Comparator declare equals?


File count limit for NFS Mounted Drive

Question: Is there a known upper limit on the number of files one can place in a folder on an NFS mounted drive on 64bit Linux systems? I have come across one excellent source that discusses related issues but there appears to be nothing specific to NFS.
Answer: I don't see how NFS is relevant. The issue here will be the actual file system of the exported partition. The fact that you are mounting it locally via NFS shouldn't make any difference at all, it is the underlying filesystem that needs to deal with the files so you will have the same limits as those mentioned in the SO answer you linked to.
So, as far as I know, there is no limit to the number of files an NFS partition can hold. There are limits for the actual file system, but not for the sharing protocol you use.
Answer: The limit would probably be on the remote filesystem not the NFS protocol,
I found these info here:
https://stackoverflow.com/questions/7722130/what-is-the-max-number-of-files-that-can-be-kept-in-a-single-folder-on-win7-mac

FAT32 (still used a lot, especially on external media): 65,534
NTFS:   4,294,967,295
ext2/ext3 (Linux): Depends on configuration at format time, up to 4,294,967,295 HFS+ (Mac): "up to 2.1 billion"
Answer: Big number of small files is usually not a problem for NFS. But it can be a problem for file systems optimized for huge files, for examples Hadoop file system: HDFS.

File count limit for NFS Mounted Drive


How do I submit hashes to a stratum URL?

Question: I have block hashes mined from bitcoind. I want to submit it to a pool. How do I do that?
Answer: If you are mining with bitcoind then you are your own pool. If you find a valid block then you have to submit it to bitcoind. You got the block template with bitcoind's "getblocktemplate" and now you submit the complete block with bitcoind's "submitblock".
If you want to mine in someone else's pool then you use the stratum protocol to get the work and then you submit the work results back with the stratum protocol. You don't need bitcoind when you are mining in someone else's pool.
You can't mix the two. You can't work in your own pool and send the block to some other pools to get paid there. They will not pay you for working on your own blocks in your own pool.


Dijkstra algorithm step in Introduction to Algorithms

Question: In the introduction to algorithms proof of Dijkstra, I don't understand why the statement "both y and u were in V-S when u was chosen". We add x before y, and so we relax d[y] with the the edge $$\omega(<x,y>) + \delta(s,x)$$, d[u] can't be smaller than delta[u], so shouldn't y be in S when we add x to S? thanks.
Answer: $u$ is supposed to be the first node to be insert in $V$ such that $d(s, u) \neq \delta(s, u)$. Before the insertion, $u$ is in $S-V$, it's clear. In addition, $y$ is supposed to be the first node in a choosen optimal path $s -> u$ to belong to $S - V$ when picking $u$ from $S - V$. So before we insert $u$ in $V$, $u$ and $y$ are both in $S - V$. Note that $y$ can be equal to $u$.
Comment: Ah, the first one, because we add u before y, the second one, because we already added x and relaxed , and this is corollary from a previous lemma because x is on the shortest path to y, and  completes it. Thanks a lot!
Comment: Ah, but I don't understand why such a y should exist based on our false assumption. That is, it seems that you can lead to the same contradiction without the false assumption, proving that you'll add y before x to S.
Comment: Based on our assumption, $y$ must exist, because in the worst case, it's just $u$. If you try to find the first node that belongs to $S - V$, and you don't find any, then you can pick $u$, because $u$ don't belong to $V$ before being inserted. For the contradiction that you give, you still need the false assumption, because $x$ and $y$ have been defined thanks to this assumption.
Comment: So I gave it another thought. What we are saying is that if $d[u] = \delta(s,u)$ when we add it, then $ \delta(s,u) = \delta(s,y)$ which is fine, but otherwise $\delta(s, u) < d[u] \leq d[y] = \delta(s,y)$ which is a contradiction because y is before  x on the shortest path. Am I correct?
Comment: You don't have to consider $d[u] = \delta(s, u)$,  because we supposed in the beginning that $d[u] \neq \delta(s, u)$. For the rest you are correct, but not for the reason you give. $y$ is before $u$ in the shortest path, and weights are positive, so $\delta(s, u) >= \delta(s, y)$. In addition, $\delta(s, u) <= d[u] <= d[y] = \delta(s, y)$, so by merging : $d[u] = d[y] = \delta(s, y)$ so $d[u] = \delta(s, u)$ which is a contradiction with the assumption we made at the beginning. But, how do we know that $d[u] <= d[y]$ and $\delta(s, y) = d[y]$ ?


Non-WIMP vs ubiquitous computing

Question: So, what would you call as a main difference between non-WIMP and
ubiquitous computing? I know that non-WIMP is seen as interface seen
after WIMP, kinda as a revolution of user interface, and that main
idea of ubiquitous computing is to get to something Mark Weiser called
"calm technology". Can we say that non-WIMP end goal is to get to be
called ubiquitous?
Answer: Well, non-WIMP really has no particular meaning. If you call non-WIMP anything that is different to WIMP, then the scenario is incredibly broad. As I told you in another answer, WIMP is a very old term and it fell into disuse decades ago. Maybe that's where the confusion lies, because WIMP is what now we call GUI (thus, non-GUI could be a text interface, a non digital interface or whatever).
Going into ubiquitous computing or ubicom, it has absolutely nothing to do with GUI, WIMP or any kind of particular interface representation. Instead, it's a model or concept.
Of course, on user's end, ubicom uses GUI most of the times, but it doesn't mean that it's necessary, or that this user (or actor, if you want to use UML models) is the only user.
An easy example: let's say you have an ubicom app that uses Google maps (an ubicom app by itself). The system will show you a GUI (or WIMP as you call it). But the backend might be used by a Sys-Admin using a console. One of them will use a GUI, one of them will use a TUI (Text-based user interface), the ubiquitous computing model is the same for both users
As for the calm technology part, it's correct that it's related to ubicom. However, while calm technology needs ubicom, ubicom does't necessarily require calm technology
Comment: interesting answer! Do you know any good sources on this conversation (books, etc...)
Answer: The problem with "non-WIMP" as term for user interfaces is best explained by analogy. Nobody talks about "non-elephant animals". That is simply a class which is philosophically well defined, but lacks biological meaning. Similarly, it's reasonably simple to say whether a particular User Interface is WIMP or non-WIMP, but it's not very helpful.
Hence, all your attempts to ascribe attributes to "non-WIMP" fail as the category is far too broad.

Non-WIMP vs ubiquitous computing


Is there any way to recover the deleted profile in sandbox

Question: I want to know is there any way to recover the deleted profile in sandbox.
Thanks,
Suri.
Answer: Unlike records, profiles are part of the metadata, therefore they are not stored in the Recycle Bin.You may be able to recover via customer support.  I'm not certain of that.
Tip:  Every so often, use the Metadata API to download your org's source and related metadata.  Reference and tutorial links:
http://www.salesforce.com/us/developer/docs/api_meta/index.htm
Answer: Unfortunitely, you cannot recover deleted profile as its not moved to recycle bin.
Nearest I can think of, you can see Audit trail to see which profiles were deleted in last 180 days and recreate it again.
Answer: Another way is that if you have deployed change sets that haven´t expired with the metadata that you want, you can redeploy the change set. This will restore the metadata
Answer: If the profile exists in Production, a refresh will restore it.
Other than that, there is no way to do this - you could raise a case with support to see if they can do anything for you.


Storing data from cloud page to salesforce using Rest API

Question: We want to fetch the data from the cloud page and instead of storing that in a data extension we want to store that in salesforce but we don't want to use marketing cloud connect.
Is it possible to do using Rest API and how?
Comment: Do you like to write the data from the CloudPage code to Salesforce CRM or do you like to fetch it from Salesforce CRM code from a Data Extension?
Comment: Hi Markus, I want to write the data from the CloudPage code to Salesforce CRM
Answer: In order to interact with / write to Salesforce CRM from CloudPages without using Marketing Cloud Connect, you can access the Salesforce CRM REST-API.
The easiest way to do so is by using the Marketing Cloud Language "Server-Side JavaScript", as you can make use of javascript objects to natively interact using JSON-format.
However, the default functions are limited to GET and POST calls that only let you create a record or retrieve data from it.
In order to be able to update, delete, etc. you need to use Script.Util.HttpRequest and create your own requests, that suit your needs:
Approximate example code for an update:

Related documentation:

Salesforce CRM REST-API Documentation
REST-API - Working with Records
Server-Side JavaScript Syntax Guide
SSJS Post function
SSJS Script.Util.HttpRequest documentation
Answer: You can create Schedulable apex class which runs every 1 hour(you can change interval as per your requirment). Create a Batch apex class and call from Schedulable class where it will make callout and you will get the response. Process the response you got from callout and insert the records using DML statement in batch. Keep the batch size as 100 as you know we can make only 100 callout per transaction.


If I have a public wallet address can I calculate its other wallet addresses?

Question: Lets say someone else generates 100 bitcoin addresses using the same private key.  If know one of the public addresses, can I deduce the other 99 public addresses?
Comment: No I don't think you can get other addresses with one address and it's public key. You will need XPUB.
Answer: Public keys are generated from a Private key in a one-way function. Public and private keys have a 1-to-1 relationship. A given private key can only generate one public key.
However, as specified in BIP 32, extended keys can be used to derive child keys (public keys). An extended private key (xprv) can derive many child private keys, each of which can derive a single public key.
This child key derivation (CKD) function is a one-way function, so child keys cannot be used to calculate their parents. Nor can child keys be used to calculate their sibling public keys.
Check out this article on wallets and public/private keys for more info and a visual explanation: https://river.com/learn/terms/h/hd-wallet/
Check out BIP 32, which has some useful graphics: https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki
Answer: Relationship between private and publiic keys is ( in practicality ) unidirectional and 1:1 so given an algorithm; a private key will always, generate the same public key.
There's an advanced concept, that has to do with your question and is deterministic wallets. Here private keys are created from a seed and there are scenarios where you could guess them from knowing one; but you would need to know the private key, not the public.


Chrome NET::ERR_CERT_REVOKED for Trusted Self Signed Root CA

Question: I'm having an issue where Chrome raises NET::ERR_CERT_REVOKED for some sites signed by our internal self signed ca but not others. Can't figure out what's different. I am on MacOS Catalina so it sounds like superuser.com/questions/1492207/neterr-cert-revoked-in-chrome-chromium-introduced-with-macos-catalina but in my case my system is set to trust our internal ca and it works on several of our other internal sites. Obviously the internal cert that no one knows about can't possible be in a CRL. Firefox works fine and Safari complains because our Root CA is signed for 10 years. If that was the issue though Chrome shouldn't work on any of the internal sites.
Here's a screenshot
Answer: Check these requirements:.
https://support.apple.com/en-us/HT210176
Maybe one or to many of them are not matched?
You could also post Your cert chain to allow others to check it.
Comment: And that's what it was. Apple changed the max time to 825 days and these certificates were all new. Chrome really should give a better error message though. Like hey this certificates are valid for too long. Safari tells you they don't match requirements anymore.
Comment: Yes probably Apple introduced new return code for such cases (as Safari can tell a quite exact reason). Firefox ignores this (for now) and for Chrome a fall-back verification-handler kicks in that just forwards the "fatal" code to to user and behaves as such.


Is there a particular UX benefit of the Android Marshmallow boot screen preloader?

Question:
Was pretty amused with the boot screen for Android Marshmallow, using 4 separate animations as preloaders. Would you consider this an overkill or is it better than having a single loading animation?
Comment: Compared to a stale loading animation,this requires some attention and looks pretty interesting in general. Its also more satisfying to watch and therfor takes less perceived time
Comment: @downrep_nation initially that may be the case. but after seeing it for a while the wow factor will wear off. at this point compared to a subtle animation that had a less wow factor, this could potentially feel annoying or amatur unless it has no functional purpose
Answer: This type of loader may not be overkill, in it's context. If this was a loader for something within an application for completing a simple task, then I'd say there is definitely too much going on, but this is for the boot screen, and it may work in this case.
I think one of the main reasons this works well is that it changes the user's perception of time by distracting them. When you try booting up some of your most used devices, you may say that it takes a long time to do so, but in reality, it's not that long. This is because our perception of a long time in that situation is actually only a few seconds to 1-2 minutes, mainly trained from today's age of instant gratification. When boot screens, and other applications/software, use simple loading screens with not much activity going on, the user can get easily bored and also frustrated when it does not load within a few seconds.
An excerpt from an article on Usabilla:

Unoccupied time feels longer than occupied time
First of all, make sure your visitors don't get bored while waiting. If we have to wait for something in the offline world, there are (1) a lot of things to distract us and (2) we usually wait long enough to have time for distraction.

Taking that into account, here is a few benefits I see in Android's loader...
It avoids bordem and frustration by giving the user something to look at
By using a fancy looking loading screen it definitely changes moods, and get's the user looking. Bright colors, happy things, and fast moving parts — perceived as "Hey, there is a lot fast things going on, check out all these nifty animations," changing their alternative mindset of "When is this going to finish?"
There is a lot going on in the loaders, and it may be initially confusing — but maybe that's the point? You need to engage the user in these loaders to achieve the change of their perception of time, so making a lot of crazy looking loaders may be the attraction, and poses a challenge to the user to figure out what all of them are doing, occupying their time while they wait, going back to the previous point made about unoccupied time.
By the time the user is finished evaluating all the flashy loaders, the boot screen is finished and they are thinking, "Oh, it's done already!"
It gets the user excited by giving them a preview of what to expect
Going back to the description of the loaders, they are fast, and do cool things, these may not be full on features to what the operating system can offer, but it's easy enough to understand that they can expect those same qualities in the tasks they do in the device, which can get new users excited to jump in and start messing around with stuff.

With that however, there is certainly concern that these benefits are great for the initial user, but won't have much affect to a person that has seen it a dozen times over, but I'll consider this; the boot screen is usually a process that isn't done every day, only every so often the user needs to be faced with that screen, so in this case, unless they are restarting their phones everyday, the screen won't seem so familiar when they next see it, and even so, it still provides a distraction method, in hopes of still achieving the change in our perception of time.
That is my wrap up, hope it covered most of what you were looking for.
Answer: I liked it more than

or older Android default preloader theme because

Window 7 theme is pretty boring after a few views since there is only one animation and it does the same thing every time.
This Android theme is more engaging since I was naturally hooked to observe what it was trying to do, more because older Android themes were pretty much same (brand name will brighten up and start fading)

Also, I found it confusing because

Even after a few watches I couldn't figure out what it was trying to do. I mean that are these 4 animation signify something or are these synchronized in anyway?

Having said this, I don't own an Android Marshmallow phone so cannot comment on whether (or how) it has any impact on booting performance. For all I know, this theme might be put in place to set the tone for rest of Android Marshmallow experience since we all know how much Google has been pushing for Material design and has been making subtle changes in their apps since last couple of years.
Answer: The only risk of using this more complicated animation is that users might think it is redundant.
"Is the OS actually still loading or is this just a fancy, gratuitously long launch screen?"
I feel a simpler animation would eradicate any doubt about that, particularly if it's consistent with the other preloaders Google's using now.
Answer: To me it is an overkill, at least in terms of the cognitive load. The app hasn't started yet, and the user was already loaded with four pretty complex different processes merging into yet another one.
It is quite pleasant to look at for entertainment, on purpose, but not when you are waiting through something fairly ancillary to get to something important.
I personally would trade it for 3-sec (or was it 5?) fade-in-fade-out Windows animation any minute.
Answer: I would say the fact that you got amused was exactly what Google were aiming for. Instead of deep sighs waiting for your phone to load this distracted you as the phone booted up, which therefore created a positive experience! :)


Routing an owned /24 IPv4 block to a single server

Question: How would one go about routing a recently-purchased /24 block of IPv4's to a single server? The server is colocated in Zare.com, and the BGP session is already set up and announced.
Comment: Welcome! You will need to provide more information for people to be able to help you with this. For example: *how* is this BGP session configured? On what device, and how is it connected to the server?
Answer:
"the BGP session is already up"

Between which peers? We might need some more information here.
On the other hand, it will all depend on how the network admins built their network. You'll have to ask them, I'm afraid. I can think of these cases, and I'm certain that there's plenty more ways to do this:
Same Subnet: --> ARP Alias
If that server and its default gateway are in that very same /24, you probably won't have to actually route.  In that case, you may add additional IP addresses (sometimes called "secondary IPs", "ARP alias" or "IP alias") to the NIC that server already has in that /24 subnet. The upstream router will ARP for these addresses, and the server hear and ARP-reply to them.
Multinet: --> ARP Alias
If the server is on a different IP subnet than the /24 AND if the network admins have implemented "ip address secondary" for that broadcast domain/VLAN (i.e. running multiple IP subnets on the same broadcast domain, sometimes called a "multinet"), you may do as above: add ARP aliases on the given NIC of the server.
different Subnet: --> Routing
If the server (resp. the given NIC) is on a different subnet than that /24 AND if there is no multinetting, some routing will have to be involved, and you'll have to talk to the network administrators.

ask the network admins to add a static route for the /24, with your server's IP as the next hop, and - if needed - to take appropriate action on their further upstream routers, such as redistributing/injecting that static route into whatever (internal) routing protocol they may be using.
implement a dynamic routing protocol on your server (probably External BGP), and have it "talk" to the upstream router(s), announcing the /24 as being reachable via your server. That requires even closer cooperation with the network admins.

In either variety of routing, the IPs themselves will also have to be implemented on the server. That might be as additional addresses on the NIC or some subinterface thereof, on some kind of internal loopback device; that all depends on your servers' capabilities, and would be beyond the scope of this board.


Who has asked the most questions?

Question: Just curious as to who has asked the most questions. Lots of people answer questions, but who has asked the most? How do you find out who has asked the most questions?
Comment: @Valorum - if it's not you, i'll eat my mouse.
Comment: I was like, "Aww DVK has a pet mouse!" Then I was like "Oh."
Comment: I wonder who's *answered* the most.
Comment: @Valorum good question.
Answer: Taken from the main meta

You can use the Stack Exchange Data Explorer (SEDE) to query sites for information like this. It does require knowledge of SQL to write queries (there is a tutorial under the SEDE help pages if you're interested), but there are a lot of existing queries you can use and you can search SEDE as you can any other Stack Exchange site.
An existing query that should give you what you're looking for:

Who Asked the Most questions

Here it appears to be @DVK-in-exile, with 664 questions.
Comment: Awesome, thanks for that/
Comment: Isn't it technically Satellite of Sin [now]?

Who has asked the most questions?


Does upgrading Yosemite risk losing material on HD?

Question: What are the risks of losing what's on my hard disc--especially programs--when I update Yosemite?
Comment: It is always advisable to do a backup before upgrading the OS, even if the procedure *should* (and usually does) leave user data and installed applications untouched. If you don't have a spare external drive, running at least your usual backup just before the upgrade should have you covered.
Comment: TM backups applications stored in /Applications as well.
Comment: The problem is that Time Machine won't protect apps like Adobe Design and Web Premium CS6. Will I need something like Carbon Copy Clone to save that sort of app?
Answer: Low as this does get tested before.
What is not tested as much is does each application work with the new OS. Apple can only test major apps it is the responsibility of the App's developer to test their app with the new OS and not all have the time to do this.
But as always with any computer change take backups as there is always a chance of something going wrong.
I assume that changing something will break something else. When it does I have to work out if it is worth living with that problem or revert to the old version.
Another example is professionally in enterprise software when I install a new version of my program I have to provide a tested process that will reverse the change as stopping something working can cost money
Comment: Like Mark said upgrading the O/S should not effect (or delete) any apps. And as long as they are compatible with the new O/S, it should work fine. As always, though, make a backup first just in case.
Comment: Amen to testing, backups and expecting/anticipating that things might not go swimmingly so that if they do, your mindset is already established.
Answer: The risks to a system being able to boot again are highest when you make fundamental changes to the OS as part of an upgrade.
This is due to three factors:

some parts of the code base are only called when swapping in and out a new system so those code paths don't run normally. Any degradation of the files (bit rot) will be discovered, but not necessarily caused at upgrade time.
any change has a very small chance of failing - more change, more chance
you run new migration steps from the vendor and they could run poorly or not anticipate the changes you have made to your system and cause data loss. This is mitigated by storage idioms like putting all user settings and app data in folders where the OS doesn't install any files. (i.e. the system is designed to not break thusly)

The risk to your data files is no higher during an upgrade than otherwise. If you don't care to back up the files anyhow, then you're always one disk error or drive failure away from losing data. If you have a good backup, you mitigate all the risk of upgrading other than the time you spend wiping and reinstalling the old system / old backup.


How do I create an asynchronous wrapper for log4net?

Question: By default, log4net is a synchronous logging mechanism, and I was wondering if there was a way to have asynchronous logging with log4net?
Answer: If you go to the log4net website, you can find some examples, at least one of which is an asynchronous Appender.
http://logging.apache.org/log4net/release/example-apps.html
Note that I have not used any of these examples, so I cannot vouch for them one way or the other.
Here is a link to the actual asynchronous appender from the log4net Examples area in their code repository:
http://svn.apache.org/viewvc/logging/log4net/trunk/examples/net/2.0/Appenders/SampleAppendersApp/cs/src/Appender/AsyncAppender.cs?view=markup
I looked at it briefly, and it apparently acts as a wrapper around one or more "conventional" Appenders.  On each logging request (containing one or more LoggingEvent objects), a ThreadPool thread is used to forward the LoggingEvents to the list of wrapped Appenders.
Comment: In the end, this is pretty much what I ended up doing. I switched it so my logging is on it's own thread, rather than running on the thread pool.
Answer: Just wanted to provide my complete solution for reference. Couple of important items, the FixFlags let you capture the thread that's actually doing the logging. The Blocking Collection is in the ReactiveExtensions. The jist here is that your forwarding appender handles the Asynchronous stuff and then just forwards on the LoggingEvent to a standard Log4Net appender, which lets Log4Net do all of the things that it's good at. No re-inventing the wheel.
///
/// Provides an extension for the log4net libraries to provide ansynchronous logging capabilities to the log4net architecture
///
public class AsyncLogFileAppender : log4net.Appender.ForwardingAppender
{
    private static int _asyncLogFileAppenderCount = 0;
    private readonly Thread _loggingThread;
    private readonly BlockingCollection<log4net.Core.LoggingEvent> _logEvents = new BlockingCollection<log4net.Core.LoggingEvent>();

    protected override void Append(log4net.Core.LoggingEvent loggingEvent)
    {
        loggingEvent.Fix = FixFlags.ThreadName;
        _logEvents.Add(loggingEvent);
    }

    public AsyncLogFileAppender()
    {

        _loggingThread = new Thread(LogThreadMethod) { IsBackground = true, Name = "AsyncLogFileAppender-" + Interlocked.Increment(ref _asyncLogFileAppenderCount), };
        _loggingThread.Start();
    }

    private void LogThreadMethod()
    {
        while (true)
        {
            LoggingEvent le = _logEvents.Take();
            foreach (var appender in Appenders)
            {
                appender.DoAppend(le);
            }
        }
    }
}

Then, in your log4net.xml you setup the appenders thusly

<appender name="StandardAppender" type="TSUIC.Logging.AsyncLogFileAppender">
<appender-ref ref="StandardAppenderSync" />
</appender>

<appender name="StandardAppenderSync" type="log4net.Appender.RollingFileAppender">

    <file value="log\Log_" />
    <appendToFile value="true" />
    <rollingStyle value="Date" />
    <maxSizeRollBackups value="-1" />
    <maximumFileSize value="5GB" />
    <lockingModel type="log4net.Appender.FileAppender+MinimalLock" />
    <staticLogFileName value="false" />
    <datePattern value="yyyyMMdd'.txt'" />
    <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%date [%thread] %-5level %logger - %message%newline" />
    </layout>
</appender>

Update:
If you want to use context in log4net like "log4net.ThreadContext.Properties["CustomColumn"]"
Then you need to update above code like
loggingEvent.Fix = FixFlags.All;

Comment: Instead of running the thread in the background, you could register a callback with the repository ShutdownEvent.  Then in the event you would simply call

    BlockingCollection.CompleteAdding().

To consume in this manner, don't do a while(true), instead use

    foreach(var le=_LogEvents.GetConsumingEnumerable())

Then it automatically exits the tread when the collection has been emptied.
Comment: How does this work in a production environment? Is there any noticable performance improvement? Obviously the improvement would depend on the number logging statements. I'm wondering if there is a reason to do this other than benchmarks in a dev or test environment.. thanks in advance
Comment: How does that handle LoggingEvents that are queued up when an application exits?
Comment: I am adding some other fields to log in to the log file by using log4net.ThreadContext.Properties["CustomColumn"] = "value"; but this is being printed as null if i use the above code
Comment: The code above does not work. If you try to debug it s not getting inside the for loop.
Answer: This is how I do it:
Task.Factory.StartNew(() => log.Info("My Info"));

That way log4net performs logging on a separate thread, asynchronously...
BTW, Task class is in System.Threading.Tasks namespace.
Comment: This is *dangerous* especially in ASP.net applications http://blog.stephencleary.com/2013/08/startnew-is-dangerous.html
Comment: This is a very very bad idea, not only you need to `Fix` the thread name, message and the exception but also the log entries will be persisted out of order.
Comment: @Dean Unfortunately that solution also is not correct resulting in out of order entries again due to the usage of `ThreadPool`. the throughput is also terrible.
Comment: This is much less efficient than using an AsyncAppender.
Comment: It will also totally throw off the thread id if you are logging that.
Comment: @MaYaN What is your recommendation in that case?
Comment: @Dean, I have added an answer.
Comment: For tasks like this, Task.Run should be used, not Task.Factory.StartNew.
Answer: Some of the ideas here are incorrect and result in invalid/stale data, out of order logging or very bad performance. For instance the accepted answer suggests using the log4net AsyncAppender which uses the ThreadPool resulting in out of order entries which might not be a problem for some but I certainly want my log events to be one after another it also can have terrible performance and put too much strain on the ThreadPool also it does not batch the log entries. The answer suggested by Jonathan is certainly a much better solution but still lacks optimal performance.
A good example of how this should be implemented can be found HERE and the benchmarking results and the explanation HERE.
Another good feature of this solution is that it has been implemented as a Forwarder not an Appender allowing the user to include more than one Appender and log to each of them at the same time.
Answer: I came across this problem this week however I did not want to keep firing requests off to the thread pool because it could end up starving the rest of the application of threads so I came up with an Asyncronous appender that runs a dedicated thread for appending which is fed through a buffer. Check it out here: https://github.com/cjbhaines/Log4Net.Async
Comment: Hi afif. This appender is used by every single product in my company. The application I work on has a massive amount of throughput running anywhere between 300-1500 worker threads. N.b, I am taking no responsibility for the code. If you want to test its safety you can write some extra unit tests with an invalid connection string, bad config, etc so it throws exceptions on the background task. The background task has a continuation to log any errors which will stop any unobserved exceptions throwing in the finalizer when the thread is cleaned up.
Comment: @Chris, I just ran the benchmarks for this appender, both the `Log4Net.Async.AsyncForwardingAppender` and `Log4Net.Async.ParallelForwardingAppender` and they have terrible performance, please look at the my answer below. You can run the benchmarks yourself just add your own NuGet package to the project.
Comment: Thanks Chris, this is exactly what I was looking for. Can you please indicate if this is production ready code? Have you used it in production under sufficient load to know its battle hardened?
Answer: https://github.com/cjbhaines/Log4Net.Async
We have asynchronous log4net methods available now. For the people who are looking for updated answers.
https://www.nuget.org/packages/Log4Net.Async/


How would you represent the $T$ gate in terms of rotations around the $X$ and $Y$ axes?

Question: I know that the $T$ gate is equivalent to a $\frac{\pi}{4}$ rotation around the $Z$-axis, but what about $X$ and $Y$?
Comment: @FranklinPezzutiDyer the determinant is not everything. The restriction is stricter than that. Even if you want to get $T$ up to a global phase it is still not possible from $X$ and $Y$ only (they generate the Pauli group which does not include anything close to $T$).
Comment: Not entirely sure if this is what you're asking, but you can't write the $T$ gate only as a product of $X$ and $Y$ gates. For a simple proof, notice that $\det(X)=\det(Y) = -1$, but $\det(T) = e^{\pi i/4}$. The determinant is multiplicative, but we cannot get $e^{\pi i/4}$ by multiplying copies of $(-1)$.
Answer: A $T$ gate is a rotation of $\pi/4$ around the $Z$-axis of the Bloch sphere. Meaning that $T$ is equivalent to $R_Z(\pi/4)$ up to a global phase (where $R_Z(\theta)=\exp(-i\theta Z/2 )$ is the rotation operator around the $Z$ axis).
You cannot get a $T$ gate from $X$ and $Y$ only. Multiplications of $X,Y,Z$ form a closed group (Pauli group), that does not include $T$.
Nevertheless you can write a rotation in $Z$-axis as multiples of rotations of around the axes $X$ and $Y$ axes: $$R_X(\pi/2)R_Y(\pi/4)R_X(-\pi/2)=R_Z(\pi/4).$$
Answer: As already mentioned by Mauricio in his answer, you can express the $T$ gate as a combination of rotations around the $X$ and the $Y$ axes (up to a global phase).
In particular, this is how you can see this transformation by using the Qiskit transpiler:
<code>from qiskit import QuantumCircuit, transpile

qc = QuantumCircuit(1)
qc.t(0)
qc = transpile(qc, basis_gates=['rx', 'ry'])

qc.draw('mpl')
</code>


IE7 on Win XP does not show download file dialog for PDF file

Question: I'm testing the following JavaScript in our Umbraco site:

I've removed some stuff to leave the core function code. The actual web service call is working fine for both of the scenarios that I discuss below, it's opening the documentUrl that is causing an issue.
Now when I test the web page in a VM running WinXP and IE7 I encounter a problem. Instead of the script opening up a new window and showing me a PDF (or at least giving me the option to save it) all I see a quick flash of a new IE instance opening and then closing.
When I test the same page in IE9, on my Win7 laptop, and change via Web Dev Toolbar to render in IE7 mode, I can see the PDF file displayed (since I have Foxit Reader installed).
My question is this: Why in WinXP does the window.open() result in a quick show of a new IE window which then just closes?
I looked at Fiddler on both systems and found a difference. Under the VM, when requesting the PDF, the HTTP status is 206, which is partial content. Under IE9 the request is a HTTP 200, not a 206.
Can someone explain what might be going on here and how I can fix the WinXP IE7 instance?
I've been looking at the pop-up blocker settings in WinXP/IE7 and have told it to allow pop-ups for the domain I'm working with. I also disabled the blocker and I still get the same problem anyway.
Answer: I had not included async : false in the AJAX config. After doing that, IE was happy and now displays the download file dialog.


Euler-Lagrange expression uniformly non-negative

Question: If you form the Euler-Lagrange equation for some calculus of variations problem in x, f(x) and f'(x), and the resulting expression is always non-negative over the domain of x (because the expression is independent of f(x) and f'(x)), does that mean any perturbation that increases f(x) increases the functional? This seems natural given it should be some generalisation of
what one gets when the first derivative of a function is always non-negative...
Answer: Given a functional $L(x,f(x),{f(x)}')$, the Euler-Lagrange equation provides the solutions which make $L$ stationary (not extremum). It is given by:
$\frac{\partial L(x,f(x),{f(x)}')}{\partial f(x)}-\frac{\mathrm{d}}{\mathrm{d} t}\frac{\partial L(x,f(x),{f(x)}')}{\partial {f(x)}'}=0$
The resulting expression is thus always zero. If $L$ is independent of $f(x), {f(x)}'$, then the derivatives in the equation vanish and you have the identity $0=0$. Maybe you should clarify a bit more your question.


Tips on basic electricity and magnetism books

Question: I would like to study basic electricity and magnetism with a book with a good mathematical language (e.g. to calculate the thing with gauss law the book explain the importance of parametrization, coordinate change inside integral and so on...)
Answer: Try 'Introduction to Electrodynamics' by David Griffiths. The first chapter gives an intuitive explanation of the mathematical concepts used. Furthermore, there are lots of examples, and many good exercises.


Find the fixed points of a function $f(x) := exp(x - 2)$ using a recursive algorithm

Question: I need to find the fixed points (i.e. when $f(x) = x$) of the following function  $f(x) := exp(x - 2)$. I understood that the fixed points should be the intersecation points between $f(x)$ and a function $g(x) = x$, which has infinitely many fixed points (if I am not wrong).
So, to find the intersecation points I decided to do: $$f(x) = g(x) \\ f(x) - g(x) = 0 \\ f(x) - x = 0 \\ exp(x - 2) - x = 0$$.
Plotting $f(x)$ and $g(x)$ in the same graph (in a range $-5$ to $5$), it seems that $f(x)$ intersects twice $g(x)$, so it might have 2 fixed points:

I have also discover that the fixed points of $f(x)$ are the roots of the following function: $$h := f(x) - x$$
So, I can simply find the roots for the function $h$, and I am done.
Apparently the following algorithm (which I don't know how is it called. Can you tell me?), should find the roots of a function:

Let $f: [a, b] \rightarrow \mathbb{R}$ be continuous function with $f(a) < 0 < f(b)$. Then we can find a $c$ with $f(c) = 0$ with any desired accuracy $\epsilon > 0$ as follows:

Let $a_0 := a$, $b_0 := b$ and $ k := 0$.
Let $c_k := (a_k + b_k)/2$ and:

$a_{k+1} := a_k$ and $b_{k+1} := c_k$, if $f(c_k) > 0$
$a_{k+1} := c_k$ and $b_{k+1} := b_k$, if $f(c_k) \leq 0$, and incremenet $k$ by $1$.

If $|b_k - a_k| < \epsilon$, then return $c^{'} := (a_n + b_n)/2$, otherwise continue with step $2$.

This algorithm recursively defines the terms of the sequences $a_k$ and $b_k$ in such a way that $a_k$ is increasing and $b_k$ is decreasing. As the the sequences are clearly bounded, they both converge.

This algorithm should find the roots of function, but I am not seeing why. Could you please explain me why this algorithm should find the roots?
Also, could you explaing me why the fixed points of $f(x)$ are the the roots of $h(x)$?
Basically, I need to find the fixed points of f(x) with that algorithm. I need to implement it using Maxima, but before I need to understand it.
Comment: http://en.wikipedia.org/wiki/Bisection_method
Answer: To mention another possible solution: You can use the Lambert W function $W$ which is the inverse of the function $x \mapsto xe^x$. So you have:
$$\begin{align}
e^{x-2} & = x \\
-e^{-2} & = -x e^{-x} \\
W(-e^{-2}) & = x
\end{align}$$
Note, that there a two branches of $W$ in the range $[-\tfrac 1e, 0[$. Thus the above equation will give two solutions...


I started to play Skyrim, but all the save files are deleted

Question: So, I used to be around level 40 on Skyrim and I haven't played the game in ages but now i've started to play it again, and when I played it, it took me back to Level 18. I went onto 'LOAD GAME' and all my progress had been deleted. If there a way to recover them so I can get back to playing at level 40?
Comment: What playform are you on?
Comment: I play on Xbox 360
Comment: 1.) Make sure you're on the same account 2.) Load the same save 3.) (unlikely) You deleted it
Answer: Did you use a different account, or even accidentaly delete all your save files? If the following is false, try to see if you can retirve the saves back, though you didn't deleted them. As mentioned here:

It may be possible but not easy. It depends on how much data was actually erased and how much has been written since then.
Restoring deleted files on an Xbox is similar to restoring PC files. When files are deleted, they are often not actually deleted- the system just erases any reference to the file and may overwrite it when it needs to.
To get the data back, you would need to somehow connect your hard drive to a PC and access it, then run some disk rescue software on it. This might retrieve your save.