How well do mosquito clothing patches work?

Question: They are said to be little patches that simply stick onto clothing, lasting for 12 hours. They are made from "natural ingredients" and DEET-free so I have my doubts. How well do they work?
Comment: Don't have time to answer buat as a starting point for others https://www.npr.org/sections/goatsandsoda/2018/06/30/623865454/a-guide-to-mosquito-repellents-from-deet-to-gin-and-tonic https://academic.oup.com/jinsectscience/article/15/1/140/2583458 https://www.consumerreports.org/insect-repellent/do-natural-insect-repellents-work/
Answer: Science says they don't work.
Citation from Charlie and another about citronella in general.
(It's possible all these studies are funded by the DEET companies, but I don't wanna get into mosquito-industrial complex conspiracies.)
That being said, there are people who swear they keep mosquitoes away, so if you like anecdotal evidence, maybe the patch is for you!
Answer: If you have a look through the information provided by the page you linked, they state:

Our patches are made out of citronella, lavender, and lemon

These three plants all exude chemicals (citronella, geraniol and linalool respectively) that are known to inhibit the ability of mosquitos to find people in certain concentrations, however in general these are thought to have more or less the same efficacy as DEET and similar compounds, but are more volatile and thus are effective for a shorter time.
The general advice for mosquito repellents is to apply thoroughly to all exposed skin. Tests done with patches and other devices that do not cover the whole skin indicate that these are usually less effective than contact repellents (see table 1 and text for description of devices). The only substantially effective personal wearable in the above study was one that fogged the spray in an area around the wearer. Patches and similar devices were about 25% effective (i.e. not very).
In conclusion: These patches will work to some extent, but likely to be very limited, possibly only to the area where they are applied. Although "natural" ingredients do work, they need to be applied to the exposed skin regularly for them to be effective.
Comment: I think you might be too generous to the wearables. From the article cited: "None of the bracelets we tested caused any significant reduction in mosquito attraction. Although the active ingredients in some bracelets may be mosquito repellents, we hypothesize that the concentrations that are emitted by all of the bracelets that we tested were too low to have an effect."


Are there any tests that can be used to measure the quality of an UI?

Question: What are the criteria that are used to measure the quality of an UI, or compare different UIs with each other?
Answer: There are several methods of measurement and I'm assuming your goal is to measure usability.
The first method would be doing usability tests.
Set the major goals you want your users to achieve and let them accomplish them in all the UIs you have. See which UI facilitates reaching those goals better.
I suggest you read Jakob Nilsen's: Why You Only Need to Test with 5 Users on this topic.
This method is usually used in early stages where you present completely different UI schemes and have the luxury to change almost anything.
At later stages of the application or site, if you just want to make small changes, you can do A/B testing - display two versions of the element you want to test (usually in a random manner) and test the results (time spent, the desired link clicked, etc.)
A great article about the limitations of A/B testing can be found in Jeff Atwood's blog Coding Horror: Groundhog Day, or, the Problem with A/B Testing
Don't forget to run accessibility tests on the UI as well and make sure that people with disabilities can use the UI properly (the simplest problem to imagine is color blindness).
There are other ways of measurement, for example bringing in a UI expert to do a report, but it's just one person's opinion. It can be very helpful, but cannot replace actually putting the UI to the test by multiple users.
Answer: There's always the hallway usability test if you want a quick result :)
From This blog post by Joel Spolsky:

A hallway usability test is where you grab the next person that passes by in the hallway and force them to try to use the code you just wrote. If you do this to five people, you will learn 95% of what there is to learn about usability problems in your code.
Good user interface design is not as hard as you would think, and it's crucial if you want customers to love and buy your product. You can read my free online book on UI design, a short primer for programmers.
But the most important thing about user interfaces is that if you show your program to a handful of people, (in fact, five or six is enough) you will quickly discover the biggest problems people are having. Read Jakob Nielsen's article explaining why. Even if your UI design skills are lacking, as long as you force yourself to do hallway usability tests, which cost nothing, your UI will be much, much better.

The hallway usability test is useful for some quick guidance in the right direction before you (maybe) start doing more intensive UI testing. It can get you close to the mark, if not right on.
Answer: Alex Faaborg from Mozilla has a nice blog post about Usability in user interfaces:
http://blog.mozilla.com/faaborg/2010/04/22/dont-talk-about-users/
He starts with some mistakes to avoid, then comes the part relative to this question: a list of rules that every user interface should follow. Put simply, every violation of these rules is a usability bug. (At the very end of his post is a link to this list, under the heading "Next Steps")
Personally I find this very helpful to evaluate interfaces, especially when comparing interfaces or while building your own.


Increase the limit of 6 questions in 24 hours

Question: Is there a way to augment the limit of 6 questions in 24 hours ?
Comment: What sort of questions are you asking if you need to ask more than 6 in a 24 hour period?
Comment: I suggest instead augmenting the quality of your individual questions, such that you gain more knowledge out of the answers. Requiring more than 6 questions in a 24 hour period is ludicrous.
Comment: @JeffAtwood - this limit makes me sad :( I usually get about 1.5 upvotes per question, at least from looking at my last few Qs ... so people agree my questions, on average, benefit the system. So why the urge to limit me from asking more? When diving into a new topic, asking a lot of questions is sometimes ... necessary. Was there a blog post about the reasons for this limit?
Comment: If augment means Evade/Stretch then I hope not.
Comment: @Henk Holterman: No, it doesn't mean that, it means, allow the users to ask more then 6 questions in 24 hours. @ChrisF: I guess it's the same type of those questions that you ask more then 5 times in 24 hours. And, it's the same type of those you ask 3 times in 24 hours... and so on. I'm really sorry for stating the obvious, but perhaps it is, just that, a simple question. :)
Answer: No, this is a hard limit by IP and account.
Attempts to work around this limit will be viewed.. unfavorably.
Comment: Explaining why the limit exists and how the OP can resolve his/her alleged need to evade/augment would make this a better answer.
Comment: I would prefer that that limit wouldn't exist, or that it could be augmented, by having more points, or that it could be smart and exist only on those cases where the need is not to get answers but to spam, or, that it could at least tell us, "you are reaching your daily limit, think well this question". (but perhaps this will force the user to ask 10 in 1 question and that may not be good. Anyway, neither of those exists, and we seemed to jump directly to: "Let's cut the access!" It's like a punishment for someone who have just asked questions. We may call this mechanism: Cicuta. ;-p
Comment: @Jeff: Can you please tell _since when_ we have such a limit? There was talk all over with those 70 - erm, no, 50 - question in 30 days. Now we have in addition 6 per day? Is this new?
Comment: @Jeff: *"Attempts to work around this limit will be viewed.. unfavorably"* Literally laughed out loud on that one. Do agree with Tomalak, though. [Hey, @Tomalak, I finally spelled your name right. ;-) ]
Comment: How about a holding queue?  http://meta.stackexchange.com/questions/123077/holding-queue-for-questions-over-the-rate-limit
Comment: And that is why I want websites with paid models to stay. SO is great, but there are limits. Sometimes, you just need that extra question or two to breakthrough and sail ahead. But you are stuck for another day because - You can only ask 6 questions in a 24-hour period.
Comment: this... really sucks! wtf. I've used the site for years and have accrued 42000 rep from 600 answers (and 400 questions), and now I can't ask a few questions when I'm learning a new tool? cmon...
Comment: well ok upon review and thinking about it it makes sense. ah well
Comment: @MEM: A warning seems reasonable.
Comment: @Jeff Atwood: I've accepted the answer but as others have pointed out, Tomalak Geret'kal comment should be taken into consideration. :-)
Comment: Just discovered that using a VPN does not bypass this, which means that the limit is PER USERNAME, and not per USER/IP.
Comment: Excuse me my ignorance. Assuming that it is a golden rule that can not be crosses and there was no need for further explanation on why was it so, could I at least have an explanation of what IP and OP is?
Answer: This limit needs to be changed. I legitimately run into this limit all the time, and I'm currently waiting with one question in the hopper waiting for questions to be accepted again. I would urge StackExchange to be slightly more nuanced here, and to consider things like

How well the questions were accepted on the network.
The OPs reputation on the site and the network.
Whether or not any of the questions were self-answered, or have accepted answers.

I'm diving into OSX for the first time, and I have a lot of legitimate questions on what I'm learning -- and they're new and original contributions to the site. (see my recent activity here)
I think a more reasonable question limit would be the greater of,

6 questions a day.
One question every 15 minutes.

I'm not principally against limits in this case, but I think four questions an hour when you're learning a novel technology is quite possible, and I can probably go at that level for six hours before retiring.

Google for problem
There is nothing on Google
Ask question
Rinse and repeat
Comment: In addition, 4 questions an hour is a *lot* considering one should have prior research and then a well written question showing that research. If researching and then writing a question takes less than 15 minutes, in my personal opinion, that question *cannot* be well researched. Even to *find out* that there is nothing on Google will take time, because to find out, one has to actually read all those pages´that do come up.
Comment: I haven't looked at your questions, but when looking into a new technology for the first time, I find it hard to believe anybody comes up with 6 unique, never asked before questions. I would suggest you get a better book/tutorial or teacher if the one you use leaves you with that many questions.
Comment: It might be better to link to your [questions sorted by newest](https://apple.stackexchange.com/users/76955/evan-carroll?tab=questions&sort=newest).  That gives a good idea that your questions aren't being well received.  You've got a net score of +1 over the last two days, so I think @nvoigt is quite correct.
Comment: Wow you wrote two comments lecturing me about research and you don't want to click the link that I provided to get informed about my case? Talk about gleeful ignorance
Comment: I also just got a bunch of a downvotes this morning. So it stands to reason a few people followed the questions from this site into Apple.SE and downvoted.


Would like to know some Machine Learning applications for Civil Engineering

Question: I've thought about some, like predicting Concrete's strength by it's composition, but would like some ideas on the theme. One other thing I was thinking was traffic, how would I use machine learning to predict something related to streets and roads... Maybe something related to hydraulics on a building. I'm really beginning on machine learning so I'm not so good at knowing how can I use it. Although it's amazing. Thank's a lot guys.
Answer: This is something I've experimented with substantially.  Predicting the compressive strength of concrete based on it's mix design is a great example, and fairly challenging - I used the concrete compressive strength dataset available at the UCI ML repository to help me better understand how to manipulate ML algorithms to get good predictive results.  Here's a guy who took the same data set and used BigML's tools to do an analysis.
I've also used ML to predict payitem bid prices on highway construction contracts to help improve cost estimation.  For example, I set out to determine the unit price for a specific item, like guardrail, that contractors are likely to bid given a contract's location, time of year, total value, relevant cost indices, etc.  It took a lot of time and experimentation, but I achieved surprisingly good predictive results.
Regarding your specific topics of interest, I imagine you'll have to just do a lot of searching and reviewing master's theses on related topics, taking time to learn the mathematical models and theories that they employ.
If you want to know where to get started, my learning path involved the following:

Learning whatever I could about neural networks and machine learning
theory (guys like Jeff Heaton proved helpful, here)
Taking Andrew Ng's Machine Learning course on Coursera.
Downloading and getting to know the R scripting language
Reading whitepapers / theses on applying ML to specific
applications.
Developing a solid, rigorous understanding of basic statistics theory, especially linear regression.

There's a lot of different ways to approach the topic, though.  Websites like BigML.com are a good way to get your feet wet doing ML without having a rigorous mathematical / statistical understanding, but it'll only get you so far.
In the end, you'll really need to devote time to getting your head wrapped around the math and programming in languages like R or Python.  It's also important to remember that while ML seems to favor complexity over simplicity (in many cases non-linear problems benefit from novel and sometimes non-intuitive approaches), sometimes the simplest approach is best.  It's really a question of how accurate the final model really needs to be.
Comment: Nice, that really helped. Could you show me your predicting bid program? Or is it commercial? Tks for the answer.
Comment: The code wouldn't be helpful.  And I don't really have easy access to it, anyway.  Suffice it to say you need to write code that will clean and prep the data for machine learning, then write code to test different algorithms.

R has a few good ML packages built in.  I used random forest and SVM quite successfully.  The neural network packages seemed lacking.  Otherwise, I relied on external packages (like H2O) to see what I could get done.

In the end, it was more about using creative combinations of algorithms (ensembles and stacks) than it was any piece of code or particular algorithm.
Answer: You are touching some issues that border simulation, namely Discrete Event Simulation (Simpy, Simul8). For instance Traffic Simulation is mainly done using this, and others, techniques with the purpose of either testing the consequences of a change to the system, or to conceptualize the system altogether. Some commons areas you might like to consider exploring are:

Mobility scenarios (traffic, etc.),
Optimization of mining or construction operations,
Consumption scenarios (water, electricity, etc.)

These are all fields that can be broadly put inside Operations Research (Python for OP). In the event that you might want to tackle issues related to this, the first thing you need to decide is "What Problem are you trying to solve?":

I need to see if a buildings water distribution will hold the requirements (simulating water use scenarios against water/pressure availability).
I need to optimize the transportation and mobility in a construction site.
I need to conceptualize the emergency exits plan against possible evacuation scenarios.
etc.

Than, what information do you need to build your models? Data analysis, image analysis, data management and manipulation. What are my variables? What are my scenarios? Can I predict phenomena (it's easier to predict water usage peaks than earthquakes, for example)? Can I create a model using those predictions?
Answer: Machine Learning is not an answer for the question of Life, the Universe, and Everything or silver bullet but a very powerful tool which you should apply to a problem (and corresponding data). So, the short answer to your question is 'depends on data you have and problems you see'.
Generally, the best way would be taking any available introductory course into Data Science / Machine Learning, given you have at least some programming experience so that you could complete assignments: that would not address your question directly, but you will get your hands on some examples of data and will see how ML is used to tackle that data. After you will grow some understanding, you will see potential applications of ML when you see any particular blob of data.


How do I set my login avatar in MATE? (Fedora 17)

Question: Plain old GNOME 2 had a panel applet which displayed your user long name. It had a menu option under which you could set an image to be a login avatar, and it appeared in the gdm display manager (aka login screen). Now I'm using F17 with MATE and I don't see such an option anywhere. I believe there's some package from the MATE repository I need to have to have this functionality, but which one?
Answer: are you using gdm 3.x ?
if yes, then the account properties are managed trough the dbus service org.freedesktop.Accounts.
You can install d-feet, which is a python+gtk browser of the dbus, conect to system bus, search for the org.freedesktop.Accounts service, then the object path "/org/freedesktop/Accounts/UserXXXXX" (with XXXX your uid), interface "org.freedesktop.Accounts.User", method SetIconFile and give a path name (in "quotes") to an icon.
or with the command line tool:
<code>dbus-send --system --dest=org.freedesktop.Accounts  \
    --type=method_call --print-reply \
    /org/freedesktop/Accounts/User$(id -u) \
    org.freedesktop.Accounts.User.SetIconFile \
    string:"/some/path/to/tour/file.png"
</code>
also, MATE people could backport the old gnome2 applet, and change it to use that dbus service (hint: copy files um-user.c and um-user.h from the gnome3 control-center and then all needed functions will be easily available)


Wrong Date Format (locale) in SQL Server Reporting Services Report

Question: I am unable to solve an issue with a report on SSRS.
The scenario is : a link in a Silverlight application loads a new web page containing a Microsoft.Reporting.WebForms.ReportViewer control. The parameters for the control are passed in the QueryString to the new page.
If the date is passed as 01/08/2011, when the report is loaded, the date control on the report viewer has the value 08/01/2011 (i.e. it has been converted to American format and my locale is United Kingdom, en-GB).
This only happens in the deployed solution, it works correctly in Visual Studio on my local PC. My IE settings are correct.
I made the following checks on the deployed IIS Server (IIS 6.0):-
The security account for the relevant Application Pool = Network Service.
In the Registry the SID for this user is S-1-5-20 so I checked the registry settings under HKEY_USERS > S-1-5-20 >  Control Panel > International . These were all correctly set for United Kingdom. Although theoretically they should not have an effect I also checked the same settings for Local System (S-1-5-18) and NT-Authority (S-1-5-19) which were identical.
I next checked the Culture and UI Culture on the web site. These were as follows: -
Right-click website - Properties > ASP.NET.
Click the 'Edit Global Configuration' button to check the server config settings.
Select the Application tab and view the values for Culture and UI Culture.
Both values are set as en-GB. So OK.
Did the same in the 'Edit Configuration' button which is for viewing/editing the specific application's config file.The websites' config file values will supercede the server config file values. These are also OK.
Next I checked the SSRS Server.
Service runs under 'Local System' account.
Registry settings for this account are correct HKEY_USERS > S-1-5-18 >  Control Panel > International.
S-1-5-19 and S-1-5-20 also correct
ReportServer.aspx page directive includes Culture="en-GB"
(e.g. <%@ Page Language="C#" AutoEventWireup="true" Inherits="Microsoft.ReportingServices.WebServer.ReportViewerPage" Culture="en-GB"%>)
In code the parameter values are set using Microsoft.Reporting.WebForms.ReportViewer.SetParameters(ms.ReportParameter []) and ms.ReportParameter(name, value, visible) and the parameters are definitely the correct value when they are received at this point by the web app (when I step through in code).
I don't know where to turn next. The one difference I have but I can't imagine it is the problem is that the querystring is URLEncoded on the deployed version but not in the design on my local PC i.e
[server version]
  ...%26END_DATE%3d31%2f07%2f2012+00%3a00%3a00%26START_DATE%3d01%2f08%2f2011+00%3a00%3a00
[local version]
...&END_DATE=31/07/2012+00:00:00&START_DATE=01/08/2011+00:00:00
I would be extremely GRATEFUL for any suggestions that anyone has.
Answer: If you pass in the date in dd-mmm-yyyy format then it is unabiguous, eg  18-Jan-2011.
Comment: This seems like a good idea. I'll try it as soon as I can get chance and let you know if it fixes it.


Would Hapkido be a good choice for transitioning from Tai Chi (Yang) to a form with with more locks, strikes, falling for an older adult?

Question: I currently practice Tai Chi (Yang style--short and long form) at an intermediate level. I enjoy it, but have recently wanted to explore a martial art form that still stresses economy/fluidness of movement, is well suited for an older adult (60's and in good health), but has a greater emphasis on not-too-demanding strikes, blocks, locks and learning to fall properly.
I'm uncertain if Aikido fits this description--as I would like to pick up some practical self-defense techniques in the near(er) term--and Aikido classes are not offered near where I live.
Hapkido, however, is. Would this martial arts form address many of my interests and needs? Would my practice of Tai Chi likely help or hinder a transition to learning this form? Finally, (and primarily out of curiosity), does Hapkido include forms for using a staff or other traditional weapons?
Comment: Sardathrion: Let's reduce this to 1 question: I practice Tai Chi (Yang style--short and long form) at an intermediate level. I enjoy it, but have recently wanted to explore a martial art form that still stresses economy/fluidness of movement, is well suited for an older adult (60's/good health), but has a greater emphasis on not-too-demanding strikes, blocks, locks and learning to fall properly.I would like to pick up some practical self-defense techniques in the near(er) term (and Aikido classes are not offered nearby). Hapkido, is. Would this martial arts form be a good choice?
Comment: Could you edit your question to include the above comment? That would be a nice focused question. Thank you for taking the time to make your question better.
Comment: You have three questions in there. Which one do you want answering here and which are are better asked as separate questions?
Comment: Would Hapkido serve as a good martial arts form to explore for an older adult (middle 60's) wishing to learn different types of locks, blocks, strikes, falling techniques that seem less strenuous and offensive than karate and martial arts forms similar to it? If it helps as a baseline, I have a Tai Chi background and enjoy its fluid form and emphasis in economy of movement.
Comment: We are not a forum but a Q&A site. Please see the [help] to see how we work.
Comment: Sardathrion: I did read through the Help Center entry on "subjective questions"--assuming this is your concern about this question. (If not, please let me know.) I think my last rewrite serves as and acceptable question that (for the most part) can be quantifiable. Still, in the spirit of learning and collaboration, I will try this again and see if it falls closer to how you (and others) feel a constructive question should be written in this venue. Best, Charles.
Comment: Please **edit the question!** to reflect what you want answering.
Answer: So you're in your 60's, in good shape, and you've been practicing Yang style Taiji. You're now looking to branch out and train in a style that has an emphasis on practical self-defense, which should have a mix of striking and grappling.
First let's talk about the age thing. Being in your 60's doesn't necessarily mean you can't do certain martial arts or are limited to just a small amount of low-impact styles. But it does mean you have to be aware of your own limitations when you're doing them. That's all.
You just have to make sure your instructors and partners know what you're comfortable doing. Either directly or through peer pressure, you're going to be tempted to do stuff that you know might be risky. Your best defense against this is to know your limitations and say "no" when you need to. Speak up. They can't read your mind.
That advice actually applies to people of all ages.
Second, let's talk about Hapkido and Aikido, since you brought them up. No doubt you've seen both styles, either in person or on the web. You're attracted to the joint locking and the effortless looking throws. You notice the fluidity of the motion and how everything seems to come from the "center". And you've seen a good number of older people in Aikido classes. This might evoke comparisons with the Taiji you've learned.
While Aikido has some strikes, Hapkido has a lot more and is much more developed. Hapkido generally borrows its striking from Taekwondo. While Aikido's strikes are merely used to distract ones opponent, Hapkido's strikes are done with the intent to harm.
As for its grappling, Aikido also has less number of grappling techniques compared with Hapkido. Though, some would say that Aikido's grappling is more refined in comparison. This judgment is biased, however, because Hapkido's objectives and training methods are much different from Aikido's.
Aikido's training method revolves around the notion of not resisting. And Aikido's highest moral code is to leave their opponents unharmed. Both of these contrast with Hapkido, which trains with a greater amount of resistance and with no reservations about harming their opponents. As a result, Hapkido will look a bit more "clunky" compared with Aikido. That arises directly from their values.
Aikido students are taught not to resist anything done to them. As a result, the throws and joint locks can look a lot more fluid and soft compared with Hapkido. Students of Aikido are often guilty of being too compliant, even throwing themselves into their partner's throws.
I talk more about Aikido and its weaknesses and strengths here:
What are the main uses of aikido in self-defense?
The "do no harm" principle of Aikido means that hard contact beyond distracting strikes are not practiced in class. And without the need to develop striking technique, there just aren't a lot of strikes taught in Aikido.
So if your goal is to learn more practical self-defense, I would steer you towards Hapkido more than Aikido. Hapkido trains with more resistance than Aikido does. And Hapkido's striking and grappling is more complete. Based on that, I believe Hapkido is more practical for self-defense.
As for how Taiji training might help or hinder learning Hapkido or Aikido, I think you'll find that the styles are so different from Taiji that it won't matter at all. It will not be confusing.
I will say that Aikido is more compatible with Taiji, and that's because Aikido is a style that bases itself on internal mechanics, just like Taiji does. But Hapkido does not. As a result, you might find that transitioning to Aikido will allow you to continue to develop and apply your internal mechanics that you're learning in Taiji.
But a word of warning regarding internal mechanics: It takes 10 years under a very good, qualified Taiji instructor to learn internal mechanics properly and begin to use them in real life self-defense. It might take 20 years before you reach the point where you can actually use it "for real". So, are internal mechanics important to you? Or is it more important to learn practical self-defense that still uses good mechanics and leverage, but just not "internal" mechanics? That's a question you have to answer for yourself.
Your last question has to do with whether or not they teach staff and other weapons. Yes, Aikido teaches primarily the Jo staff (a 4 foot long staff), and also teaches sword and knife. Whereas Hapkido also teaches those along with stick, cane, long staff, and other weapons.
Finally, I'd like to recommend that you also check into Gracie Jiujitsu. It is very grappling oriented and specializes in ground fighting (wrestling). It also has a good number of take-downs, throws, and standing grappling. And while at first you'll learn defenses against strikes, later on you'll develop your striking a lot more. Their striking methods come from kickboxing and western boxing. Their primary focus is on self-defense, while competition is also done later on. They train with much more resistance, so it's going to be more practical and realistic compared with Hapkido. Will it be too physically demanding for you? That's a question you'll have to answer yourself.
Ultimately, which martial art you decide to take is going to be a completely subjective choice. My recommendation for anyone in this position is to go ahead and dive right in. Go to a school near you, and ask to try some classes. Usually schools let you try a class, or a week or a month of classes, without paying much or committing to anything. So go try some classes in one school, then another, and so on. This is going to answer all of your questions much more than reading what others have to say. In the end, you'll select the school that felt better to you.
Hope that helps.
Answer: Part of training in Hapkido is being on the receiving end of the joint locks and throws.  We practice various weapons, but each Instructor and school could vary.
Answer: I would not stop your tai chi practice . Hapkido is perfect cross training . If your tai chi teacher understands application you will finds locks in your forms . Hapkido will get to it sooner . Both share circular motion and blending and flowing as well as deflections . As time goes on you will see the connections and both will compliment each other .


Make my facebook app (not a canvas app, its on website) work on https as well as on http, and with www. as well as without www

Question: I have facebook app which is non canvas,the website URL of the app is specified without www.
Now when i goto my app with www.myapp.com the app doesnt work. and also also the same problem with protocol as well. so how do i solve this problem?
Answer: It sounds like two thing.
1) Your DNS records are mixed up not allowing www.domain.tld and domain.tld being the same IP.
2) Your facebook app settings might have only www.domain.tld and not domain.tld specified in the App Domain setting.


Cognitive turn and social turn approaches in cognitive psychology

Question: What is cognitive and social turn in (cognitive) psychology?
Googling has not returned anything on this. I came across this in my lecture slides listed under possible Cognitive Psychology approaches, along with Introspection and Behaviourism.
Answer: There isn't a single, simple answer to this question. Instead, I would expect as many answers as there are theoretical foundations/schools within psychology. From a point of view grounded in Theory of Action I maintain:
Behaviorism posits that behavior is learned in response to both combinations of stimuli (Pavlov) and reinforcing stimuli (Skinner). The processes inside the individual are reduced to the concept of reaction.
The cognitive turn opened the so-called "black box" of the subject, reasoning, feeling and so on. Scholars left the "reliable terrain" of directly observable attributes of behavior and gave birth to theories which aim to both describe and explain the invisible internal psychic processes. One subjectively important milestone is the work by Miller, Galanter and Pribram: The individual is an actor. As such he/she doesn't just behave in reaction to external stimuli. He/she tests (plans) various alternative ways to act in a given situation within a cognitive representation of the environment.
One example for a social turn in psychology is Social Cognitive Theory by A. Bandura. The actor does not just rationally plan the best way to reach a goal – he/she also reproduces the ways of acting experienced in the social system. Rationality is bounded by the norms of the social system. In other words, an actor acts in a particular way, even if it is dysfunctional, because it mirrors what significant others have done before.
References: Literature about the cited works is readily available in both university libraries and the Internet. (Also: One of the textbooks by P.
G. Zimbardo et al.)


Can a rigid-prop quadcopter hover upside-down?

Question: Most small quadcopters use rigid rotors with some fixed pitch.
In principle, I can imagine it might be possible for such a rigid-prop quadcopter to hover upside-down,
but that apparently requires reversing the direction of rotation of all 4 motors.
(This is very different from the way some "standard" single-rotor model helicopters can hover upside-down by continuing to spin that rotor in "the same direction", but moving the swash plate to give negative blade pitch).
Is it possible for a rigid-prop quadcopter to hover upside-down?
When I build a quadcopter so it can switch from flying upright to flying upside-down and back again in mid-flight, what do I do differently than a normal quadcopter designed to always fly right-side-up?
(Related: "Can you run a BLDC motor backwards without damage?" )
Answer: You can run a brushless DC motors backwards using a H-bridge. Basically you'll need to program your microcontroller/microprocessor onboard to flip the polarity of 2 of the wires of the bldc (a bldc has 3 wires, switching any 2 wires will reverse the spin)


A word describes a place where people from different nations live

Question: I want to know if there is a word that has similar meaning to "community", "society", "country" or any word describes a group of people. but the special thing in this group or this place is that there are people from different nations or different communities happened to be in that same place. Is there a word like that?
Comment: If it is a small place (neighborhood) I think you're heading towards something like _"international village,"_ but there may be a better word out there.  ...
Comment: Just edited the question. Please tell me if it's still not clear.
Comment: Much better. - You should get some good answers.
Answer: Such a place is said to be cosmopolitan. Here are examples from the Oxford Learner's Dictionaries:

A cosmopolitan city/resort
Jamaica is a very cosmopolitan island
Answer: Below I've collected some phrases I was able to find in various documents as well as ones that I thought of. I've roughly sorted them from phrases I could imagine being used in common speech to those found in government or clinical contexts.
Generally the words "global" or "international" will put emphasis on people who have traveled from another part of the world, or still have strong connections to other regions while "diverse" focuses more on the cultural differences and does not address the people's origins.
International Village
I've a friend from a village in India called Auroville. He would refer to it as in international village, thought the government seems to prefer to call it a universal city. http://www.auroville.org/
Global City
I think most people would understand global city to mean a city with many international connections, but I think it comes from a well defined ranking based on financial connections. Wiki: Global City
Melting Pot
This term is discussed in American studies to refer to the idea that people would assimilate into culture—melt into uniformity. Depending on region this may be true to an extent, but I feel that there is always a merging of culture. This phrase must be used carefully because of the extensive background of its usage.
Wiki: Melting Pot
Diverse Neighborhoods
In American English diverse usually means "not white", and is often used in media to discuss demographics. I've linked to a Forbes post about which cities in America have the most diverse population. I'm from San Jose and I can attest that residents of San Jose generally consider there to be three major racial groups: White, Mexican, and Asian. Forbes: Diversity in America
Diverse Populations
Diverse populations is a term that comes up in technical and social work contexts to discuss groups of people. Office of Adolescent Health training material, Psychologists: Reaching Out to Diverse Populations: Opportunities and Challenges
Comment: Looks like International village is the closest to the meaning. Although i was expecting a word not a phrase. Thanks for the effort.
Comment: No need to accept so soon, others might find something better!
Answer: Although it does not directly mean 'a group or society' perhaps multicultural would suit your needs. "The community was multicultural","This is a multicultural society".


how Ripple Xcurrent works?

Question: As per my knowledge Xcurrent uses blockchain. But how it uses blockchain ? Xcurrent or Interledger protocol talk about ledger entries, I would like to know, do they really use blockchain as a ledgers in bank transfer or they use banks native ledgers like (databases or anything)? What is difference between general and sub-ledger?, is sub-ledger is referred as bank's native ledger ?
Answer: Taking a look at the "How it works" document on their website, the first time they mention the word "Blockchain" is in their "About the company" section in the end of the document. So I would be quite sure that their product has absolutely nothing to do with any kind of blockchain technology.
Comment: so it means that they are using pure messaging layer with Xcurrent, nothing else related to blockchain? and they ledgers are databases kinda things??
Comment: xCurrent has the same problem that blockchains like bitcoin have. You have parties that don't trust each other and are not willing to trust any single central authority, yet you need them to reliably agree on whether or not a payment took place.

how Ripple Xcurrent works?


What are the practical uses of low speed continuous drive (shooting) mode?

Question: The Canon 7D has two types of continuous shooting mode: High speed continuous and low speed continuous. What are the practical applications (scenarios, photography style, uses, etc) of using the low speed continuous mode?
Comment: You aren't going to discover some revelation here. If you want less FPS you shoot at the lower speed. Do you want more images or less per x time?
Comment: A school I went to would actually hire uncle bob (whose name actually is Bob!) to shoot school events. He would walk around with his fast camera holding down the high speed mode. I prefer to just press the button more than once, unless I'm having trouble getting a steady shot.
Answer: My camera has high speed 6.5 fps and low speed 3 fps. I use 3 fps most of the time. This is good for normal slow situations of people. 333ms is enough time to allow facial expression/eye blinking to change between the shots. 6.5fps is just way too fast and I fill up my CF card too fast and sorting the photos in the end will waste too much time. So I consider slow speed to be default and high speed for special occasions.
Highspeed is useful for fast action, like animals running fast and catching a ball in the air, or sports where you want the exact moment the soccer player hits the ball, or the puck in hockey flies past the goalie's head and he looks surprised, or a horse jumping the tallest hedge, runners touch the final goal strip, etc. Or maybe you just want to make a stop-motion-like film of a phenomenon.
Answer: The main advantage is easier continuous shooting over an extended time.  Generally, continuous shooting goes in to a buffer and eventually that buffer fills.  When shooting high quality RAW images particularly, that buffer can fill within 6 to 12 shots on many cameras.  Even my 5D Mark III fills the buffer (with two very high speed cards) in about 17 shots at top speed.
By shooting in a slower mode, the shooting is easier to control and can also run continuously for a longer time since the buffer is filled more slowly.  It also may be a quieter shutter.  On the 5D Mark III it is continuous shooting, high speed continuous shooting and silent continuous shooting.  Silent goes the slowest, but has a quieter shutter, continuous fills in the middle and high speed is like a machine gun. (Ok, not quite, but you get the idea.)
Answer: The most common reason for me to use that mode is when I think I may want to get a couple of shots in a row, but I don't want to accidentally squeeze off two or three shots when I really only want one.  At 8fps, the shutter is really sensitive, and it's really easy to wind up taking extra shots unintentionally.  I used to experience that even at 6.5fps on my 40D.
The low-speed continuous mode, on the other hand, is slow enough that I only get multiple shots if I'm deliberate about holding down the shutter, so I'll use this mode for casual snaps, candids, and so on.  In most of these cases, I'm just shooting one frame at a time, but if there's a reason to capture more than one, I can.
Answer: Sometimes the 8 fps of the 7D is a little faster than you want to go in burst mode. Even with the Version 2 firmware, you can fill the buffer in about three seconds when saving to RAW files. By reducing the frame rate to 3 fps you can stretch that to around 8 seconds with a fast CF card.
I shoot a local high school band. During their halftime show at Friday night football games I sometimes want to shoot a stop action sequence for use in a multimedia presentation at the end of the year. I prefer the slower rate for this as it makes it obvious it is a series of stills, even when played back in real time. The 8fps looks more like slow, jerky video. An eight second sequence also allows me to capture an entire move such as a spinning toss and catch of a flag, rifle, or saber. Since the lights in the stadium pulse at a slower rate than the Tv I need to freeze the action, I need to use RAW format to correct for the changes in WB and brightness from frame to frame.


Determining a Block's Extranonce Value

Question: How do I determine a block's extranonce value? I know they are in the coinbase transaction, but where? How do I decode the part that is the extranonce?
Answer: In Bitcoin v0.1.5 to v0.3.24, the coinbase scriptSig contained:
txNew.vin[0].scriptSig << nBits << ++bnExtraNonce;

In Bitcoin v0.4.0 to v0.5.3, it changed to:
pblock->vtx[0].vin[0].scriptSig = CScript() << pblock->nTime << CBigNum(nExtraNonce);

Bitcoin v0.6.0 to v0.6.3:
pblock->vtx[0].vin[0].scriptSig = (CScript() << pblock->nTime << CBigNum(nExtraNonce)) + COINBASE_FLAGS;

Bitcoin v0.7.0 to current:
txCoinbase.vin[0].scriptSig = (CScript() << nHeight << CScriptNum(nExtraNonce)) + COINBASE_FLAGS;

So the coinbase scriptSig field has changed over time, but it has always placed the extranonce as the second item.
In Bitcoin v0.1.5, the size was unlimited, and in more current versions, it's 4 bytes or less. (This is caused by the switch from bnExtraNonce to nExtraNonce.)
There are some other odd details - in v0.1.5, it only resets when the client resets, but in later versions it resets on every new block.
Of course, this is just what the core client does. Other miners or pools might do something else.
Answer: From here:

The extranonce can be found in the coinbase data from a coinbase
  transaction. This data can be interpreted as a script pushing data
  onto the stack and the extranonce is the second value.
{
   ...
    "vin" : [
        {
            "coinbase" : "03443b0403858402062f503253482f",
            "sequence" : 4294967295
        }
    ],
   ...
}

can be interpreted as:
03 = push 3 bytes onto the stack
443b04 = 3 bytes pushed onto the stack <-- Block index
03 = push 3 bytes onto the stack
858402 = 3 bytes pushed onto the stack <-- The extranonce
06 = push 6 bytes onto the stack
2f503253482f = 6 bytes pushed onto the stack <-- arbitrary data

EDIT: This format was defined by BIP34 and applies to blocks with
  version >= 2. I'm not sure how the extranonce was managed before that.
Comment: This answer says extranonce is 3 bytes, yet it's 8 bytes…
Comment: Actually that last bit did use to have a meaning - it's a [vote for P2SH](http://bitcoin.stackexchange.com/questions/2749/what-is-an-explanation-of-the-p2sh-voting-in-laymans-terms).

Determining a Block's Extranonce Value


What effect does portafilter diameter have on espresso?

Question: It seems like a lot of the higher end espresso machines come with a 58mm portafilter.  Does this larger diameter have some improved effect on the resulting espresso?
Answer: MT San talks about portafilters and the resulting differences between these portafilters that are different in size.
I will discuss the differences in espresso extraction as a result of portafilter and subsequent basket size.
Assuming that the only variable here is basket diameter:
Scientifically, pressure applied is normally measured as force per unit surface area. Taking the bed of coffee as a flat surface, and we assume traditional extraction pressure of 8 bar, the net extraction force upon the coffee puck will be greater for larger surfaces.
8 bar = 800kPa = 800 kN/m2
Now given two different basket sizes 58mm and 52mm which are 0.058m and 0.052m respectively, the net force on the surface of bed of coffee will be:
For 58mm:
Surface area: πr2 = π*0.0292 = 0.00264207942
Net force on area: pressure x area = 800*0.00264 = 2.11 kN
For 52mm:
Surface area: πr2 = π*0.0262 = 0.00212371663
Net force on area: pressure x area = 800*0.00212 = 1.7 kN
We can see that the larger portafilter is subject to a higher force of extraction with the same pressure. What this amounts to in practice is very much dependent on your setup. However it should tend towards better consistency in extraction. This is because, for a given dose, more of the coffee is in contact with the water at the bed surface and so has a more direct pressure gradient through the puck as it will be comparatively thin in the larger basket helping to reduce probability of channeling.
However, observable effects in practice will be slight to none as many other variables will take precedence in affecting shot flavour and the difference in extraction force will be accounted for silently in your dial-in process.
Comment: This is the type of answer I was looking for.  Thanks!
Comment: Thanks for your take on this. It seems logical. But there's a thing I probably don't understand. Isn't the net surface of the ground beans at least just as important? That remains the same regardless of the dimensions of the container. Why the net pressure on the sides of the container should matter? It's not the container we're extracting. Thanks in advance.
Answer: Short answer, yes. There are a few differences. Does this differences affect the home brewer? It depends. Let me explain the differences in between the three main types of portafilters:

pressurized: This type is mostly smaller in diameter (52 mm) and made partly from plastic material. As the diameter is smaller, keeping the pressure is easier even if the tamping is not even, grounds are not homogeneous in size. Thus, this is why this is the most common portafilter for entry-level home machines. It hides the irregularities. An example photo from Wikipedia is below.

regular: This type is mostly made from pure metal and larger in diameter (58 mm). Thus, their heat retention is greater. This makes temperature equally distributed. This type is more sensible to tamping errors or grinding irregularities. But, this may be a good thing actually if you are looking for the perfect cup. Here is a photo I copied from La Marzocco website.

naked: This type does not have the bottom spouts. This is very good if you want to see what's going on inside the portafilter. E.g. if you have problems in tamping, you can see if one side is flooding rapidly than the other side. Very good for education. Also, very nice for photographers. One nice photo including tiger stripes from wikipedia, again.


Where does the water to transform Dune into a non-desert planet come from?

Question: In the first three Dune books, where does the water come from to terraform Dune into a non-desert planet? They have the qanats (water tanks/reservoirs), but where does the water come from?
Also, aside from spice, what do the Fremen eat? (I doubt they could raise crops with so little water.)
Comment: IIRC, the water was locked underground as part of the life cycle of the sandworms/sandtrout. Once Leto II disrupted that cycle, the water was released. There might also have been a remark about the Fremen using sealed hydroponic gardens to grow food but I'm not quite sure.
Comment: im pretty sure that paul/leto 2 where also actively bringing in water from other planets to speed up the teraform process, im pretty sure i remember them ordering guild ships literally filled with water to constantly be coming to dune.
Comment: @Himarm Madness! I'm sure the Sigma Draconis system harbors comets. Thes Emperors and their money printing devices...
Answer: Where did the water come from?
The Fremen were collecting water, under the guidance of Dr Pardot Keynes from the atmosphere into secret catch-basins using hidden wind traps.

Station tools began finding their way into the sietch warrens -
especially cutterays which were used to dig underground catchbasins
and hidden windtraps.
Water began collecting in the basins.
Dune - Appendix I - The Ecosystem of Dune

And from various plants, cactus like in their retention of water:

There was a rare native root plant that grew above the 2,500 meter level in the northern temperate zone. A tuber two meters long yielded half a litre of water.
 Dune - Appendix I - The Ecosystem of Dune

The water extracted from the deathstills† would also be available to be added to the catch basins.

When it was done, Muriz clapped his hands once. Attendants came and removed the bodies, taking them to the deathstill where they could be rendered for their water.
 The Children of Dune

Sandtrout were also placed into deathstills to extract their water.

And She thought Sandtrout? Many times in this flesh and other had she played the childhood game, poling for sandtrout, teasing them into a thin glove membrane before taking them to the deathstill for their water.
 The Children of Dune

This method as well as getting water will also remove some of the Sandtrout that are part of the Spice>Sandtrout>Sandworm cycle. Though this cycle would not be able to be broken until the terraforming of Arrakis begins with vast quantities of water
Using all of these methods Pardot Kynes estimated that it would take:

In the manner of a teacher answering a child who has asked the sum of 2 plus 2, Kynes told them: "From three hundred to five hundred years."
 Dune - Appendix I - The Ecosystem of Dune

Following the establishment of a proto-ecosystem of various grasses and a few hardier plants in some protected areas of Arrakis the estimate was locked down a little further:

From the charts emerged a figure. Kynes reported it. Three per cent.
If they could get three per cent of the green plant element on Arrakis
involved in forming carbon compounds, they'd have their
self-sustaining cycle.
"But how long?" the Fremen demanded.
"Oh, that: about three hundred and fifty years."
 Dune - Appendix I - The Ecosystem of Dune

Then came Muad'dib, who upon becoming emperor increased the pace of water collection to create the terraforming needed for the vision of the golden path he had, including the use of weather satellites to bring rain. This was taken up by Leto after his father could not continue on the golden path, until only a small personal desert, the Sareer, was left in God Emperor of Dune.
As for food.
There is the Muad'dib mouse that Paul took his Fremen name from, this could be eaten, as could anything that preys on it.
There are also carrion birds mentioned by Liet Keynes

There are carrion eater birds over me. Perhaps some of my Fremen will see them and come to investigate.
 Dune

These again could be eaten, and presupposes there is an active enough ecosystem on Arrakis to support them.
Plants are shown to be tended in the sietches using plastic sills to gather water and funnel them to the roots.
One could also guess that mushrooms would grow in the various caves that make up a sietch.
Following the establishment of the nascent ecosystem the following fauna and flora were introduced:

They turned then to the necessary animal life - burrowing creatures to
open the soil and aerate it: kit fox, kangaroo mouse, desert hare,
sand terrapin . . . and the predators to keep them in check: desert
hawk, dwarf owl, eagle and desert owl; and insects to fill the niches
these couldn't reach: scorpion, centipede, trapdoor spider, the biting
wasp and the wormfly . . . and the desert bat to keep watch on these.
Now came the crucial test: date palms, cotton, melons, coffee,
medicinals - more than 200 selected food plant types to test and adapt
 Dune - Appendix I - The Ecosystem of Dune

All of which would conceivably be used to supplement their existing diet.

†a machine used to extract water from a body, usually dead, though the person can be alive when used as Fremen capital punishment
Comment: I would add that as I recall, there's a bit in the Appendix in Dune where Kynes studies Arrakis and notes that obviously there had once been a great deal of water on the planet, and that while much of it had been removed somehow, not all of it was. From here the appendix discusses the discovery of the lifecycle of the sandworm and how it sequesters water.
Comment: @Broklynite I toyed with that but the question doesn't ask where has the water gone, but where does the water come from and what are the Fremen eating. If I can make it fit within the question i'll put it in, but didn't think it was relevant to what was asked... actually i think i know where it can fit. Thanks!
Comment: Hm, you make a good point. I suppose I read it as a broader question of the water cycle, but it doesn't actually ask that.
Comment: In addition to the water sequestered in the sand trout & catch basins, there's fresh water in the polar ice caps on Arrakis.
Answer: The sandtrout encapsulates free water, binding it.  Once sandtrout starts dying in numbers that water gets released.
As for food: the Fremen trap water, mainly from the atmosphere, and presumably use that for irrigation or hydroponics.
Comment: In the appendix to the first book, Pardot Kynes *explicitly states* that there is enough water.
Comment: @Himarm: what makes you think that the planet doesn't have enough water?  The author's premise, if I remember correctly, is that it did - just as much water as any other Earth-like planet.
Comment: -1 you didnt explain, A. how or why the sandtrout are dying to release water, and B. how or if, the fremen actually had enough water saved up to terraform an entire planet.
Comment: @DougWarren the point is where did they get enough water to make the entire planet wet, that wasn't answered here, which is what the question is about. despite killing all of the sandworms, the planet still literally does not have enough water to have full on rivers, lakes ect. so thats the part of the question i haven't seen adequately addressed.
Comment: @Himarm The sandtrout aren't actually the ones doing the dying, at least not directly.  Water is toxic to the adult sandworm, however, and sandtrout are (I think) their offspring.  So if you kill enough sandworms, the supply of sandtrout starts to dwindle.
Comment: If I remember rightly there was also talk at some point (third book, probably?) of planting sandtrout on other planets, turning *them* into desert worlds.
Answer: I don't think the sandworms and aquifers has much to do with the arrival of water on Arakis. At the end of the film when Paul overcomes the empire and thus becomes emperor, a quick glimpse of the waters of Caladan are shown to the viewer. I believe that Paul, after drinking the water of life, acquired the ability to bend space and thus transport water from Caladan to Arrakis with his mind.  For a moment there is a shot of Paul looking into the distance as if meditating as the rain arrives.
Comment: OP was referring to the books, not the awful (awful) film.
Comment: @Spencer - I liked the mini-series. It was a lot truer to the books
Comment: @Valorum Which one? There have been a couple of awful ones.
Answer: Arrakis has enough water locked in the atmosphere, also underground in permafrost. Vast man-made underground cashes were replenished almost exclusively from atmosphere.

Station tools began finding their way into the sietch warrens -
  especially cutterays which were used to dig underground catchbasins
  and hidden windtraps. Water began collecting in the basins. (Dune -
  Appendix I - The Ecosystem of Dune)
Comment: Is this stated in one of the books? Can you add any references to this answer to support your statement?
Comment: The ice caps are specifically ruled out by Pardot Keynes in Dune - Appendix I - "The polar caps (disregarding the false sense of water security they gave the pyons) held far too small an amount for his project . . ."
Comment: @Cearon O'Flynn, I mentioned permafrost and atmosphere, you can't see "the ice caps" nowhere in my post.
Comment: @Jason Baker, "Station tools began finding their way into the sietch warrens - especially cutterays which were used to dig underground catchbasins and hidden windtraps. Water began collecting in the basins."
(Dune - Appendix I - The Ecosystem of Dune)


ASP.NET 3.5 Facebook iframe application Library or Samples that work? Specifically tagging photos

Question: Brief version: I am looking for an example or a decent library that will run on .NET 3.5 for uploading and tagging images in an iframe web application for facebook
I have tried several libraries and code samples. I have brute-forced through various iterations and attempts at configuring various requests. I can achieve working versions of what I want to do in PHP as well as .NET 4.0 but unfortunately I need this to work in .NET 3.5
I have tried using a couple of development toolkits (one from MS and one that came as a VSI for Visual Studio) as well as just using the facebook.dll and facebookweb.dll with raw code. My closest iteration involved me being able to create an album and upload photos to it but I am unable to tag because Facebook expects a certain ID and this ID causes the older dll's to crash.
I can go into specifics but I have over a dozen projects each with a small stumbling block somewhere (one won't list photos in the albums, one won't produce an authkey that Facebook will accept for tagging, etc). I have tried straight graph requests, but when I take a working url from Facebook's generator it fails from my code (though it works in the .NET 4.0version)
I am tearing my hair out. Does anyone have any tips, or information - does this even work with any 3.5 libraries? Any help is appreciated.
Answer: I found out that the Facebook Graph Toolkit http://computerbeacon.net/ has a .NET 3.5 compilation availble, which has been my saviour.


What are the cons to using a PC/cheap PCs as servers vs hardware designed to be used for servers?

Question: What is it with server hardware that actually makes it more suited for server hosting vs. say a collection of cheap PCs or a good PC?
Comment: I am basically asking what makes server hardware special? - If you really need more information - lets go with clustered apache and mysql servers. Cheap PC would be a 400-500$ PC - Duel core, 50$ motherboard, 1 TB hard-rive setup with possibly distributed redundant storage as far as the implementation of the storage, but I really want to emphasize I am asking for more of a general answer.
Comment: Possibly a duplicate - does this answer your question Josh? http://serverfault.com/questions/15040/rack-mount-servers-build-them-or-buy-them
Comment: He seems to be looking at custom machines with server hardware vs. commercial servers [with server hardware]. My question is asking about the comparison between server hardware and normal PCs. That provided some good information though. Thank-you for the link.
Answer: ""What is it with server hardware that actually makes it more suited for server hosting vs. say a collection of cheap PCs or a good PC?""
When hosting a lot of servers, holding them up, getting power to them, being able to get to them all makes a difference. The typical way to do that is in a rack ( http://images.google.com/images?q=server%20rack ) and servers are shaped so they fit in standard rack sizes and come with rails and cable tidies which all fit the right spacings Lego style. If you build your own machines, you will have to deal with putting them on the floor or standing them on shelves, running cables to them neatly, a way to keep them behind a locked door or to lock the front of them to prevent fiddling, etc.
On the electronics side, servers are built with more expensive memory which can correct errors (Google's study on their own kit showed error rates of >3,500 errors per memory stick per year). Not much good having a database spread over cheap machines if it's content gets quietly corrupted and it was important.
They are built with higher quality disks which can run 24x7 and stay in warranty, run at faster speeds (15,000 RPM compared to standard 7,200 RPM), connect to faster interfaces (Serial Attached SCSI), and they often have high end disk controllers which handle spreading load over disks in a way that works even if a disk fails, and have on board memory for buffering disk access to make it faster.
Electrically, they are more carefully designed so they can have disks removed and reconnected while switched on - and in bigger servers, have component cards and processors, power supplies and expansion cards added and removed while on.
They usually use more expensive processors such as Intel Xeons which have more integrated memory and faster processor-to-memory interface ability, more cores and beefier cooling.
Often they have simply more of everything - a dozen memory sockets, half a dozen disk slots, multiple processors, multiple power supplies, four network connections, all for handling more work at the same time, or handling a single failure.
All their electronic components such as capacitors and resistors, will have higher quality material so they can stand being run at full load for longer in a hot environment, and not fail. They aren't competing on being cheap, they are competing on being trusted to work for a long time.
Airflow will matter - inside they have carefully laid out components and cabling, and many fans. It's not uncommon to see two rows of fans one blowing through the other to shift more air and tolerate fan failure.
More sensors - how many home computers warn you if the case was opened recently?
All the changes are designed to answer the questions:

What happens if we need 50 or 500 of these things, how do we manage them?
How much more work can it do at once than a normal computer?
How can we have servers without needing a huge team of employees to run them?
How can we avoid a failure taking out everything?
How can we avoid causing more problems while we fix it?

OK, your clustered MySQL server - what happens when the $50 motherboard fails? Do you get a support guy on site within 4 hours with a guaranteed correct replacement or do you have to order a replacement from NewEgg and hope it's still being made? Will it take out any other components as it fails? Can a $50 motherboard hold enough memory to make a good MySQL server? Can it shift enough information from disk->RAM->CPU to make a good database server?
How much effort are you spending on clustering, when instead you could buy one big machine and not cluster?
What if it's not MySQL, but instead a system which doesn't support clustering, like a random company's document store, and you need it to serve 500 users on one server?
There's nothing magical about servers is quite right - they are more expensive heavy duty computers. Bulldozers, cranes and Formula1 cars instead of a fleet of Mondeos. Are you a business which needs a really fast car and can afford a team of mechanics? Are you working on your own and you can't afford a fast car but you can afford to spend all weekend fiddling with things to make them work?
You are asking at a time when distributed servers are all the rage - if you have a redundant storage system and you can point thirty cheap Apache servers to it and have a site simple enough that you can load balance between them without needing an expensive load balancer, then you're onto a good thing. Cheap machines will do fine.
Answer: Servers generally have support of a vendor; that is, you pay to have replacement parts on your door in hours. As such, they usually are higher quality, and tested out the wazoo.
These are also tested configurations, so certain hardware is "known good" with stable drivers.
Servers generally are also able to accommodate things like multiple processors, multiple power supplies, etc. And often are configured to fit a proper standard rack. And there's always calling your vendor and knowing they can tell you that XYZ component will work with ABC system.
In general there's nothing magical about servers. If you don't mind scrounging parts, you can assemble a system just as good as what comes from Dell. But when something breaks, you can't get the exact same part or certified part to replace it and have it be someone else's concern, it's yours. For most businesses/admins that added cost is peace of mind, knowing that your vendor is to get things replaced instead of adding to the sysadmin's problems.
Even Google used commodity systems for servers. I think they supposedly still do.
Answer: Reliability, Availability and Servicability.
This means better components designed to run 24/365 for years, oceans of alarmed sensors to spot prefailure, lights-out control and remote ISO booting, higher performance chipsets designed to remain responsive under extreme loads, 'hot' swapping of disks, CPUs, memory, adapters, redundant PSUs, better air management, memory failure protection, power management plus things like >2 CPU support, huge local storage support, much larger memory capacities, rack mountability/physical size.
They're the ones I can think of from the top of my head anyway. Ultimately it depends on how important the service provided by your server to your organisation is.
Answer: I would say the biggest cons of PCs are reliability, stability of the hardware platform and availability of hardware.  PCs tend to use cheaper hardware and not implement redundancy of components.  PCs also tend to have a much wider range of hardware components than servers, so firmware and driver updates can fluctuate more than with a solid server hardware platform.  PCs also tend to not have redundancies such as fans, power supplies, hardware RAID, etc.
But in the end, it all comes down to what you are running on the box.  I could see running a lightweight server app on a PC if that is all your buget afforded you.  Although virtualizing would still be a better option.
Answer: Low end servers can come in a tower form factor that closely resembles desktops.  But even these servers will have a few things that set them apart from desktop equipment:

Nicer power supplies.  These will provide more power on the 5V rail vs the 12V rail (to support more hard drives), but even if you only need one drive (or two for RAID 1) they will also be more reliable over time.
Faster, more reliable disks that will stay under warranty longer
Buffered memory
Support for Server class CPUs (Xeon/Opteron)
Answer: Servers are for enterprises only, not for mere mortals. Enterprises usually want to set up their business-critical applications and let them run, without changes and without expensive and downtime-risky migrations to new hardware, for 5-10 years. True servers do last that long and even have spare parts available after that time, and commodity desktops don't.


Why are double strokes (with the same hand) taboo on timpani but recommened on snare?

Question: I've played snare for a few years now and all my teachers recommended that I use double strokes for evenness of sound and speed. When I started playing some more difficult timpani parts however and used double strokes so I did not have to move my arms from drum to drum, thinking it would be a clearer and more even sound, the conductor told me to use single strokes instead. Why is this? What causes the difference in preferred sticking for snare and timpani?
Answer: The primary reason for this difference is in the mechanics of the drums themselves. Snare drums have both top and bottom heads that are usually tuned to a relatively high tension compared with the timpani. Hence, all the notes are extremely staccato and have virtually no sustain to them at all, so it is very difficult to play a note on the drum in the 50 or so milliseconds while the precious note is dying away. If you want to get exactly the same sound, you need to produce the next note under identical conditions to the first, which means hitting the drum in the exact same place with the exact same force.
Timpani have no bottom head and are designed to resonate, and it is easy to play notes that take almost 2 seconds to fade completely. This occurs because the parabolic nature of the timpani shell reflects the sound inward where it can continue to bounce around the inside of the drum. Since these vibrations last so long, if you play a second note in the exact same place on the drum the strokes will sometimes cancel each other out by producing the same vibrations going in opposite directions. When this happens, the net result of the second hit is not a new note, but stopping the vibration of the first and producing very little of its own. In order to replicate the conditions of the first note, you need to play the drum in a different place so it is starting in a spot with no directly opposition sound-waves.
Since double strokes tend to occur exactly in the same place, they work very well for producing even notes on snare where this type of accuracy is required, but not so well on timpani where different notes tend to have a canceling effect.
Answer:
all my teachers recommended that I use double strokes for evenness of sound and speed.

a lot of teachers might claim that for evenness of sound you should use alternating sticking. this is highly variable and does not always apply though.
i think you're partly right in your answer, but there's nothing wrong with playing repeatedly in the same spot on a timpani head. there might be a bit of sound-cancellation but that can be useful and not necessarily noticeable.
the short answer is simpler: there's no reason not to use double-strokes.
in fact, there are times when you must. (so don't worry if you come across a place where you think you have to; do it!) there are places where some players use singles and others use doubles (the end of beethoven 8, for ex.). it can simply be preference.
a more technical answer would be that a bigger/higher stroke will give you a fuller sound.
say you have two notes to play. with two sticks, both sticks can start, say, 10 inches above the head and you'll get two "full" strokes. with one stick playing both notes (double-stroke), the stick will have to start lower/closer to the head, presumably 1/2 as close, so the height of the strokes will be shorter and you will get a less full sound. (this assumes a tempo that's fast enough to ensure you cannot raise a single stick as high as two sticks; hopefully you get the idea.)
all of this, as usual depends on the ability of the player, tempo, dynamics and all the rest. but i think it's mostly a question of sound from the player's perspective, more than the physics of the drums, although you make a good point in your answer.


what is the block in crypto

Question: Sorry for the newbie question, I'm new to cryptography and I have difficulty in understanding what the block is. For example for DES algorithm the block size is 64 bits and the key is equal to 2 to the power of 56.
Can someone explain this to me ? Maybe like I'm 5?
Comment: [This article by Matthew Green](https://blog.cryptographyengineering.com/2016/08/24/attack-of-week-64-bit-ciphers-in-tls/) may be of help.
Answer: A "block" is the amount of bits your block cipher operates on (There are other kinds of ciphers, see below.)
Think of a Caesar cipher, it replaces letters in words. For example with key 13 it replaces as follows:
A -> N

and
HELLO ALICE -> URYYB NYVPR

Note that the same letter will always be replaced by the same letter. So it operates on one letter at a time.
A block cipher operates not on individual bits 0/1 but on blocks of them so if your block size was four your cipher might do the following replacement:
0100 -> 1110

(Of course what exactly is replaced with what depends on the key.)
Essentially, a block cipher behaves like a permutation of the blocks. Which permutation exactly, is determined by the key. So it is a bijective map $\{0,1\}^{blocksize} \rightarrow \{0,1\}^{blocksize}$.
It is not strictly required for a cipher to have a block size. A stream cipher for example operates on individual bits.
Note, that since a block cipher is a permutation it will replace the same input-block with the same output-block just like the Caesar cipher always replaces the same input-letter with the same output-letter. Because this makes for bad cryptography block ciphers are used in different modes of operation.
Answer: Lots of practical cryptographic algorithms operate on variable length inputs.  E.g., if you're encrypting messages, you want your algorithm to be able to handle messages of any reasonable length.
But the way cryptographers usually build algorithms capable of handling variable length inputs is by splitting the problem into two parts:

Design a basic algorithm—often called a primitive—that works only on fixed-size inputs.
Design an extender algorithm (often called a mode of operation or just a mode; or sometimes called a construction) that uses a primitive as the basic building block for processing variable-length messages.

DES and AES are block ciphers, which means they are primitives.  Their block sizes (64 and 128 bits respectively) is just how much data each one of them processes in a single call.  An extender algorithm will typically break up the inputs into blocks of the primitive's supported size, and use a separate call to the block cipher to process each block.
Block size is often an important security parameter for cryptographic algorithms, because it tends to put a limit to how many times you can securely call the primitive with the same key.  For example, since DES has a 64-bit block size, when you use DES or 3DES many extender algorithms' security breaks down if you process more than $2^{32}$ blocks with the same key.  So AES' 128-bit block size is a big improvement, because you tend to run into problems after $2^{64}$ blocks instead, but even there many cryptographers worry that's uncomfortably low.
Comment: You need to surround the exponent with braces: {} in Tex, otherwise only the first character / digit will be in superscript.
Comment: Thanks @MaartenBodewes.  Not the first time I make that mistake, and I doubt it'll be the last.
Answer: There are stream ciphers (which operates on 1 character a time) and block ciphers - algorithms are supposed to obtain a fixed-length block and encrypt/decrypt it as a whole.
You may see some parallel in operations of decimal numbers: you may double every digit (throwing out tens), or double the number as a whole. The last is more complicated.

what is the block in crypto


How is gluten-free beer made?

Question: I have a friend who is Celiac and can only drink gluten-free beer. How on earth is it made?
Answer: These beers are made with non-gluten containing grains such as millet, rice, corn, or buckwheat as opposed to glutenous grains like rye, barley, or wheat.
As a side note, I recommend your friend give Omission Beer a try. I had it once on accident at a social event. I couldn't tell the difference between it and the real thing until I got home and looked it up online.
Comment: As a side note Omission is made with barley, they add a protein to break down the gluten. My wife has to avoid gluten and has drunk this beer a few times.
Answer: In general, such beers are made with grains that contain little or no gluten, such as buckwheat, sorghum, rice, or corn.
Incidentally, this also means that cheap macrobrews such as Coors and Budweiser, which are often brewed with the cheapest of cheap adjuncts in their grain bills (such as corn and rice) end up being gluten-free (or very low gluten, legal definitions of the term vary) as well. Because the legal definition of 'gluten free' in many jurisdictions allows for very low quantities of gluten (generally less than 20 ppm), many gluten free beers include small quantities of rye malt for flavoring purposes. While some individual with celiac are able to tolerate these low levels of gluten (as are non-celiacs pursuing a gluten free diet for other reasons), tolerance can vary, so be careful when selecting a gluten free beer, and be aware that they can vary wildly, both in quality, and in gluten content.
Comment: Your final warning is important, and not just in beer!  The tern *gluten free* allows for small amounts, so celiacs are always minded to read the ingredients!
Comment: I highly doubt that any macrobrew is anywhere close to gluten free. Except obviously Redbridge and Bard's.
Comment: Some of what you said is wrong.  Please read my answer.
Answer: Some Gluten Free beers actually have gluten at the moment they are made, for example in the case of Omission the beer goes trough a proprietary "gluten removal process" and then they use a gluten test called the Competitive R5 ELISA, which is used to test foods that are "hydrolyzed," or broken down. This test looks for a specific lengthy fragment of the gluten protein and returns a negative result if it doesn't finds it.
If the test results are under 20 parts per million (as @LessPop_MoreFizz's answer says), then it can be called "gluten free" in the USA... and yes, I've heard people with serious celiac disease can still "feel" that concentrations and get sick.
"Estrella Damm Daura" on the other hand has only 3 parts per million, they also have a proprietary process to remove gluten. Estrella Damm Daura is an European beer made in Spain, obviously in Europe health and food standards are way better than USA's. In Europe, food authorities will never allow a brewery to call a beer with 20 ppm "gluten free". Just don't confuse it with "Estrella Damm" which is their regular beer.
Comment: Not all, if made with a non-gluten containing material then no gluten will be present. This is how Redbridge make stheir beer.
Comment: Thank you, I added "Some" at the beginning to clarify :D
Comment: Some of what you said is wrong.  Please read my answer.
Answer: Just to add to this topic, I think it's important that the correct terminology is used since a number of answers contain false information.
In the beer world, "gluten free" can only be used if the ingredients used to brew the beer do not contain gluten. Sorghum appears to be the most popular ingredient for making GF beers, but there are a number of other options including buckwheat, millet, honey, even chestnuts. Redbridge is an example of a GF beer. I have yet to taste a GF beer that I enjoyed as sorghum specifically gives off a cider-y flavor and all the beers end up thin.
There are also beers that are brewed with traditional ingredients that contain gluten, but an enzyme called Brewer's Clarex is added to primary fermentation. Clarex was originally used to clear beer and prevent chill haze, but it was discovered it also breaks down gluten such that, when tested with traditional gluten tests (ELISA), the beer is well within the "gluten free" threshold (<20 ppm). These beers cannot be called "gluten free", though, rather they can only be labeled "crafted to remove gluten". Omission brews "gluten reduced" beer (which are very good IMO), but apparently Yards also uses Clarex in some of their beers (even though they don't label their beer as "crafted to reduce gluten").

How is gluten-free beer made?


Applications of quadratic forms

Question: It seems that a lot of great mathematicians spent quite a while of their time studying quadratic forms over $\mathbb{Z},\mathbb{Q},\mathbb{Q_p}$ etc. and there is indeed a vast and detailed theory of these. It usually qualifies as part of Number Theory as in Serre's book "A Course In Arithmetic" half of which is devoted to the topic, but it is also claimed to have applications in other areas such as differential topology, finite groups, modular forms (as stated in the preface of the aforementioned book). I would like to have examples of such applications just for general education in order to truly appreciate its universal importance.
Answer: If $X$ is an orientable manifold of dimension $n = 4k$, then the Poincare Duality pairing induces a unimodular integral symmetric bilinear form on $H_{2k}(X,\mathbb{Z})$.  This form turns out to be a deep and useful invariant of $X$.  For instance, I quote (or rather, closely paraphrase) the following result taken from Milnor and Husemoller's Symmetric Bilinear Forms:
Theorem: Let $M,M'$ be closed, oriented simply connected $4$-dimensional manifolds.  The following are equivalent:
(i) There is an orientation-preserving homotopy equivalence from $M$ to $M'$.
(ii) The symmetric inner product space $H_2(M,\mathbb{Z})$ is isomorphic to $H_2(M',\mathbb{Z})$.
There are other amazing results along these lines: see e.g. Rokhlin's Theorem.
(In fact, I want to say that something about the intersection form defined above gives an obstruction for a topological manifold to admit a smooth structure, which is about as deep as it gets in this subject.  Unfortunately I am not remembering the actual statement now: perhaps someone can remind me.)
I have always assumed that geometer-topologist extraordinaire John Milnor
got interested enough in symmetric bilinear forms to write an entire book about them because of this application to manifold theory, although I looked just now and didn't find any explicit statement along these lines in the book.
Note finally that Chapter V of Milnor-Husemoller is called "Some Examples" and the entire discussion above concerns $\S V.1$: Homology Theory of Manifolds.  I suspect that one could write two more good answers using $\S$ V.2: Rings of Smooth Real Valued Functions and $\S$ V.3: The Discriminant of a Field Extension.
Answer: Offering a very different application: the Gold sequences (used in e.g. GPS-navigation to track changes to the distance of one of the satellites) can be viewed and analyzed using the fact they are gotten by evaluating certain quadratic forms (depending on the id-number of the satellite) defined on a 10-dimensional space over the field of two elements.
Several other widely used synchronization/scrambling sequences (but not all) are gotten in the same way.
Answer: Let $A$ be a finite-dimensional algebra over a field $k$. Then there is a distinguished linear functional $\text{tr}(L_a) : A \to k$ given by computing the trace of the left multiplication operator $L_a : x \mapsto ax$. This induces a distinguished symmetric bilinear form $\langle a, b \rangle = \text{tr}(L_a L_b)$, the trace form. This is a useful tool for studying $A$.
Theorem: $A$ is a semisimple algebra if and only if the trace form is nondegenerate.
In this case, $A$ is equipped with the structure of a Frobenius algebra, so it can be used to define 2-dimensional topological quantum field theories.
Theorem: Similarly, let $\mathfrak{g}$ be a finite-dimensional Lie algebra over a field $k$ of characteristic $0$. The trace form in this context is called the Killing form, and by the Cartan criterion $\mathfrak{g}$ is semisimple if and only if the Killing form is nondegenerate.
When $k \to A$ is an extension of number fields, the trace form can be used to define the discriminant of a number field, which alone has many applications in algebraic number theory.
Answer: You mention simple groups. Quadratic forms are a quiet element in the classification of Lie algebras, through Weyl chambers and so on. The short version is that any discussion involving Euclidean spaces that talks about reflections is using a quadratic form, typically the ordinary dot product.
So, finite simple groups were discovered as integral automorphism groups of integral lattices, meaning other positive quadratic forms over $\mathbb Z.$ This area has always used the language of Lie algebras, Dynkin diagrams, etc. However, the defining example is the Leech lattice, which is a peculiar object. Furthermore, the main calculation taking the Leech lattice to simple groups takes a detour through indefinite forms.
Three good references are ERROR CORRECTING, LATTICES AND CODES, and any edition of SPLAG.  I am especially fond of section 4.5 in Lattices and Codes by Ebeling. In the preface to the second edition, he points out that section 4.5 is new in that edition. Amazing, as there was no other place that explained what I needed. There is something a little off about availability and price, I think I bought it for about $35.00 from the AMS, but now AMAZON.  I wrote to Prof. Ebeling, he quite liked my application of the section. Known to Conway and Borcherds, of course, but worth publishing as a separate item. We will see.

Applications of quadratic forms


Where does the energy of a lightning strike go?

Question: Lightning contains a lot of energy, so where does this energy go after lightning has hit the ground?
Does it travel all the way to the core? What happens after that?
Answer: The majority of the energy is dissipated in the travel through the air from the cloud to the ground. The energy goes into heating the air and generating the shockwave that we hear as thunder. I can't give you a single definitive refernce for this, but Googling "energy dissipation lightning" will find lots of relevant articles.
You can understand why this is because the energy dissipated by a current $I$ travelling through a resistance $R$ is given by $W = I^2R$. In a lightning strike the current is constant, because the charge flowing in one end has to flow out the other end, so the power dissipated is proportional to the resistance. The resistance of air is a lot higher than the resistance of the ground/tree/person or whatever the lightning hits, so the majority of the energy dissipation is in the air.
The electrons flowing from the cloud through the lightning bolt end up in the ground, but with an energy only slightly greater than ambient. They will presumably flow into the surrounding area until the potential difference around the point of strike falls to effectively zero. This is likely to be within a few metres, so they wouldn't get anywhere near the Earth's core.
Comment: A lot of the energy in heating the air is also used for Nitrogen Fixation.
Answer: Lightning is generated by dielectric breakdown of air. So it is essentially a collection of electromagnetic waves. Now any material on earth has a notion of permittivity and permeability attached to it. Now when the electric field corresponding to the EM wave touches the surface of earth then depending on the nature of surface one can write down maxwell's equation, with proper boundary condition. And then calculate the transmitted electric field whose magnitude gets attenuated with distance.


easy backup solution from windows to ubuntu server 11.10

Question: Well for the last week I tried setting up amanda on my Ubuntu server 11.10. I should start out by saying that i'm fairly new to Ubuntu. Well I googled and read guides and tutorials and no matter what I kept running into issues all over the place. I'm way too frustrated and ready to quit. Is there something that's easy to setup that can backup my server, backup network shares(windows server 2003) and also help me backup windows desktop clients. Hopefully something that use ubuntu specifically on it's installation guide. For now I setup scripts to backup the shares and stuff on the server, but I would really love to be able to get everything in a single place and be able to backup windows clients.
Answer: BackupPC is pretty simple to set up. If you know how to share the folder you want backed up as a network share, setting up BackupPC is trivial.
You can also use rsync or ssh to back up remote machines, and tar to back up the server but if they are Windows PCs using network shares is easiest.
The only thing I found hard to understand is that BackupPC is intelligent about backing up machines.
At first it appears that it backs them up more or less when it wants to, you set targets like you want daily backups and do them after 6pm, but it allows for machines being turned off, so if it attempts a backup which it fails as the PC is off, it will try again later. Once you get it, it is actually pretty handy thing to have though.
You get emails letting you know if a machine has been off for a while and backups are old.
And it uses hardlinks on the server which means it doesn't have to store the same file more than once.


web page performance testing in Internet Explorer

Question: How can I get the web page load speed and data amount in Internet Explorer? I know about PageSpeed and Firebug for Firefox, is there an equivalent of those tools for IE?
Answer: I would recommend dynaTrace AJAX Edition for testing page performance in IE6+. It's a free download (although you have to create an account to actually get a download link) and hooks in to the browser to get low-level timings of network, rendering, scripting, and other metrics.
It can also show you what is taking the largest amount of time.
This tool has been invaluable to me since the site I work on requires IE6 support and performance is critical.
Answer: Firebug can be very handy most of the times in Firefox. IE also has this kind of simple application that helps you edit or see the codes in a webpage. Try hitting F12 to see it.
Comment: Which versions of IE have this? I want to see how long it took to load the resources of the page and which resources were loaded.