Is the GOST block cipher broken?

Question: Is the GOST cipher broken or known to be weak and insecure? Please try to simplify your answer since I am a programmer and not a cryptographer and please consider this web page and tell me if the claims made there are true.
Comment: People shouldn't use GOST if possible, because of its short block length. GOST is broken in an academic sense but not practically (according to [Wikipedia](https://en.wikipedia.org/wiki/GOST_%28block_cipher%29#Cryptanalysis_of_GOST))
Comment: (1) `do you know any … cipher … not supported by nsa? ` – Do you know anyone having access to governmental agency secrets? I'm pretty sure no one here knows (or is willing to admit he/she knows) anything about the actual cryptographic capabilities of agencies like the NSA. (2) 'any recommended cipher or public key algorithm by them they know how to crack it' – the NSA doesn't recommend ciphers. NIST is responsible for such standardisation. Besides, most current ciphers recommended by NIST haven't been developed in the USA. Eg: AES (Rijndael) was created in Belgium by civilian cryptographers.
Comment: All (good) modern ciphers are developed independently of the NSA, except for those kept secret and later published by them (like SIMON). AES (a.k.a. Rijndael) is indeed a belgien design, ChaCha and Salsa20 (stream ciphers) are made by Bernstein who has already publicly fought the NSA, Threefish is a modern block cipher whiches design is rather simple and design choices can be verified quite easily. @e-sushi, Snowden leaks indeed showed that the NSA only had some slight cryptanalytic advantage which isn't practically relevant. And that NIST is responsible doesn't mean NSA has nothing to say ...
Comment: @SEJPM do you know any block cipher that is not supported by nsa? I mean I suspect that after Snowden leaks  that any recommended cipher or public key algorithm by them they know how to crack it. sorry for asking another but if you may help please :)
Answer: There are two ways a cipher can be broken: theoretically and practically. If the paper is correct then:

Our fastest attack achieves a nearly feasible $T=2^{101}$ (cf. Section 28.6 and [28]). (emphasis mine)

If we verify this claim then we get:

If we have a diverse population of at least $2^{79}$ different 256-bit GOST keys generated at random, with access to $2^{32}$ CP per key, one can recover one of these 256-bit keys in total overall time of about $2^{101}$ GOST encryptions.

Now theoretically a cipher is broken if there is an attack that is (significantly) faster than brute force. Simply said, $2^{101}$ is a darn amount better than $2^{255}$ required for brute force. So if this claim is correct then the theoretical security of GOST is much lower than it is supposed to be. Hence it could be called broken.
Of course, nobody is going to invest that amount of memory or processing power to even prove this point. SHA-1 has a much lower effective security margin and it took a very long time before a collision could be found.

Given the structure of the paper I would say that it's likely that the claims are correct. That doesn't mean that anybody can now attack GOST. It does show that too many kinds of attack are possible. These kind of attacks will only get stronger. This is a clear indication that it would be wise to upgrade. The block size of 64 bit is also a good reason to upgrade, 64 bit is considered on the low side for many applications of block ciphers.
That all said, these specific cryptographic attacks can not be used to attack any well designed / implemented protocol that makes use of GOST. 101 bit security is generally thought to be out of reach, i.e. the "nearly feasible" in the text.
Comment: This looks like a related-key attack where it's already (quite) infeasible to store all the required related keys, let alone getting the $2^{79+32=111}$ chosen plaintexts...
Comment: Yep, my idea exactly. This is why I mentioned the "amount of memory" even before the "amount of processing"... Still, it seems this cipher loses even more security than DES for specific attacks. It's that they start off at 256 bit that saves the cipher against practical attacks.
Comment: I agree particularly with the block size assessment. [Sweet32](https://sweet32.info/) alone is reason enough to discontinue the use of 64-bit block ciphers.
Answer: That a cipher is supported by the NSA doesn't mean that (publicly) it is developed by the NSA. The NSA can insert a backdoor requirements into the endevors of third parties, then promote this as a "secure" cypher.
AES was indeed not "developed" by the NSA, but it was promoted by the NSA. And still is. This is particularly important as this is the cypher used for cryptocurrencies and most "encrypted" info on webpages and messaging.
Furthermore the NSA does have a history of inserting backdoors. Even is less cryptographically/mathematically secure, all attacks on GOST are practically unfeasible without quantum computing, so in practice it is as secure as AES.
And it has a greater advantage: It was never intended for public use, so it woudn't be backdoored.
Comment: "[GOST] was never intended for public use, so it woudn't be backdoored"; actually, GOST was intended for use within the USSR; it has been speculated that it may have been backdoored (so that some parts of the USSR government could track what other parts of the USSR were doing...)
Comment: Well, When NSA says it is secure we believe immediately. I would rather say weakness to cipher, and that is going to be discovered as soon as published. If you add the attacks with their methods and success rate it will be a nice answer.
Comment: That isn't really very likely since as you yourself say it was all secret government issues, and there wasn't an internet back then; so, even with the cipher, the "Other parts of the USSR government" woudn't have access to the transmission channels by default, and thus not incentivized to use such "Backdoors".

Is the GOST block cipher broken?


Under what circumstances are general relativistic coordinate transformations physically meaningful?

Question: Although the field equations of GR are covariant under arbitrary coordinate transformations, such as the transformation given by Dirac (in Princeton Landmarks pp 34) that eliminate the singularities in the Schwartzschild metric, is it necessarily the case that the new coordinate system, with its singularity free metric, is physically meaningful?
    This question comes up because there are transformations that are clearly meaningless. For example, if an observer makes a transformation from his coordinate system to that of his virtual image seen in a mirror, general covariance is preserved, but there is no actual space that can be entered by an observer.
     Is there a criterion for deciding when the transformation is strictly virtual in the sense above?
     Does the world inside a black hole as described, by say Kruskal coordinates, have the operational meaning of consisting of a world that can be entered and explored by an observer, or is it a strictly virtual construct like the mirror image coordinates?
Answer: The coordinate transformation is never physically meaningful. Certain coordinate systems have some specific meaning.
The coordinates are just sets of arbitrary numbers with some convenient mathematical properties (a smooth and invertible map) but they are otherwise arbitrary. Coordinate transformations are just bookkeeping required to go from one set of such arbitrary numbers to another.
Because the numbering is largely arbitrary, a physicist may arbitrarily choose to assign them based on some physically meaningful quantity, but they are not required to do so. Any physical meaning (or lack thereof) comes from that choice.
Edit: regarding the question about the meaningfulness of the coordinate transforms on the interior of the event horizon in Schwarzschild coordinates. Again, the coordinates are not in themselves physically meaningful; instead physical meaning comes from the invariants. All of the curvature invariants are well behaved and finite at the event horizon, so there is no GR-based reason to think that it is any kind of a barrier. Geodesics crossing the horizon are well behaved and so forth. All invariants are reasonable, so absent unknown quantum effects and direct experimental evidence, the assumption is that the interior is physically meaningful based on invariants, not on coordinates.
Comment: "how does one make this choice when the physical meaning is unclear?" It is completely arbitrary. There are no constraints to making the choice (other than smoothness and invertability). It can be for mathematical convenience, or aesthetic preferences, or whatever. Coordinates in themselves have no physical significance.
Comment: This really doesn't answer the question, which is, how does one make this choice when the physical meaning is unclear--as in the case of the alleged world inside a black hole which might be operationally meaningless in the sense described above.
Comment: If you want to look at physically meaningful quantities then you must look away from coordinates entirely and focus on invariants.
Answer: As an example of a possible virtual coordinate transformation consider the Schwarzschild metric for r less that the Schwarzschild radius. Then make the space-time switching transformation indicated to get the inside metric:
$$ds^2=\Big(1-\frac{r_s}{r}\Big)\ c^2dt^2-\Big(1-\frac{r_s}{r}\Big)^{-1}dr^2$$$$d\rho = cdt$$ $$dr =cd\tau$$
$$ds^2=\Big(\frac{r_s}{c\tau}-1\Big)^{-1}\ c^2d\tau^2-\Big(\frac{r_s}{c\tau}-1\Big)d\rho^2$$.
$$(dr/dt)(d\rho/d\tau) = c^2$$
Notice that the speed of the same particle in both coordinate systems is related by an inversion transformation using a sphere of radius c.
This means that if the speed of a particle in the r,t coordinates must be less than c, then in violation of relativity, it must be greater than c w/rspt to the $\rho, \tau$ coordinates
Conversly, if $d\rho/d\tau$ is less than c, then $dr/dt$ must be greater.
This seems to imply that the transformation must be virtual because it inevitably leads to a contradiction of relativity. A similar problem can be shown to exist using Kruskal coordinates
Comment: All this means is that you have picked odd *names* for your coordinates: this is no more strange than if I picked unusual names for the normal Minkowski coordinates and wrote $ds^2 = c^2dx^2 - dt^2 - dz^2 - dy^2$: this is a perfectly valid set of coordinates but (I hope) no-one will ever write a book using this convention.
Comment: A perfectly valid set if t now plays the role of a spatial variable and x the temporal variable. It follows that the same relativity violating inversion appears under the space-time switching transformation above. The issue is the transformation not the initial choice of coordinates,
Comment: I should add that t and x must play those roles or on a null geodesic, the speed of light would be 1/c.
Comment: @H.Cooper It has no meaning to call speeds both $dr/dt$ and
$d\rho/d\tau$. First, in both cases these are "coordinate speeds",
devoid of any physical meaning and therefore not constrained to be
${}<c$. Moreover, whereas $d\rho/d\tau$ in the inner region is at least
correct in being (space coord. increment)/(time coord. increment),
this isn't true for $dr/dt$.


Installing Linux on a Windows Dell XPS 13

Question: I can buy from a retailer with employee discount. This means that I can get a Dell XPS 13 with Windows for considerably less money than the Dell XPS 13 developer edition from Dell directly. I would like to know whether I can install an Ubuntu on that laptop, or whether this would be a problem (i.e. I would like to know if Dell ships a modified ubuntu or the one from ubuntu).
Answer: I was looking for the same laptop, to run linux on it, but it's not that great...
Linux support for Dell XPS 13 9343
Fortunately, if you're looking to run Ubuntu on it, this month, Dell announced a developer edition running (modified?) Ubuntu 14.04 LTS
XPS 13 Developer Edition


What are the differences between a bible encyclopedia and a bible dictionary? Why would someone use one versus the other?

Question: What are the differences between a bible encyclopedia and a bible dictionary? Why would someone use one versus the other? This is in relation to doing hermeneutics.
Comment: I think they're basically the same. An actual dictionary of the Biblical languages is however called a lexicon instead. Rather confusing.
Comment: thanks for answering
Answer: This may help you: Think of a regular dictionary such as Websters or any that you are familiar with. You use it to look up words that you don't understand. The same is true of a Bible dictionary. Use it to look up words that you aren't familiar with. Regarding a "regular" encyclopedia, you use it to read more details about a certain subject. The same is true about a Bible encyclopedia. You use it to read more about a specific subject that is in the Bible. An encyclopedia has much more detail and has a different purpose than a dictionary does. Hope this helps you know which one to use at various times as you study the Word of God. God bless YOU.
Comment: thanks for answering


do candles burn wood varnish fumes?

Question: A friend of mine bought a piece of furniture that had a super smelly wood varnish, so I read around online and someone said to light candles to burn the chemical fumes.  Does anybody know if there is a chemical reaction between the wood varnish fumes and the candles? they are unscented candles and I assume they can break down the hydrocarbons in the wood varnish fumes.  Are there any other harmful chemical in the wood varnish fumes that are not oxidized by the candles that cannot be smelled by humans?
Comment: He should let it stay to dry. Simply keep it on sun or in a dry, warm well-ventilated room for some time (days to weeks).
Answer: Smelly compounds can be destroyed by free radicals wich are formed by a burning candle. But you need a lot of candles for your apartment.
Candles were also used in bathrooms (after toilet visits) to destroy smelly compounds like $\ce{H2S, CH3-SH, CH3-S-CH3}$ ...
A better and safer way is it too use cyclodextrin. This compound absorbs molecules and destroys them as well. Cyclodextrin is the working stuff inside the air-improvement sprays.

do candles burn wood varnish fumes?


Redundant Fixed Effects Test

Question: What is a Redundant Fixed Effects Test? How do I run it in Stata?
I can't find much about it using the help in Stata or by searching on the Web.
Any help will be appreciated.
Thank you
Comment: *To me*, the term `Redundant Fixed Effects Test` seems to simply imply testing whether or not one should include a particular fixed effects term in his/her model. There are numerous resources on the web (and on this forum) regarding `variable selection in mixed effects models`, `hypothesis testing in LME`, etc. See for example glmm.wikidot.com's [FAQ](http://glmm.wikidot.com/faq). If this is not what you want can you please provide more information/references on what you try to achieve?
Answer: I also don't have enough reputation to either vote up or comment on @marquisdecarabas post, so I'm writing here:
There are two types of tests that you can run.
1) You can compare the pooled regression vs the fixed effects model, by using a simple F-Test or LR test. The FTest is done by Stata with the areg command (the last line of the table).
2) The other test is as marquis said the Hausman test, which you can run with the hausman command (see the help, you need to previously run an xtreg command)
Answer: I do not have enough reputation, so I cannot post this as a comment. A cursory glance at both Google and Google Scholar for "redundant fixed effects test" (use the quotation marks in your search to get hits that contain the exact string) produced search results that you may find usable:

http://www.google.com/#q=%22redundant+fixed+effects+test%22
http://scholar.google.com/scholar?hl=en&q=%22redundant+fixed+effects+test%22

Here's another link http://ebooks.narotama.ac.id/files/Introducting%20Econometric%20Financial/Chapter%2010%20Panel%20data.pdf
The term "redundant fixed effects test" is not used in my field.  From what I gathered, this test is associated with a software called Eviews.  At any rate, based on the search results I linked to above, it seems the term is used to refer to what we call the Hausman test.  You should establish first whether the two tests are the same.  If they are, search in Stata or Google for the Hausman test in Stata, and you will have no problems figuring out how to do it then.
Answer: The redundant fixed effects test in EViews is not what is commonly called the Hausman test. The Hausman test, at least in my research area, refers to a test between fixed and random effects.  Eviews implements fixed effects with dummy variables and the redundant fixed effects test in Eviews is simply an F-test if all dummies are equal to zero.
Comment: What is EViews?

Redundant Fixed Effects Test


What is the curve fitting model used for the "Qubit™ Protein Assay Kit"?

Question: This is the example of Qubit™ Protein Assay curve.
It relates the concentration (x-axis) with fluorescence (y-axis).
What is the curve-fitting method it use for this? Clearly this is not simple linear fitting model.
Comment: It is clearly a non-linear fitting but the method used can not be identify only from the graph.
Comment: The caption to Fig. 3 in your link calls it a "modified Hill plot" without clarifying what modifications were made. I thought a Hill plot was of the log-log type which isn't the case here, so maybe they instead mean the fitted curve is an approximate solution of a (modified?) Hill equation.
Answer: This is Fig 3 of the PDF that you linked to us, as pointed out in the comment by Anyon. The captio to Fig 3 is:

"Figure  3. The  curve-fitting  algorithm  used  to  determine  concentration  in  the  Qubit™  protein  assay.  The  Qubit®  2.0 Fluorometer  generates  concentration  data  based  on  the  relationship  between  the  two  standards  used  in  calibration. This  plot  shows  the  line  corresponding  to  the  curve-fitting  algorithm  (a  modified  Hill  plot)  used  in  the  calculation  of concentration data for the Qubit™ protein assay. For reference, the positions of the standards and a set of data points from an actual experiment are shown superimposed onto the line, demonstrating that the curve-fitting algorithm gives accurate values for quantitation."

The curve in the figure also does look like one that you would get from a Hill equation (nevermind the log-log "Hill plot" which is different). The word Hill only appears in that 9-page PDF document once (in precisely the quote figure caption above), so it doesn't look like they wanted to tell you any more.
At the top of the PDF we see two commercial company's names, Invitrogen and Molecular Probes and the PDF is about the trademarked "Qubit™ Protein Assay Kits For use with the Qubit® 2.0 Fluorometer", which means that we're probably not dealing with open accecss science, but with commercial products, which rarely disclose details about calculations to the extent that (some!) scientists more often do when publishing academic papers.
You can try to contact them (all this information was fortunately in the PDF to which you provided a link), but I'm not sure if the model matters to you enough to bother with that:

"Molecular Probes, Inc.
29851 Willow Creek Road
Eugene, OR 97402
Phone: (541) 465-8300
Fax: (541) 335-0504
Customer Service:
6:00 am to 4:30 pm (Pacific Time)
Phone: (541) 335-0338
Fax: (541) 335-0305
firstname.lastname@example.com
Toll-Free Ordering for USA:
Order Phone: (800) 438-2209
Order Fax: (800) 438-0228
Technical Service:8:00 am to 4:00 pm (Pacific Time)
Phone: (541) 335-0353
Toll-Free (800) 438-2209
Fax: (541) 335-0238
firstname.lastname@example.com
Invitrogen European Headquarters Invitrogen, Ltd.
3 Fountain Drive
Inchinnan Business Park Paisley PA4 9RF, UK
Phone: +44 (0) 141 814 6100
Fax: +44 (0) 141 814 6260
Email: email@example.com
Technical Services: firstname.lastname@example.com"

The link that you gave us happened to be hosted on a Smith College server for the "Center for Molecular Biology (CMB)"  but there's not "About us" or "People" page so it's hard to see who the contact person would be. Likely the PDF was not on the Smith College website due to them having anything to do with product, other than the fact that it's equipment that's available to their students. The only email address I found on the CMB website after a few clicks and a couple minutes was given here:

"Users will need to be granted access to use the equipment by the CMB's ITI, Riccardo Racicot, at email@example.com"


What are the some common practices for storing and accessing data for analytics?

Question: If you need to access application information from a database for analytics, is it better to copy the information first and analyse the copy or is it okay to analyse the data retrieved fresh from the application database?
Comment: Does anyone know if posts can be marked as "community" posts here or is that just on SO?
Comment: CW functionality has been greatly scaled back from the old days. I think you have an option when creating the post to start with, otherwise, you would need to flag it for a moderator to change it.
Answer: The problem with nosql is that it's like going back in time to the age of tape drives. From my experience - it completely sucks for almost everything which is small to medium sized, especially statistics where you have very large number of rather small rows, you need to access many of them and REAL DECIMALS, not CRAPPY FLOATS are sometime nice to have. I was recently thinking where the hype is comming from... i think it's from multi-million corporations which could afford thousands of servers running nosql because this way is much cheaper and safer to scale large size apps than hiring large engineerig teams to design and maintain SQL cluster.
And it's really suprising that their devs who need to manually sanitize every piece of data are not ending up in mental asylums in large number ;)
I was doing some tests of some best available nosql solutions like 2 weeks ago, when putting 1 day worth of stats (about 2-3 million row) - what was taking less than 100 ms in mysql, unoptimized - it took over 10 seconds in nosql with some optimizations on same hardware. Not to mention that standard aggregating functions were failing because there was "too many rows to agregate" and i had to write map-reduce jobs. Which is really pathetic. Some simple, aggregated/sorted query which took 20 seconds to write in SQL will take days to do in nosql "database" and still suck.
I think if you don't have 5-10 servers and you're not extremely bored looking for something stupid to do with your life - then forget it. JSON is crap for storing large amount of data. The time needed to parse this bloated shit and send it around will kill you. There are some legitimate cases for a tape drive, and for nosql... but it's short signed to use a tape drive just because "it has more space".
If you worried about additional statistics - store most of the data in normal columns, and use JSON columns to store everyting exotic. At least users won't wait 10 minutes to get a report on commonly used metrics, and for less used stuff you can put a warning it'll take eg. 20 minutes to generate.
For using application DB for doing long, analitical queries... you can design it "properly" using some MVCC based engine wich is very hard to do and then you could make your queries alongside application queries without locking data, or you can use standard way of "moving data around" called replication in master-slave mode, do analitical queries on replica, and save yourself couple months of life plus days of application's downtime when it'll turn out that MVCC isn't always as great as advertised :)
Answer: This is pretty broad
First you need to design a database for day to day features.
Likely you will want some real time analytics for the user that you most likely just use existing database design.
For historical data you could have dedicated table where a column is the feature name so you can perform analytics on features. And / or look for correlations across features. That can be an external NoSql like database but I would not call it unstructured data as need enough structure to run the analytics.


To find out last activity/access date and date of expiration for all SQL Server 2012 User login

Question: Apart from enabling the Audit on the SQL server, is there any other way we could find out the last activity/access date and date of expiration for all SQL Server User login ?
Also using DMVs wont work as the information provided by them are limited.
Comment: Did you checked the default trace. If the activity is was recent you could be lucky.
Answer: For logins, only things that are like server audits.

A SQL trace to capture logins. This is more intensive than using an audit though.
An extended event session to capture the logins.
You could cobble up a login trigger to record logins to a table, but this is probably the worst of them all.

For expiries you can use Loginproperty('login_name', 'DaysUntilExpiration') but I think this is for SQL logins only. You may need to run a separate PowerShell script to determine the same for AD logins.
Neither of these will help when it comes to an AD account access through an AD group. For those it's much more complicated, either through impersonating each login and selecting the list of activate groups from sys.login_token and recording it all to a table; or cycling through AD with PowerShell again.


How do I deploy my own GPG keyserver?

Question: I would like to set up a read-only key-server operating by me where I am going to publish my own keys. So that anyone could do gpg --keyserver example.mymedia.su --receive-keys 0xDEADBEEF and get the key. Is it some sort of HTTP server? Is it enough to just put static files and serve them via Apache? If so, in which directories they should be located?
I do not like that I cannot make any changes of my key on so-called public key-server network. However, it would be nice to support a GnuPG compatible protocol to share the key.
Answer: There are many keyserver implementations available like Hockeypuck.
Answer: The sequoia-pgp team built Hagrid the "keeper of keys". It is written in Rust and is based on Sequoia-pgp which itself is a an OpenPGP Library.
I did not deploy an instance yet, due to lack of time, but it might be a solution to your requirements. It is already running on https://keys.openpgp.org

How do I deploy my own GPG keyserver?


Inverse Fourier Transform of $e^{-ck}/k$

Question: How do I find the inverse fourier transform of a function of the form $$\hat{f}(k)=\frac{e^{-ck}}{k},$$ with $c$ being some constant (can be complex)? The definition of the inverse fourier transform that I am using is $$f(x) = \frac{1}{2\pi} \int_{-\infty}^{\infty} e^{ikx}\hat{f}(k)\, dk$$
I have tried direct integration which has led nowhere, and I cannot come up with some function which gives this as its Fourier transform.
Thanks :)
Comment: There are some integrability issues. Are you sure $\hat{f}$ is in the given form? My bet is on $$\widehat{f}(k) = \frac{e^{-c|k|}}{|k|},$$ instead. In such a case the inverse Fourier transform is related with $2\gamma+\log(1+s^2)$.
Comment: If $\int_{-\infty}^{+\infty}e^{isk}\hat{f}(k)\,dk$ is not a converging integral, there **has** to be a mistake somewhere.
Comment: I am fairly sure$\hat{f}$ is of that form, since it comes from taking the Fourier transform of the 2D Laplace equation with respect to one of the spatial co-ordinates, which gives $f''-k^2f=g$ where $g$ is just the Fourier transform of the forcing term. I then used Green's functions to solve which gives exponentials (some imaginary, some not).

Can you see any mistakes in that?
Comment: Also I am not familiar with the Fourier transform you mentioned: $$2\gamma + \log(1+s^2)$$ so could you please elaborate on this? Or send some link where I can see it?
Comment: Hmm, OK thanks I shall run through the question again, hopefully the mistake is amended. :)
Answer: If $c$ is a pure imaginary number, then the transform can be computed in the sense of tempered distributions. Let $1/k$ be the distribution defined by
$$\left( \frac 1 k, \phi \right) =
\operatorname{v.\!p.} \int_{-\infty}^\infty \frac {\phi(k)} k dk,$$
then
$$\mathcal {F} \!\left[ \frac 1 k \right] =
\left( \frac 1 k, \frac 1 {2 \pi} e^{i x k} \right) =
\frac i 2 \operatorname{sgn}x, \\
\mathcal {F} \!\left[ \frac {e^{-c k}} k \right] =
\mathcal {F} \!\left[ \frac 1 k \right](x + i c) =
\frac i 2 \operatorname{sgn} (x + i c).$$
If $c$ has a real component, the answer will depends on the chosen space of test functions. Gelfand and Shilov's book defines the Fourier transform for distributions acting on functions with finite support. Then $\mathcal {F}[e^k]$ is well-defined, but it is a distribution living in a different dual space (acting on entire functions with a certain condition on their rate of growth). For the Schwartz space of functions of rapid decay, $e^k$ is not a valid functional, because the integral of $e^k \phi(k)$ does not necessarily converge.
Comment: Thanks, this is an interesting answer. I haven't really studied anything about distributions in much detail, so I don't fully understand what is going on here. I will hopefully be reading about this area soon though, after which this answer may be more accessible for me. What I can say is, I am fairly certain I wouldn't have been required to know about these things when given the original problem, so I probably made a prior computational error (unfortunately I can't quite remember or find the work). I will accept this answer since it would be the answer that future readers would be looking for
Answer: As in @JackD'Aurizio 's observations, there was probably a prior computational error: in solving $(\Delta-s^2) u=\delta$, taking Fourier transforms gives (up to constants) $(x^2+s^2)\widehat{u}=1$, and $\widehat{u}(x)=1/(x^2+s^2)$. The inverse Fourier transform can be evaluated by residues (unsurprisingly), and/but this technique gives slightly different results depending on whether $x$ is positive or negative...
For that matter, we should certainly expect that a Fourier transform of a tempered distribution is a tempered distribution, and something like $e^x$ is definitely not a tempered distribution, whatever virtues it may have. So any computation that produces this as an alleged FT of a tempered distribution is ... doubtful.
And, then, indeed, something like $e^{-|x|}$ (with constants) is surely the right thing...
Comment: Unfortunately, I can't quite remember what the prior computations were. Looking at the question again now, I can see the issues with performing this integral too, so it is probably true that I was looking for a different Fourier transform.
Comment: @paulgarret Another formal 'indeed.' Another hackey formalism. Lose that and I'll upvote.


How can I check if handlebars are straight?

Question: My motorbike took a pretty hard hit and I am suspecting that my steering is crooked. How can I check if my handlebars are straight/does not deviate from its originally intended shape?
Comment: Is it a single piece handlebar or "clip on" style handlebars?
Comment: Single piece motocross-style handlebar.
Comment: Are you just focusing on the handle bars themselves or is this all part of a total answer toward your other question posted.
Comment: @spicetraders This question here focuses on the bars themselves. The other focuses on steering.
Comment: Hold the handle bars straight and see if the bike goes straight. Also, I mean, does it *feel* off? That's all that matters: safety and feel. So if it feels off, trust your feeling, and adjust until it doesn't. If you've been riding your bike for a while, your brain is essentially keeping it straight with a well-trained neural control feedback loop (colloquially, "second nature"); your feeling is probably correct, even if you can't put a finger on it.
Answer: Since your motorcycle has handlebars that are a single piece, remove them from the motorcycle and remove all accessories from them.
Check for obvious signs of bending such as cracked paint or metal. If these are present, the bars are definitely bent.
The bars should be symmetrical, so you'll want to get an angle finder. You can place it on each bend of the handlebar and compare the angle with the same bend on the opposite side. If they do not closely agree, then something is bent somewhere.
Replacement aftermarket handlebars are generally not very expensive, so the easiest course for correcting bent handlebars is replacement.
Here is the type of tool you need
Comment: How closely is closely? :)
Answer: Find a flat surface

Take all of the accessories off of you handlebars, strip them clean of any additional item.
Set the handlebars on the flat surface.
You should have 4 points of the handlebars that are hitting the flat surface.
If you have 4 points all hitting the flat surface at the same time, you handlebars are straight.  If you DO NOT have 4 points touching the flat surface continue to step 5.
If you have only 3 points on the handlebar touching the flat surface at the same time you; rock the handlebars on the two highest points. You may be able to figure out how they are bent.  The point being, they are bent.

Granite countertops are good flat surfaces.  Sometimes within .005 inches.  Cement slabs frequently are not good, having too much variance in height.


How to consolidate config files in asp.net solution?

Question: I have several projects and a website in a large asp.net solution.  In some of projects I have an app.config file and in the website I have my web.config.  In each config I specify several things that are common amongst them, such as connection strings.  Is there any way to consolidate these common items into one config file and reference that from the various projects that need them?
Answer: You can do this with a pre/post compile step in the project to copy the config file from say the solution directory.
A few years back, this was the solution that was recommended by scott hanselman.
Answer: You can reference external config files in any .config (see http://www.peterprovost.org/blog/post/External-Config-Files-in-NET.aspx).
Each project has a config file; these files get published to the bin directory as [output name].config (e.g., MyCode.DLL, MyCode.DLL.config).  You can then reference these .config files from your main config (web.config).
If you find your config files aren't finding their way to the correct destination, you can do the post compilation step that ChanChan mentioned.


Why do atomic radii decrease across a period?

Question: It's said that atomic size/radius decreases across a period, in spite of addition of electrons. But how does this actually work?
Answer: Atomic radius decreases across a period because  electrons are being added to the same energy level and at the same time the nucleus is increasing in protons. The increase in nuclear charge $$Z_{eff}$$ attracts the electrons more strongly, (addition of one proton has a greater effect than addition of 1 electron) thus pulling  electron closer to the nucleus and making atomic radius decrease.


WPF: Master - detail view with two datagrids and in MVVM

Question: I'm trying to write a master - detail control that consists of a master datagrid and the detail datagrid. My scenario was following - I used the SelectedItem and bound it to a property in ModelView. The problem is - the SelectedItem in ViewModel is never used, so I can't get the information which item is selected in a master datagrid and cannot fetch the data for thos selection.
The code is below:
<toolkit:DataGrid ItemsSource="{Binding}"  RowDetailsVisibilityMode="VisibleWhenSelected" SelectedItem="{Binding SelectedItemHandler, Mode=TwoWay}"></toolkit:DataGrid>

And in ViewModel
private CustomerObjects _selectedItem;
public CustomerObjects SelectedItemHandler {
                get { return _selectedItem; }
                set
                {
                         OnPropertyChanged("SelectedItem");

                }

            }

The code in SelectedItemHandler is never used.
What could be the problem? Should I use another approach to create master - detail in MVVM?
Answer: You might be interested in the EventToCommand behavior provided by the MVVM Light Toolkit which will allow you to work with for instance the LostFocus (or any other for that matter) event of the Master control. Other approaches might include using a DataGrid for the Master and TextBox or TextBlock controls for the Detail.


Are there new states of matter at ultrahigh temperatures and densities?

Question: Under extreme energetic conditions, matter undergoes a series of transitions, and atoms break down into their smallest constituent parts. Those parts are elementary particles called quarks and leptons, which as far as we know cannot be subdivided into smaller parts.Is there exist new states of matter?
Comment: Have a look at the Wikipedia article on [degenerate matter](http://en.wikipedia.org/wiki/Degenerate_matter) for an overview of the various suggestions, but note that this is all speculative. We not even know for certain the equation of state for the matter in neutron stars, let alone the more exotic forms of degenerate matter.
Comment: There is QGP - http://en.wikipedia.org/wiki/Quark%E2%80%93gluon_plasma
Answer: Yes, collider experiments show that a new state with properties of a perfect fluid is entered.
"RHIC [Relativistic Heavy Ion Collider] experiments and Brookhaven National Laboratory (BNL) have recently announced the discovery of a new form of matter with the properties of a 'perfect fluid', i. e. of a strongly interacting plasma-like state of nuclear matter with an extremely low ratio of shear viscosity to entropy". Quark Matter 2005.
"When heavy ions (the nuclei of heavy atoms such as gold and lead) collide at high energies at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) and Europe's Large Hadron Collider (LHC), the components of the nuclei (protons and neutrons) melt to form a hot soup of their constituent particles, quarks and gluons. A new model that accurately describes the experimentally observed patterns of particles flowing out from this "quark-gluon plasma" (QGP) suggests that the effective shear viscosity, or resistance to flow, is close to the ideal limit used to define a 'perfect' fluid." RHIC's Perfect Liquid a Study in Perfection
Comment: It's debatable if it is an ideal fluid or not (or even if it is a fluid), since there is strong evidence that there should be some kind of viscosity, though small. Anyway, it is trully some kind of new, collective, state of hadronic matter
Answer: Motivation
Since matter consists primarily of baryons, which are bound states of quarks and gluons described by quantum chromodynamics, one can ask the question in the framework of this theory. The so-called QCD phase diagram is actually a huge field of research, both experimentally and theoretically. Below you can see a schematic version of this diagram, where $\mu$ represents baryon chemical potential which can be thought of as a density parameter, while $T$ is temperature.

As you can see, the phase structure is highly nontrivial, and there is even more to it than one can see from this structure. For a more thorough explanation, you might consider this review article, I will only give a quick description of the most important points.
High Temperature
At low temperature, QCD is confining, which means that quarks and gluons are bound into hadrons and do not exist as free particles. At high temperatures, however, matter undergoes the so-called deconfinement transition, beyond which quarks and gluons no longer exist as hadrons, but can move more or less freely. This state of matter can be described as a viscous fluid, also referred to as quark-gluon plasma. Its existence has been verified experimentally at LHC and RHIC (as mentioned in the other answer).
High Density
While high temperatures are relatively easy to study in colliders, systems with high densities are hard to capture in experiments. One natural system in which extreme densities are expected to occur is inside neutron stars.
At very high densities, the coupling is weak, which allows one to describe matter in this region as a weakly interacting quark liquid. This allows pairs of quarks to condense, essentially forming a superconductive phase, also referred to as colour superconductor. Another concept relevant at high densities is so-called colour-flavour locking. In this phase, both colour and flavour symmetry are completely broken.


Full Product Catalog Import Possible?

Question: Is there any possible way to make one spread sheet that generates configurable products as well as all the simple products, link them and also contain image data? I have found "MAGMI" but it says I should do them in pieces of simple products then configurable products. I am trying to import a configurable tshirt product with simple sizes and colors.
Comment: With magmi you can do it in a single file.you just need to put simple items before configurable ones. no need to split in two files.Read carefully magmi wiki about configurable & needed columns.
Comment: [What have you tried?](http://whathaveyoutried.com/) Did you read? http://www.magentocommerce.com/knowledge-base/entry/tutorial-using-magentos-import-export-profiles
Answer: Here's another answer with some information how to import simple/configurable keeping the associations using Magmi.  Looks like it should be possible:
https://stackoverflow.com/a/16766075/1117286
Answer: In my opinion and as far as I've used both Magmi and the native Magento Import/Export tool (introduced since v 1.5.x), the latter can represent an effective replacement to Magmi.
I suggest you to start from the following resource by Vinai Kopp
http://netzarbeiter.com/media/MagentoImagine/ImportExport.pdf
Regards,
Alessandro
Answer: Yes it's possible, but not out of box in Magento. If you consider paid extensions in order to accomplish this task check out  this one.
Here is a sample format, check out tab "Configurable".
With this extension, you can import only configurable products, and it will automatically generate and auto-link needed associated simple products variants.
I think this might be a good alternative to the enterprise solution mentioned by Serpyre.

Full Product Catalog Import Possible?


Why is $\{0,1\}^n$ referred to as the Boolean hypercube?

Question: I used to view it just as a set of bit strings of length $n$. What does it mean for it to be the Boolean hypercube? Does viewing it from the hypercube perspective give a useful insight in a certain context?
Answer: Strictly speaking, the "Boolean hypercube" is not the same as the set $\{0,1\}^n$. The Boolean hypercube is the graph whose vertices are the set $\{0,1\}^n$, and whose edges are defined as follows: two strings are adjacent if and only if they differ on exactly one bit.
I guess that usually, people refer to $\{0,1\}^n$ as the Boolean hypercube when they really want to think of it as a graph and not as a set of strings.
Comment: I am not sure this is really the entire picture. I think the Boolean hypercube also evokes the polytope whose vertices are $\{0, 1\}^n$. Then one can talk about the faces of this polytope and identify each face with the vertices that span it (the subcubes Yuval talks about). Or one can talk about the 1-skeleton of the polytope, which is the hypercube graph you mention.
Answer: The name "Boolean hypercube" is mainly just that — a name. But in one context it is useful to talk about subcubes. Namely, Harper's isoperimetric inequality shows that the set of measure $2^{-k}$ with least edge boundary is a co-dimension $k$ subcube.


DTM, DSM, DEM, LiDAR and Lakes

Question: Would any of these show the depth of lakes in great detail? i.e. a LiDAR at 25cm, will it show the detail of the lake bed?
Comment: sonar (Echo Sounding) is better than lidar for this if the device sending the sonar can be accurately measured with location (gps).https://en.wikipedia.org/wiki/Sonar#Echo_sounding
Answer: Standard LiDAR collectors do not do a good job at measuring water depth; they aren't designed to. The pulses are often either reflected in a different direction or absorbed by the water feature. The only pulses are returned to the sensor are those that are perpendicular to the water's surface. Except for some very shallow features in relatively clear water, the pulse returned often represents the surface.
To map a lake bed you would want to conduct a bathymetric survey, using sensors designed for that specific purpose. The sensor would use wavelengths optimized for penetrating surface water and its contaminants. Even then its effectiveness is limited. The best results from a bathymetric LiDAR sensor I've heard of is only up to two times Secchi depth.

DTM, DSM, DEM, LiDAR and Lakes


Pseudo Random Number Generator test uniform distribution

Question: If I test the Pseudo Random Number Generator(PRNG) and it satisfies the serial test, it is not sure that the resulting distribution is uniform. Is my observation correct? If it is correct, what test should I do to be sure?
Comment: \[not an answer!\] I guess you consider this [serial test](https://nvlpubs.nist.gov/nistpubs/legacy/sp/nistspecialpublication800-22r1a.pdf#%5B%7B%22num%22%3A448%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22Fit%22%7D%5D). Are you looking for the [monobit test](https://nvlpubs.nist.gov/nistpubs/legacy/sp/nistspecialpublication800-22r1a.pdf#%5B%7B%22num%22%3A141%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22Fit%22%7D%5D)? That tests uniformity, but not independence of the bits. Nothing can reliably test the later.
Comment: I'm trying to understand mainly if the PRNG passes the serial test then it is not uniform distribution and why?
Comment: @fgrieu Standard permutation testing can identify independence :-)
Comment: @Paul Uszak: consider the PRNG based on a 64-byte hash $H$ (e.g. SHA-512) and 32-byte constant $C$, that given 32-byte seed $S$ first outputs 64-byte $R_0=H(C\mathbin∥S)$, then 64-byte $R_{i+1}=H(C\mathbin∥R_i)$. New output $R_{i+1}$ is dependent on $R_i$, and nothing else variable. Yet no test will identify that, or otherwise distinguish from random the outputs of the PRNG for different seeds, unless the test somehow knows $C$. Testing can sometime identify dependence. Does that mean _"testing can identify independence"_? You decide on that semantic issue.
Comment: @fgrieu Oh I see! I thought that you were referring to the output sequence since we're talking about tests on that...
Answer: I assume the question is about this serial test.

If I test a Pseudo Random Number Generator (PRNG) and it satisfies the serial test, it is not sure that the resulting distribution is uniform.

Yes. No test on the output of a PRNG is able to make one sure of anything about the distribution of the output of the PRNG, for the mathematical definition of sure. Argument: it's possible that the PRNG outputs 0 starting at the $2^{160}$-th bit that it outputs, and it's trivial to make such PRNG that passes all practical tests ever devised. I admit this argument of correctness of the quoted proposition is not helpful.
For something more helpful, we must reach some functional interpretation of "sure that the resulting distribution is uniform". For a start, we should decide if this shall hold for a PRNG which computes it's first output bit from the seed, then always outputs the complement of the previous output bit (so that it's output is either 0101010101… or 1010101010… depending on the seed).

If we accept that for such pathological PRNG "the resulting distribution is uniform" can hold (perhaps, according to how the first bit is computed), then we are interested only in the frequency of bits in the output. The best possible test on the output of a PRNG for the property that we want is the monobit test.
Otherwise, our interpretation of "sure that the resulting distribution is uniform" likely is that we want assurance that the output bits of the PRNG are indistinguishable form a uniform distribution of independent bits.

For both meanings, the quoted proposition is true if parameter $m\ge2$ in the serial test. That parameter defines the length of the bit subsequences which frequency is assessed. Argument: it's possible to construct a PRNG which output sequences for a given (nontrivial) length both reliably pass the serial test, but reliably fail the monobit test. The idea is that the subsequences of $m$ bits have a distribution within the tolerance of the serial test, but overall the bias of individual bits is above the threshold of the monobit test.
For $m=1$ the serial test aims at being equivalent to the monobit test, and I think it is; that would arguably make the proposition false for meaning 1 and $m=1$.
For meaning 2, the proposition is true because it's impossible to experimentally reach the desired conclusion by any test on the output of a PRNG, even if we accept a high residual probability of falsely reaching that conclusion (say, 25%). Argument: even if many PRNG tests succeed, it's always possible that the output sequence(s) of the PRNG have some regularity that escape these tests, but is caught by a test tailored to the internals of the PRNG. The best a PRNG test can do is disprove, within an extremely small residual probability of the contrary (say, 0.01%), that "the resulting distribution is uniform" for meaning 2; and that can occur only when the test fails.

What test should I do to be sure?

Again, for meaning 1, the monobit test is the best there is. And even if that passes, you can't be sure.
For meaning 2, no test on the output of a PRNG can give a meaningful assurance (much less certainty) "that the resulting distribution is uniform". Only examining the construction of the PRNG can give such assurance.


Openswan/IPtables: Faking a unique IP address over VPN tunnel

Question: Context:
I'm running a set of instances within the Google Compute environment. Each host has a single interface with single ip in the 10.0.0.0 private range.
I have external customers who require connectivity to these instances over a VPN connection. Due to their size however, these customers require a unique "real" ip address as the target.   (To avoid the risk of having multiple clients with the same private address.)  Before moving to Google we were able to hairpin these connections into our internal network using a Cisco ASA and virtual public interfaces but the static external addresses that attach to GCE instances can't be hairpinned using the Google VPN/Routing tools so that's not an option.  (I've confirmed this with Google Support.)
Proposed Solution:
I've reserved a static address in the GCE environment (Let's call it 5.x.x.x)
Within the GCE environment, I have an Openswan box that can route into the internal network over an ipsec connection from a remote ASA.  I'm able to ping hosts within the private GCE network from that remote network.  (when 10.0.0.0/16 is configured as the target)
The goal now is to change the target for the VPN configuration to the 5.x.x.x address, then have that target translated by iptables on the GCE Openswan box to point to a specific ip address (10.x.x.x) on the private network.  I've configured iptables such that pings to the 5.x.x.x address translate correctly on the Openswan box itself but so far I haven't had any luck having this work for hosts behind the ASA.
Iptables looks like this:
<code># iptables -L -t nat
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination
DNAT       all  --  anywhere             x.x.x.5.bc.googleusercontent.com  to:10.x.x.x

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
DNAT       all  --  anywhere             x.x.x.5.bc.googleusercontent.com  to:10.x.x.x

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
</code>
Any help would be hugely appreciated.  This was a problem we didn't anticipate when we moved to Google but it's one I suspect will impact many larger customers down the line.  We're hoping they'll eventually offer a solution but in the meantime this is what we've got.
Answer: You can set multiple internal static IP addresses for every VM instance using Google Compute Engine routes. For the steps visit this article.


Futuristic prison escape movie where convicts wear linked collars

Question: In this movie, convicts are forced to wear collars linked to other convicts (they don't know which ones), and if one prisoner moves too far from the other, both collars explode. Two prisoners figure out their collars are linked to each other and escape by staying close together. One was played by Rutger Hauer, and the other by a woman.
Comment: Deadlocked: Escape from Zone 14 (TV Movie 1995)
Comment: @BatiRamirez- the movie I was looking for was before that- also it was made for theaters (I think) and I don't think Rutger Hauer was in your suggestion.
Answer: This is Wedlock (1991), starring Hauer and Mimi Rogers.  It was originally titled "Deadlock," and that name was used in a lot of promotional material (such as the trailer below); however, I do not think the "Deadlock" name was used in any actual prints of the film.
Per Wikipedia:

After stealing diamonds in a robbery, diamond thief Frank Warren is betrayed by his best friend Sam and his fiancée Noelle, who turns him into the authorities. Frank is sentenced to 12 years imprisonment at Camp Holliday, an experimental prison where each convict is given an electronic collar containing an explosive device which is electronically connected to another inmate. If any inmate tries to escape from Camp Holliday and is separated from the collar-mate by more than 100 yards, their collars will explode. Frank learns the inmate he is connected to is Tracey Rigg and Tracey and Frank both escape with their collars intact. On the run from the authorities, Tracey and Frank find they are being pursued by Sam and Noelle, believing Frank will lead them to the diamonds, which he has hidden. Can Frank and Tracey get to the diamonds without separating from each other more than 100 yards?
Comment: I saw it on VHS in the UK and I think it was called Deadlock on the box but I might be misremembering.


VW Golf 5 FSI Jerks When Driving

Question: Please help. I have recently serviced my VW Golf 5 with air cleaner,oil filter,fuel filter and plugs but the car still jerks most times when driving. I had it diagonised and gave me  error codes P1403 Exhaust Gas Recirculation :control difference & P2540 Low pressure fuel system sensor circuit (ALL IMPLAUSSIBLE SIGNALS). What exactly am I supposed to do? Change certain sensors? Change Fuel pump? Change gear box as the jerking seems to have something to do with gear box engaging.
Comment: Welcome to Motor Vehicle Maintenance & Repair! Can you better describe what you're talking about by "jerking"? Is it vering left/right? Is it engine power we're talking about? Is this a standard or automatic transmission?
Comment: It is an automatic vehicle and by jerking I want to mean behaving like being propelled forward and backwards at the same time. No it is not vering to the left or right
Comment: Why do you consider the P2540 code as implausible? Did you clear the codes and see if they came back?
Comment: Diagonistic person performed the work and results on the machine read ' Implaussible sig' yes. He had 3 errors coming up buy said he managed to delete one other and these 2 remained -P2540 & P1403
Comment: I mean he initially had 3 errors but only managed to delete one but error codes P2540 and P1403 could not be deleted. I personally saw the 'implaussible signal' result against each error.
Comment: Please don't type USING ALL CAPITALS, as in your question's title. It's seen as SHOUTING, and is disliked. It will discourage answers, not encourage them.
Comment: Thank you. I have learnt something and will avoid using capitals.
Answer: "Implaussible signal" does NOT mean the code can be ignored.  It usually means the battery is weak or there was a voltage drop somewhere when the code was stored, as is the case during startup.  You may want to have the battery tested at an auto parts store and clean the battery cables, ground straps, and other related connections.
The P1403 means the EGR valve should most likely be replaced.  It tends to get dirty and clogged with soot on VWs.
P2540 is probably a result of code P1403.  As the engine bucks and hesitates, the fuel pressure drops (or varies at least).  I would replace the EGR valve first and see if P2540 clears by itself.
Comment: Thank you so much your advise. I will first take the above steps and see if the problem will be sorted out. I then will definitely post a fed back so that others can also learn from it. God bless.
Comment: You're welcome.  I would love the hear how you resolved the problem.

VW Golf 5 FSI Jerks When Driving


Write text file from batch file with embedded vbCRLF

Question: I am using a batch file to write a vbs email file. I am unable to create the text body of the email which includes the vbCRLF code. When the TextBody is written to the Target file, it has executed the first vbCRLF, and the DOS returns errors as it tries to execute the second vbCRLF and the last string. Enclosing it in doucble quotes doesn't work.
echo.TextBody = "Hello,blah blah blah" & vbCrLf & vbCrLf & "worldblah blah", 0, "Message">>Target.txt
The vbCRLF are used to foamt a lengthy text body for the email.
Your help is appreciated.
Answer: Replace the & with ^&:
<code>echo.TextBody = "Hello,blah blah blah" ^& vbCrLf ^& vbCrLf ^& "worldblah blah", 0, "Message">>Target.txt
</code>
It's related to this: Escape angle brackets in a Windows command prompt


Scrolling has suddenly stopped working in wordpress site

Question: I have tried turning all plugins off, deleting all custom CSS, deleting new entries, etc.  Suddenly, scrolling stopped working on all single posts in my Website... My client is pissed, and I can;t figure out where the problem is.  The URL to see this not working is http://urm.case42.com/market-report-august-29-2014/
Comment: Look at the errors in console... There are like 5. Why don't you fix them?
Comment: Please DO NOT link to live sites.
Comment: Sorry - I will not link to a live site again - I'm aware of all the errors - removing the plugins eliminated most of them, with no resulting change.  I do appreciate the help, thanks.
Comment: I'd love to just delete this stupid question. I'm sorry for any time I have wasted. It was answered nicely, but I didn't realize I had posted out of the rules...
Answer: I'm not quite sure why it is happening, but you can fix it with this:
html {
    overflow: scroll;
}

As others have pointed out, there are a ton of console errors. I would imagine that a rogue javascript plugin is causing this.
Answer: on basic.css  you are define overflow:hidden at line 3 remove this attribute.
Answer: The page is riddled with errors.

I'd start by checking out line 836, where it looks like your google font import has a markup error.
Comment: Thanks - The errors mostly go away when all plugins are disabled, with no functional change - I will be going back to fix errors later, but I do appreciate the help.


Quickie ERD maker

Question: I've been using Visio to make ERD's, but I have a boatload of resource-hogging things running right now and wondered if there weren't something very light I could use instead..
Answer: I have created a python3 package that does database inspection. It includes a tool, that creates ER diagrams from the database schema. You can check it here: http://code.google.com/p/fathom/ and it also documents how to generate a diagram: http://code.google.com/p/fathom/wiki/Manual#fathom2graphviz
Answer: Depending on your Database, it may have one built in. If you're on a Microsoft stack, SQL server has an ERD tool built in.
Also check out This Post

Quickie ERD maker


How are pole-zero plots, magnitude response plots, and phase response plots related?

Question: Given that the Laplace transform of a continuous-time signal $h(t)$ is $H(s)$, what can a plot of the poles and zeros of $H(s)$ on the $s$-plane tell me about the magnitude response and phase response of $h(t)$?
Comment: Nice notes! Thank-you for the contribution. Just wondering... why? :-)
Comment: @PeterK. For so long, I struggled to understand how pole-zero plots and frequency responses are related. I've checked different books, including Proakis, but didn't get an intuitive answer. Once I saw Richard Lyons' 3D diagrams in *Understanding Digital Signal Processing*, everything clicked. It is now very easy to differentiate between a lowpass filter and a highpass filter just by looking at the pole-zero plot. Just thought this intuition should be more public.
Comment: @PeterK. there are also other intuitions: what a Laplace transform does and why magnitude and phase plots are needed in the first place. This originated from the need to visualize complex-input, complex-output functions, which is what the Laplace transform of a time-domain function is.
Comment: Cool! Thanks for the explanation.
Answer: To answer this question, it is important to understand

what a Laplace transform is intuitively doing,
how the Laplace transform is related to complex-input, complex-output functions,
how complex-input, complex-output functions can be plotted,
what a pole and zero visually are, and
what the "magnitude response" and "phase response" of $h(t)$ means.

Note: all code used to generate plots in this answer can be found here.
The Laplace Transform
Functions are the backbone of DSP theory, and in this context, it is no different. The function $h(t)$, for example, takes in a real number $t$, which could represent time, and can be assumed to output another real number $h(t)$, which could represent voltage, current, etc. Similarly, we could define another function $f(x)$ that takes in a complex number $x$ and outputs another complex number $f(x)$. We could also define a different function $g(x)$ that takes in a complex number $x$ and outputs a real number $g(x)$. Given that $\mathbb{R}$ is the set of real numbers and $\mathbb{C}$ is the set of complex numbers, then as a shortcut, we use the notation
$$
h:\mathbb{R} \rightarrow \mathbb{R} \\
f:\mathbb{C} \rightarrow \mathbb{C} \\
g:\mathbb{C} \rightarrow \mathbb{R} \\
$$
These kinds of functions are simple to understand: they take in a number (real or complex), and output another number (real or complex). However, what if we could define a function that takes in a function and outputs another function? If $h$ is a function that takes in a real number and outputs another real number, and if $f$ is a function that takes in a complex number and outputs another complex number, then we could visually represent this new "function" that takes in functions like $h$ and outputs another function like $f$ as

More precisely, if $h:\mathbb{R} \rightarrow \mathbb{R}$ is the input function and $f:\mathbb{C} \rightarrow \mathbb{C}$ is the output function, then we could define an operator $\mathcal{L}$ that takes in a function $h$ and outputs another function $f$ as $\mathcal{L}:h \rightarrow f$. We could then update the visual representation above to

This is exactly what the Laplace transform does. It takes in a real-input, real-output function $h$ and outputs a complex-input, complex-output function $f$. More specifically, if the Laplace transform of a continuous-time function $h(t)$ is defined as
$$
H(s) = \int_{-\infty}^\infty h(t) \ e^{-st} \ \text{d}t
$$
we could condense this definition to
$$
\mathcal{L}\{h(t)\} = H(s)
$$
Note that the complex-input, complex-output function $f(x)$ has been replaced by the complex-input, complex-output function $H(s)$. This is just a change of notation and is not important, since both $x$ and $s$ are assumed to be complex numbers.
Can we plot complex-input, complex-output functions?
We can easily plot the real-input, real-output function $h(t)$, since it only consists of two changing numbers: the input and the output. For example, $h(t)$ could look like this:

Here, we plotted the input on the x-axis and the output on the y-axis.
Suppose we compute the Laplace transform of $h(t)$ to obtain $H(s)$. If $H(s)$ is a complex-input, complex-output function, can we plot it like we did with the example $h(t)$ given above? The problem with trying to do so is that to represent the input to $H(s)$, we need two numbers: the real part of $s$ and the imaginary part of $s$. Similarly, to represent the complex output of $H(s)$, we need two numbers: the real part of $H(s)$ and the imaginary part of $H(s)$. That is 4 numbers in total: 2 for the input and 2 for the output, which would mean that we would need a 4-dimensional plot to represent $H(s)$!
Clearly, a better approach is needed. Recall that a complex number $a + bi$ can also be represented in polar form as
$$
a + bi = Ce^{i\theta}
$$
where
$$
\begin{align}
C &= \sqrt{a^2 + b^2} \\
\theta &= \text{atan2}\left(b,a\right)
\end{align}
$$
where $\text{atan2}$ is the atan2 function. Therefore, a good way to visually represent $H(s)$ is to

Represent the input to $H(s)$, which is $s$, as a complex number in rectangular form ($a+bi$),
Represent the output of $H(s)$ as a complex number in polar form, and then
Create two 3D plots: one for the magnitude $C$ of the output and another for the angle (phase) $\theta$ of the output. More precisely, the first 3D plot consists of $C$ on the z-axis and $a$ and $b$ on the x and y axes respectively, while the second 3D plot consists of $\theta$ on the z-axis and $a$ and $b$ on the x and y axes respectively.

To better understand this, consider the simple complex-input, complex-output function
$$
H(s) = s^2
$$
The first step in the process above is to represent $s$ as the complex number $a + bi$. For the sake of clarity later on, instead of letting $s = a+bi$, let $s = c + di$, such that
$$
\begin{align}
H(s) &= (c+di)^2 \\
&= (c+di)(c+di) \\
&= c^2 - d^2 + 2cdi
\end{align}
$$
Next, step 2 in the process above is to represent the output of $H(s)$ in polar form. Note that
$$
\begin{align}
H(s) &= \underbrace{c^2 - d^2}_a + \underbrace{2cd}_{b}i \\
&= a + bi
\end{align}
$$
Therefore, in polar form, the output of $H(s)$ is
$$
\begin{align}
C &= \sqrt{a^2 + b^2} \\
&= \sqrt{(c^2 - d^2)^2 + (2cd)^2} \\
\theta &= \text{atan2}\left(b,a\right) \\
&= \text{atan2}\left(2cd,(c^2 - d^2)\right)
\end{align}
$$
Finally, the last step is to create two 3D plots, one for $C$ and another for $\theta$. These plots are shown below

Note that the discontinuities in the plot for $\theta$ is due to the atan2 function.
What is a pole anyway?
Here is where it gets interesting. Suppose we plot the magnitude ($C$) and phase ($\theta$) of the complex-input, complex-output function
$$
H(s) = \frac{(s - 0.5)}{(s - 1.5)(s + 2)}
$$

In the magnitude plot for $H(s)$, it is clear that $|H(s)|$ shoots up to infinity near $s = 1.5$ and $s = -2$. Notice that if you take a really long and thin flag pole and plant it at $s = 1.5$, the surface of $|H(s)|$ will never touch it. This is why the point $s = 1.5$ is called a pole. It is not clear from the plot, but $|H(s)|$ does go to zero at $s = 0.5$. We therefore say there is a zero at $s = 0.5$. The corresponding pole-zero plot is

How does all of this relate to responses?
If $h(t)$ represents the impulse response of a continuous-time filter, then we say that the frequency response of $h(t)$ is represented by $H(s)$ evaluated at $s = j\omega$, which is just the imaginary axis on the $s$-plane. Recall that the Laplace transform of $h(t)$ is formally defined as
$$
H(s) = \int_{-\infty}^\infty h(t) \ e^{-st} \ \text{d}t
$$
Let us substitute $s = j\omega$ into this expression to obtain
$$
H(\omega) = \int_{-\infty}^\infty h(t) \ e^{-j\omega t} \ \text{d}t
$$
which is just the continuous-time Fourier transform of $h(t)$! In other words, the Fourier transform "lives inside" the Laplace transform. More specifically, on the $s$-plane, it lives on the imaginary axis. Therefore, the frequency response of $h(t)$ is represented by $H(\omega)$. $H(\omega)$ is still a complex-input, complex-output function, and so we can decompose its output into magnitude and phase components. If we represent the complex output of $H(\omega)$ as
$$
H(\omega) = a + bi
$$
then the magnitude response of $h(t)$ is
$$
\lvert H(\omega) \rvert = \sqrt{a^2 + b^2}
$$
and its phase response is
$$
\angle H(\omega) = \text{atan2}\left(b,a\right)
$$
Together, the magnitude response and the phase response of $h(t)$ make up its frequency response. Therefore, given a plot of the magnitude and phase of the Laplace transform of $h(t)$, to obtain its frequency response, we slice both the magnitude and phase plots along the imaginary axis. For example, suppose that the Laplace transform of $h(t)$ is
$$
H(s) = \frac{(s - 0.5)}{(s - 1.5)(s + 2)}
$$
which is the complex-input, complex-output function used above as an example. Then, we can slice the magnitude and phase plots of $H(s)$ (which are shown above) along the imaginary axis to obtain the magnitude and phase plots of $H(\omega)$, which are represented by the purple outlines in the plots below

We can even zoom into the purple outlines above and then plot 2D versions of the magnitude and phase responses using the scipy package and we can see that they are exactly the same:

Poles pull up, zeros pull down
Recall that the poles of $H(s)$ above are $s = 1.5$ and $s = -2$. Based on the plots above, we would expect that as these poles got closer to the imaginary axis, the magnitude response would be "pulled up" at $\omega = 0$. For example, suppose we move the poles of $H(s)$ to $s = 0.1$ and $s = -0.5$ such that
$$
H(s) = \frac{(s - 0.5)}{(s - 0.1)(s + 0.5)}
$$
The corresponding magnitude and phase responses are

Notice how the magnitude response is very high at $\omega = 0$ (some transparency was added to the surface plots to make the outlines clearer). This is in fact an example of a very selective bandpass/lowpass filter. We can design other filters using this method by noting that zeros "pull" magnitude responses down. For example, here is a highpass filter:
$$
H(s) = \frac{s(s + 0.5j)(s - 0.5j)}{(s - (-0.5 + 1.5j))(s - (-0.5 - 1.5j))}
$$

Further reading
It is clear from this discussion that the 3D magnitude and phase plots of $H(s)$ can be used to determine what the 2D magnitude and phase responses of $h(t)$ are. Also, since the Z-transform is just a special case of the Laplace transform, then a similar interpretation can be obtained for discrete-time signals. Section 6.2 of Richard Lyons' Understanding Digital Signal Processing extends this explanation in more detail. Finally, MATLAB's Filter Designer tool allows you to interactively move poles and zeros and observe the corresponding magnitude and phase responses, which I found to be extremely helpful.
Comment: @mhdadk I like your writing style. I particularly liked your clever sentence, "In other words, the Fourier transform "lives inside" the Laplace transform." (I wish I had written that!) Your sentence reminded me of a quote by the famous sculptor Michelangelo regarding his fabulous statue of David, "Every block of stone has a statue inside it and it is the task of the sculptor to discover it."
Comment: This is not wrong but feels needlessly complicated at times. Example: to calculate the TF of $s^2$ you can first convert to polar and then square. You will simply see that $|H(s)|$ = c^2+d^2$ and $\angle H(s) = 2atan2(b,a)$. It's mathematically equivalent, but much simpler.  It would also help to plot the amplitude on a log scale (e.g. dB) so you can much better see the symmetry between poles and zeros.  That also "prettier" in terms of  the math: If you take the log of a complex number, the real part is the log magnitude and the imaginary part is the phase.
Comment: @Hilmar thanks for the feedback. Didn't know about taking the log of a complex number, very helpful. What do you mean by "TF" in "TF of $s^2$"? Will update the plots soon.
Comment: Transfer Function. If $H(s) = s^2$  and $s = c + id$ , then  $|H(s)| = c^2+d^2$ and $\angle H(s) = 2 \cdot atan2(c,d)$
Comment: @RichardLyons WOW! it is an absolute honor to have you say this! I'm a huge fan of your book and your blog! Your book is the reason why I love DSP, and section 6.2 is the reason this post exists! Thank you so much!
Comment: Your plots are very nice that I have never seen.
Answer: As an additional simple explanation, consider that the transfer function given by a ratio of polynomials, when factored into poles and zeros the transfer function is product of phasors in the numerator divided by phasors in the denominator. For simpler transfer functions this can provide an immediate and intuitive visualization of what the transfer function is in terms of magnitude and phase based on pole and zero locations!
First, consider the difference of two arbitrary complex points on a complex plane, given as $v = p_2 - p_1$ and as shown in the graphic below; describes a phasor where the origin has been relocated to $p_1$. The magnitude of the phasor is the Cartesian distance between the two complex points, and the angle is as depicted in the diagram, with reference to 0° being in the positive real axis direction.

A transfer function when factored into poles and zeros is such a difference of points on the complex plane, and we limit $s$ to be all the points on the $j\omega$ axis when we compute the frequency response:
$$H(s) = \frac{(s-z_1)(s-z_2)(s-z_3)\ldots}{(s-p_1)(s-p_2)(s-p_3)\ldots}$$
Consider this example of a single pole filter, which results in a single phasor in the denominator. If we start at DC, the phasor in the denominator given as $s-p_1$ would be $-p_1$. With $p_1$ negative and real as shown then this denominator would have magnitude $p_1$ and angle $0$. As we sweep $s$ along the $j\omega$ axis in the positive direction, this phasor will both grow and approach $90°$. Since it is in the denominator, the transfer function as we increase frequency will reduce in magnitude and approach $-90°$. Further we now se clearly how the magnitude will asymptotically reduce 6 dB/octave, as the frequency increases, every time we double the frequency, we approximately double the length of the phasor in the denominator.

Resulting in the magnitude and phase response for a single pole filter. Thus we also now see how each pole contributes $-90°$ to the phase response and -6 dB/octave roll-off to the magnitude response (as $f\rightarrow\infty$) and each zero contributes$-90$ to the phase response and +6 dB/octave to the magnitude response.
Comment: Yes that is also good insight. What I was showing here was a simple graphical explanation using the complex values of $H(\omega)$. $H(\omega)=1$ is a phasor that has a magnitude of $1$ and an angle of $0$ radians, compared to $H(\omega) = j$ is a phasor that has a magnitude of $1$ and an angle of $\pi/2$ radians.
Comment: (+1) very insightful. If I understood your answer correctly, we could start with $H(\omega) = 1$, where the current log-magnitude is $0$ and phase is $0$. Then, we could multiply by the zero $(j \omega - (a + bj))$ with log-magnitude $$\frac{1}{2} \log \left(a^2 + (\omega - b)^2\right)$$ and phase $$\arctan\left(\frac{b - \omega}{a}\right)$$ If we let $$X = \log \left(a^2 + (\omega - b)^2\right)$$ and let $Y$ be the log-magnitude, then it is clear that $Y$ is a linearly increasing function in $X$...
Comment: ...Also, since the $\arctan$ function is monotonically increasing, then the phase is decreasing (not linearly) with $\omega$. Similarly, we could multiply by the pole $\frac{1}{(j\omega - (c + dj))}$ with log-magnitude $$-\frac{1}{2} \log \left(c^2 + (\omega - d)^2\right)$$ and phase $$\arctan\left(\frac{\omega - d}{c}\right)$$ Compared to multiplying by the zero, multiplying by a pole leads to a linearly decreasing log-magnitude and an increasing phase. I guess we could keep multiplying by poles and zeros until we build up the desired transfer function, and since...
Comment: ...logarithms turn multiplication and division into addition and subtraction respectively, then I think the magnitude response would consist of different linear regions. Is all of this reasoning correct?


How to downgrade between General Availability versions of MySQL?

Question: There are some things that are not compatible with .NET framework for MySQL version 8.0.29.
Now i want to downgrade to MySQL version 8.0.28 but unable to find the process to do so.
Any help is welcomed.
Comment: Did you try installing 8.0.28 over 8.0.29?  If so, did you get any useful error messages?  Try that on a separate copy of your data.  While I don't think it will "break", it is risky.
Answer: I think you're in uncharted territory.
As the link in the comment above says, MySQL doesn't support downgrades. That is, they don't test it, they don't guarantee it, and they don't document a process for it. This is probably a business decision on their part, not a technical reality. It might technically work, but they have decided not to offer any resources to help.
One problem is that in the MySQL 8.0 era, they are more likely to implement changes that are not backward-compatible between minor releases. This means the tablespace files you have in 8.0.29 might not be readable by some earlier version.
You can try reading the downgrade notes for MySQL 5.7 to get a feel for some of the things you need to keep in mind: https://dev.mysql.com/doc/refman/5.7/en/downgrading.html and its child pages.
The safer path to downgrade is to use mysqldump or mysqlpump to export a logical backup of your data, then import that data to a different MySQL Server version. This is much slower, but it avoids backward-compatibility changes with respect to physical tablespace format.
You might still run into logical compatibility issues, for example if you had used a data type or a function that doesn't exist in the earlier version.
By the way, I am not a .NET user, so I haven't tested this, but based on reading the Release Notes, I guess you might have to downgrade to a version older than 8.0.28. See my answer to https://dba.stackexchange.com/a/312926/2703
This situation is clearly not ideal. You need to do some careful testing and possibly export and import dump files to multiple versions of MySQL Server. It's going to be time-consuming.
The issue of .NET getting errors related to utf8mb3 has been reported here: https://bugs.mysql.com/bug.php?id=107259
I suggest you should make an account on the MySQL bugs site and click the "Affects Me" button for the above bug. The more people who do that, the more they prioritize working on fixing it.


Are there standard definitions for the terms alpha, beta and RC1 in software development?

Question: I am interested to know if the terms alpha, beta and RC1 are terms that are used
throughout the software industry or just MS.
What are the definitions?
Malcolm
Answer: Yes these are standard terms and have standard definitions, though they are often stretched a little in this Web 2.0 world...
Wikipedia has a good explanation of all the terms: http://en.wikipedia.org/wiki/Development_stage#Alpha
Answer: Yes these are terms that are in general use.  Alpha versions are generally for internal use.  Beta versions are typically given to selected customers who can use the software, recognizing that it not yet ready for release, and give feedback on problems.  Typically, these will be more sophisticated customers who are able to help find problems with the software.  Release candidates are generally the versions that one thinks are ready to ship.  These may contain small errors, but the basic idea is that this version will ship absent any  major flaws found at the last minute.
I don't think that there are any formal definitions and the usage may differ from company to company and product to product.  For instance, many Google offerings seem to exist in perpetual beta.
Answer: From http://en.wikipedia.org/wiki/Software_release_life_cycle
Here is the concise and general definition for Alpha and Beta Releases:
The alpha release is characterized by:
A1. It is the first phase to begin software testing.
A2. Developers test the software using white box techniques (sometimes called Pre-Alpha).
A3. Additional validation is then performed using black box or gray box techniques, by another testing team. Moving to black box testing inside the organization is known as alpha release.
A4. Alpha software can be unstable and could cause crashes or data loss.
A5  It is usually not available externally (except by special arrangements and needs, or for open source software).
A6. It usually ends with a feature freeze, (called Feature Complete status).
Beta Release is characterized by:
B1. It follows alpha release i.e. begins after Feature Complete status.
B2. SW will generally have less bugs than Alpha, but more bugs than completed software, as well as speed/performance issues. It may still cause crashes or data loss.
B3. The focus of beta testing is reducing impacts to users, often incorporating usability testing.
B4.  It is typically the first time that the software is available outside of the organization that developed it (for proprietary SW).
Answer: There is only one standard definition I've ever encountered that adequately covers all uses of these terms:

Alpha software is software which the originator has labeled "alpha".
Beta software is software which the originator has labeled "beta".
Gamma software is software which the originator has labeled "gamma".  (Rarely used anymore.)
RC  software is software which the originator has labeled "RC ".

No other "standard" definition comes close to this one in accuracy and applicability.


How to remove shipping price from checkout and from order mail?

Question: Can anybody help me with a solution to remove the shipping price in the checkout process, and also in the order email. Right now it is set to 0,00 but I want to remove the price, so only the delivery text is displayed.
Answer: On the orderemail go and make a new email template.
configuration --- > email.
There you can import magentos template and modify like you want.
Just remember to go under sales-emails and choose your new template.
for the frontend, i would display:none, its the easiest!
Answer: You can easily hide it using CSS because you want to keep the shipping method text.


Do you have experience with PacBio?

Question: I prepare a experiment and I found $PacBio SMRT$ as the great way to sequence my PCR products. I find the cost: library preparation 655 dollars + sequencing 435 dollars. It seems very low. Do you have some experience how much of sample can I sequence in one run (by coverage around 30) and if it is really for this price?
Answer: If it is a good idea to use PacBio depends a lot on what you want to sequence and what you want to do with the data you get. In short: you get longer reads but about 15% error per base. This means, you will have to cope with those errors, there are possibilities to do that. How much coverage you will get of you sample I cannot tell you, as I do not know what kind of sample you want to sequence.
How much data you get out of one SMRTcell depends also on the chemistry, I think it currently is between 275 Mb and 375 Mb, depending on chemistry. They have a nice brochure you could look at.
Anyway, you probably will have to get some advice by a bioinformatician of you choice on how to design and analyse your sequencing experiment.
Comment: I am planning to sequence mouse genes of MHC a TLRs (about 2000 bp). Can you get me a fictitious example about how much sample can I involved by certain covarage?? Thanks a lot! ;)
Comment: I have never done this calculation for a sample this small with PacBio. To keep it short, from one SMRTcell you would get 30x coverage of the E. coli genome (no fiction, they have done 3 cells and got 100x coverage). I guess this is a bit oversized for your short samples. Still, I would rather not do any more "virtual" calculations about this, you need to have someone you can talk to who understands your experiment and probably has seen more PacBio sequencing than I have.

Do you have experience with PacBio?


Smart-like template engine for java?

Question: Is there any template engine (open source) that is like Smart? I want to make it easy for users to design their templates in HTML and then use it, which the smarty-template really is good for. JTPL is close to my example of what I'm looking for, but I want the "foreach" part in the template to be able to let users customize the result pages as well. The more lightweight the better!
Comment: Also, check Java Server Faces (or Pages), this is exactly what you want.
Answer: Velocity is a nice, lightweight templating engine.
Answer: Freemarker is good too. Quite similar to Velocity, add some nice features.
Answer: Rythm is a Strong typed Java template engine using Razor like syntax with high performance (2 to 3 times faster than Velocity and FM). It provides a very lightweight way to do String interpolation:
String result = Rythm.render("hello @who!", "world");

This simplicity makes Rythm a good replacement for String.format() in many cases. Please be noted that Rythm.render is 2x faster than String.format
For comprehensive template you can use the same interface to pass in the file name:
Map<String, Object> args = new HashMap<String, Object>();
args.put("who", "world");
...
String result = Rythm.render("myTemplate.html", args);

Rythm has very rich feature set including java flow control (if-else, for loop, collection iteration), template inheritance, external and internal tags creation and invocation, caching, Java method extension, space compact, html escape etc. Most of the features are demonstrated at http://play-rythm-demo.appspot.com/. (And Yes, the demo is written with Play!Framework plus Rythm template engine and it is running on GAE!)
There is Play!Framework plugin created on Rythm engine, you can find documentation on how to use Rythm from there: http://www.playframework.org/modules/rythm
You can download Rythm from https://github.com/greenlaw110/rythm/downloads
There is also a Spring MVC plugin created based on Rythm Engine: https://github.com/lawrence0819/spring-web-rythm-template
Answer: StringTemplate is a popular lightweight Java template engine. Also, check out these instructions on generating web pages with StringTemplate.

Smart-like template engine for java?


Which is better for street photography on a crop: 28mm or 35mm?

Question: Canon EF 28mm f/2.8 IS USM or an EF 35 f/2 IS USM on a Canon 700D (crop sensor)? I want to use it for street photography.
Comment: We can't answer this for you - it's just a matter of which focal length suits your style better.
Comment: @unsignedzero In case you did not see it - as Inksta points out - you can link to an answer by copying the link given by the "share" link under the answer. Doh!
Answer: If I had to pick one of those two, I would go with the 28mm IS.
But, 28mm or 35mm are both too long for close in street photography. For a small and wide lens, I would recommend the new Canon EF-S 24mm 2.8 "pancake" lens, EF 24mm 2.8 IS, or even the older Canon EF 24mm 2.8 for your crop sensor camera.
One of the so called "rules" for street photography is to use a wide angle prime lens and shoot close in to your subjects. Another "rule" is to use a small inconspicuous camera and lens.
(they are not really rules but more like common practices and suggestions)
You might even consider a 20mm lens. Canon and Sigma have some 20mm lenses but they are all rather large.
Answer: I've shot with a 24mm and 35mm on the Sony E-Mount line and I'd recommended at 24mm as it grabs "more" than a 35mm can. It depends a bit on your tastes but 24mm is a good compromise between being too wide (grabbing) too much and grabbing too little, zoomed.
The 24mm is larger than the Sony 20mm pancake but I'd say that if you shoot too-wide you will distort people so keep that in mind.
For Canon, I'd say go for pancake since it makes it smaller but Mirrorless bodies end up being smaller and more discrete.


How To Install Ubuntu To A Usb Drive from an iso image

Question: I want to install Ubuntu on a 4 gb pendrive but my cd writer is not working so I wnt it to install from the iso that I have downloaded. How to do that?
Answer: Pendrive as installation medium
It sounds like you want to use your pendrive as installation medium. You can achieve this easily by using a program called unetbootin (http://unetbootin.sourceforge.net/). You can install it from the Ubuntu Software Center (or the package manager interface of your choice) or by typing sudo apt-get install unetbootin. If you are on Windows or Mac OS X right now, you can download it from the website.
To install the Ubuntu ISO on the pendrive just go through the steps provided by unetbootin. You probably need admin permissions on windows or root permission on Mac OS X or GNU/Linux.
Pendrive as installation target
If you really want to install Ubuntu to a pendrive (not using it as installation medium, but as "main harddrive" for your Ubuntu), one way is to simply install it to the pendrive via the regular installer. Theres another way providing some kind of hybrid mode between installation and Live-Medium, but I don't really know how this works.


Voltage and current

Question: It is proven only when there is a potential difference current flows in that branch between two nodes, then how we say that the current flows in a short circuit where the voltage on that branch is 0?
Comment: The answer to your hypothetical question is like asking what is 0/0.
Comment: Ohm's Law, three forms: (i) \$V=IR =0\$; (ii) \$R=\frac{V}{I}=0\$; (iii) \$I=\frac{V}{R}=\frac{0}{0} = \text {indeterminate}\$
Comment: This is one of the most frequently asked questions on the site. Unfortunately it's not the easiest to search for, but please do make an attempt. The Q&A I linked above is probably not the clearest example, just the first I could find.
Comment: Here's another example: [Why is it that current pass from charger to battery while both of them have the same voltage?](https://electronics.stackexchange.com/q/247487/6334)
Comment: In the idealized model of basic circuit analysis, wires are not physical wires, but a way of saying which is connected to which. When you connect two points together, they have the same voltage, and in fact, they're literally considered as the same point, or node. A short circuit is like saying "let +5 V and 0 V be the same thing", which is self-contradictory. Thus, a short circuit is meaningless and illegal in circuit theory, like division by zero. At best, you can think of "what would happen as a resistor between them approaches zero", but not a direct connection without anything in between.
Answer:
It is proven only when there is a potential difference current flows in that branch between two nodes ...

No, it's not.
If there's a resistor between those two nodes, then yes.
If there's zero resistance, then whatever current that there is flowing will generate no voltage. Just think of a superconductor.
If there's an inductor between the two nodes, then you can have a current flowing, with the voltage related to how fast the current is changing, not to the actual value of the current.
Answer: Superconductors aside...
In a real and practical circuit, there is no such thing as 'zero resistance', ergo no such thing as 'infinite current', which is what one might expect from the math.
Besides unavoidable wire resistance, there is also a 'source impedance'... Real voltage sources always have some internal resistance.  In an actual short circuit, the source impedance would be what limits the current to something less than infinity.
As a simple example - In an alkaline battery, there's about 1/4 to 1 ohm resistance, meaning from a typical AA, C, D etc you're limited to a few amps even if you short the terminals with a superconducting wire.
After some time, the battery starts to get hot.  That's caused by the power being dissipated over the internal resistance.
Comment: No prob.    Great question actually.   $$$$  In engineering -- Heck in many branches of science even -- the models are just simple approximations.    "First order effects" we might call them.   If one attempts to take into account all the secondary effects, the models become unnecessarily complex.   For example - If you are calculating the free fall time of a metal ball being dropped from a tower, you generally would neglect air resistance.   It's there, but the effect isn't enough to change your answer enough to make it worth considering.
Comment: @HenryBarath.M.A When you write a comment you are _explicitly told_ not to write simple "thank you" notes.
Answer:
... how we say that the current flows in a short circuit where the voltage on that branch is 0?

simulate this circuit – Schematic created using CircuitLab
Figure 1. There is a short-circuit between A and B.
I can think of two ways of looking at the problem.

A and B are the same node of the circuit. Current through that node is determined by the other elements of the circuit.
If current didn't flow from A to B then VA-B would rise to a non-zero value and the current would have to flow.

Voltage and current


How can I disable Webform submit button after being clicked

Question: I've been trying to resolve an issue with no luck. For some reason there are no submission indicators out of the box with D7 Webforms and from what i've found it doesn't look like they have any plans to implement this as they have already added this 'feature' in D8.
I wrote a handler in javascript that will just replace the input with another, but it doesn't appear that the script is firing because the actual submit is taking priority
<code>var form = document.getElementsByClassName("webform-client-form")[0];
var isValidForm = form.checkValidity();
var submitBtn = document.getElementsByClassName('webform-submit')[0];
function preventExtraSubmits() {
    if (isValidForm) {
        var fakeBtn = document.createElement("input");
        fakeBtn.value = "Submitting...";
        fakeBtn.classList.add('webform-submit button-primary form-submit')
        insertAfter(submitBtn, fakeBtn);
        submitBtn.style.display = 'none';
    }
}
function insertAfter(referenceNode, newNode) {
    referenceNode.parentNode.insertBefore(newNode, referenceNode.nextSibling);
}
submitBtn.onsubmit = function(){preventExtraSubmits()}
</code>
From what i've seen is that this might need to be handled server side, but I can't seem figure out how to override webforms submit function.
Fairly new to Drupal and this has been a very frustrating battle to get this working
Drupal v 7.77
Webform v 7.x-4.23
Answer: You could try this module: https://www.drupal.org/project/hide_submit
Be aware that this module has an issue with Ajax forms and file uploads.


MVC 3: Email notifications

Question: I'd like to integrate an email notification system into my MVC3 application. So when a users tasks are due, they get an email reminder. I'm new to MVC so I'd like to be pointed in the right direction of how to go about this.
Many thanks,
Dale
Answer: It sounds to me like you are looking for an email to get sent out as a deadline approaches, and not have to rely on a user action to trigger the email. For this to happen, you need to have some sort of external process that can check at regular intervals and send emails as needed.
There are multiple ways to set this up, but for a web app, one of my favorite is to have a script or a service that runs in the background that just hits a page of the web-app on a timer. That page is then responsible for checking the database and sending out any emails that need to be sent. You can also do fun things to make sure this process isn't taxing the core features of the MVC app too much by building in rate-limiting features. (This is what FogBugz does, I believe, to process through a queue of emails that need to be sent or ingested in a way that doesn't slow down the user)
Other ways to do this are basically the same pattern, but the logic of when to send emails and actually sending the emails gets shifted out of the MVC app and into the external process.
As far as how to send the actual emails, a quick Google search on how to send emails from .Net (assuming you are talking about ASP.Net MVC3) will give you much better information on how to go about sending an email than I can.
Answer: I would start by getting comfortable with the concepts of Dependency Injection and then develop the code that sends notifications as a generic module.
The actual notification engine doesn't need to know anything about your application, or what triggers emails, you should be able to build (or use) something that simply sends messages to a group of recipients (you can determine how functional this component will be, but it cannot bleed into the logic of your underlying application).
Once it has been tested and you are confident it does what it needs too, you can simply inject into your mvc application as required.  This way your application's controllers will perform the necessary application logic, using your notification service as required.
Ideally your MVC app shouldn't contain much business logic at all, it should simply contain references to a bunch of different services that perform the logic of your app, and your controller should simply know how to use the services appropriately.
Answer: I'm a big fan of using Quartz to execute such background processes. It can be hosted in a windows service or within ASP.NET.
Aside from sending the email you would also need to store when the notification was send to avoid sending duplicate notifications.

MVC 3: Email notifications