Problems set-up Single Sign-On using Kerberos authentication

Question: I need for Ruby on Rail application set authentication via Active Directory using Kerberos authentication.
Some technical information:

I are using Apache
installed mod_auth_kerb
In httpd.conf I added LoadModule auth_kerb_module modules/mod_auth_kerb.so
In /etc/krb5.conf I added following configuration
[logging]
 default = FILE:/var/log/krb5libs.log
 kdc = FILE:/var/log/krb5kdc.log
 admin_server = FILE:/var/log/kadmind.log

[libdefaults]
 default_realm = EU.ORG.COM
 dns_lookup_realm = false
 dns_lookup_kdc = false
 ticket_lifetime = 24h
 forwardable = yes

[realms]
 EU.ORG.COM = {
  kdc = eudc05.eu.org.com:88
  admin_server = eudc05.eu.org.com:749
  default_domain = eu.org.com
 }

[domain_realm]
 .eu.org.com = EU.ORG.COM
 eu.org.com = EU.ORG.COM

[appdefaults]
 pam = {
   debug = true
   ticket_lifetime = 36000
   renew_lifetime = 36000
   forwardable = true
   krb4_convert = false
 }

When I test kinit validuser and enter password then authentication is successful.
klist returns:
Ticket cache: FILE:/tmp/krb5cc_600
Default principal: firstname.lastname@example.com

Valid starting     Expires            Service principal
02/08/13 13:46:40  02/08/13 23:46:47  email@example.com

        renew until 02/09/13 13:46:40

Kerberos 4 ticket cache: /tmp/tkt600
klist: You have no tickets cached

In application Apache configuration I added
IfModule mod_auth_kerb.c>
Location /winlogin>
    AuthType Kerberos
    AuthName "Kerberos Loginsss"
    KrbMethodNegotiate off
    KrbAuthoritative on
    KrbVerifyKDC off
    KrbAuthRealms EU.ORG.COM
    Krb5Keytab /home/crmdata/httpd/apache.keytab
    KrbSaveCredentials off
    Require valid-user
  </Location>
</IfModule>

I restarted apache

Now some tests:

When I try to access application from Win7, I got pop-up message box, with text:
Warning: This server is requesting that your username and password be sent in an insecure manner (basic authentification without a secure connection)

When I enter valid credentials then my application opens successfully, and all works fine.

Questions:

Is ok that for user pop-ups such windows? If I use NTLM authentication then there no such pop-up.
I checked IE Internet Options and there 'Enable Integrated Windows Authentication' is checked.
Why IE try to send username and password to application apache? If I correct to understand then Windows self must make authentication via Active Directory using Kerberos protocol.
When I try to access application from Win7 and I enter incorrect credentials to pop-up message box

Application say Authentication failed (this is OK)
In apache error log I see:
[error] [client 192.168.56.1] krb5_get_init_creds_password() failed: Client not found in Kerberos database

But now I cannot get possibility to enter valid credentials, only when I restart IE I can get again pop-up box.

What could be incorrect or missing in my Kerberos setup?
I read in some blog post that probably something is needed to be done in Active Directory side. What exactly?
Answer: You need KrbMethodNegotiate on.
Without that the http client is essentially doing auth-basic to apache and apache is testing the password against the kdc.
Also, for security's sake you should really set KrbVerifyKDC on.


Being Interviewed by fellow candidates

Question: I am applying for a promotion within my department. The process involves interviewing with several managers. Upon receiving my interview schedule I discovered that some of the interviewers are actually eligible candidates that are also considering applying for the promotion. They were provided my complete resume and all of the information that an interviewer would have in preparation. This puts me at a disadvantage or allows them to poison the well if or when they decide to apply. Is this a common practice?
Comment: Why aren't you interviewing anyone?
Comment: I can't imagine why this would be illegal anywhere. Strange, uncomfortable, and probably not the best way to get the best people, but illegal? Why?
Comment: I edited your last sentence to keep the relevant meaning for us while removing the legal bits which we try to avoid if at all possible
Comment: In what jurisdiction? IANAL, but I do not believe this is illegal in most places.
Comment: While this does seem to be a rather strange situation, there is probably little you can do about it. I would ask if you could interview the other candidates for the job as well; this would at least put everyone on the same playing field. I'm not entirely sure what you are trying to find out here as your only question, `Is this an illegal practice?` is off topic here. See the FAQ: http://workplace.stackexchange.com/faq#what-questions-are-off-topic-here
Answer: This is a very odd situation, which can also be seen as quite unfair.
If you feel uncomfortable about it, or don't trust your colleagues to remain professional, then I would suggest speaking to a HR manager/senior manager to request if your colleagues could be left out of the interviews due to a conflict of interest.
Comment: +1 for this. If you raise the issue of conflicting interests to HR, they'll most likely act upon it. There's nothing they want less than allegations or a sense of unfair promotion practices.
Answer:
Upon receiving my interview schedule I discovered that some of the interviewers are actually eligible candidates that are also considering applying for the promotion.

I think the key here is 'considering'.  If the parties who scheduled the interview were absolutely aware that these interviewers were active candidates for the job, or if they were aware that these interviewers were even considering throwing their hats into the ring, then I would expect they would/should not be involved in the process.  This is also assuming that the company has a large enough number of employees to choose from for the interview panel.  At a smaller company, there may be few options.
As others have mentioned, going to HR is another option.  Depending on the size of the company, you may want to be careful about that, as you may be seen as someone who is 'telling' on your competitors before anything has even happened.  If the other candidates have not come forward yet and you only know they are considering applying from personal conversations, I'd be careful with what you say at this point.  Good luck!
Answer: I think there's a lot of potential variance on this.  I have interviewed for a position where a number of people in the team interviewing me were also qualified for the job - in tech work where the nature of seniority means that a high level engineer may be a great subject matter expert and a poor manager - or vice versa. On paper, they may even both look the same.  So just because someone in the existing group was qualified (and even interested) didn't mean that it was the perfect match.
And if they haven't applied yet, they aren't technically a candidate.
I think you have to trust that:

if everyone is ethical, even your competition will be looking for the right guy for the job and won't be trying to bury you just because you are competition.
if the hiring manager is at all savy, he can read his own people well enough to see if someone is trying to poison the well.
if both of the above statements are false (unethical employees, obtuse manager) - this may not be a place you want to work, anyway.

Being Interviewed by fellow candidates


What does horizontal scaling mean?

Question: In database context, I have come across horizontal scalability as one of the advantages of the NOSQL databases. What does the term mean?

CouchDB on Wikipedia
NoSQL on Wikipedia

How would it compare to vertical scaling?
Answer: Horizontal Scaling
Horizontal Scaling is essentially building out instead of up. You don't go and buy a bigger beefier server and move all of your load onto it, instead you buy 1+ additional servers and distribute your load across them.
Horizontal scaling is used when you have the ability to run multiple instances on servers simultaneously.  Typically it is much harder to go from 1 server to 2 servers then it is to go from 2 to 5, 10, 50, etc.
Once you've addressed the issues of running parallel instances, you can take great advantage of environments like Amazon EC2, Rackspace's Cloud Service, GoGrid, etc as you can bring instances up and down based on demand, reducing the need to pay for server power you aren't using just to cover those peak loads.
Relational Databases are one of the more difficult items to run full read/write in parallel.
I saw Damien Katz speaking about CouchDB at StackOverflow DevDays in Austin and one of his primary focuses for its creation was these parallel instances.  As this has been a focus of it since day one, it would be much more capable of taking advantage of horizontal scaling.
Vertical Scaling
Vertical Scaling is the opposite, build up instead of out. Go and buy the strongest piece of hardware you can afford and put your application, database, etc on it.
Real World
Of course, both of these have their advantages and disadvantages. Often times a combination of these two are used for a final solution.
You may have your primary database where everyone writes to and reads real time data on a large piece of hardware. Then have distributed read only copies of the database for heavier data analysis and reporting where being up to the minute doesn't matter as much.  Then the front end web application may be running on multiple web servers behind a load balancer.
Comment: Oracle's RAC is an example of Horizontal Scaling in a traditional RDBMS.
Answer: Horizontal scaling is great when you need an application that needs to handle a high amount of writes and parallel reads. Think of a website that is getting a huge amount of traffic and needs to log this, or needs to log a large number of events.
I guess the biggest plus point is that horizontal scaling can be done using commodity hardware, and there is only so much you can do with vertical scaling and trying to make your existing boxes bigger.
Traditional RDBMSs fail here for a variety of reasons (including the fact that they are ACID), and NoSQL solutions excel here because they can be easily scaled horizontally - but that's another story (RDBMS/SQL vs NoSQL.)
Answer: Some people in the industry use different terminology to mean the same thing:

scale up = vertical scaling
scale out = horizontal scaling
Answer: Make it simpler to understand. NO SQL DB means no JOIN which means your number of columns stays the same that means all you care about scaling is the number of data i.e. the number of rows which means horizontal growth or scaling.
Comment: Not true. You can definitely join in many NoSQL products.

What does horizontal scaling mean?


Will I damage my Android device by charging it using a generic microusb + car adapter?

Question: I need a second charger and a car charger for my Evo 3D and the same for my wife's Nexus S, but I don't want to pay exorbitant prices for them.  Will I fry or otherwise damage the devices using something like this couple with a cheap microUSB to USB cable priced about the same?  Will the devices complain about using "unapproved" chargers and refuse to charge like some iPhone models do with some 3rd party chargers?
Answer: USB, mini-USB, and micro-USB cables are standardized, the voltage and amperage are the same across all standard compliant USB chargers. As long as the charger is a standard, generic micro-USB charger then it should be alright.
That doesn't make it impossible for a manufacturer to create a non-standard plug that looks like a USB plug, but they're pretty much screwed up if they do so.
Comment: Looks like I just saved myself $60 then vs. buying "official" stuff.  Thanks a lot!
Comment: Some very cheap car chargers do not do well while the engine is starting (cranking), especially in old cars. That's generally only a concern on crank, though.
Answer: Micro-USB is standard size and pinout, so it will provide the power on the right pins no matter what charger that you plug into the device.  Usually any charger that has the Micro-USB plug will work.
Additionally, in most modern batteries, there are a lot of charging safeties inside the device itself, including overvolting, undervolting, charging too quickly, charging when the battery is too hot or too cold, and reverse polarity.  I would be surprised if it would be possible to buy a charger on the market today which would ruin a phone.
Answer: No it won't cause any damage. Any generic charger will do. I use a generic car charger like that all the time.
Answer: Whilst it won't cause an issue, I've noticed with my HTC Desire that non-official chargers are seen as "USB" chargers by my phone whereas the official chargers are seen to be "AC" types. This is significant because my phone will only draw 500mA in USB charger mode, but will happily pull 1A+ in AC mode.
It's all down to the USB Battery Charging Specification, which specifies that if the D+ and D- pins are shorted with a resistance of at most 200 Ω, additional current can be delivered. The short disables data transfer, but allows devices to detect the Dedicated Charging Port and allows very simple, high current chargers to be manufactured. The increased current (faster, 9 W charging) happens if both the host/hub and devices implement the new charging specification.


How often should spark plug wires be replaced?

Question: I last replaced them with the plugs about 4 years/60-70k miles ago. I bought platinum plugs I if I remember correctly, so I still got some life on those. The car (2001 Buick Regal) is still in good shape just due for a tune up, so if the wires are due to be changed I'll add those to the list.
Answer: According to the 2001 Buick Regal service manual (PDF) spark plugs wires should be inspected at 100K miles, and replaced if necessary (p. 7-15).
Comment: How exactly are you suppose to inspect them? What should you look for when you are inspecting? For me this is not a complete answer.
Comment: Thanks, I don't even know if I have this manual anymore. I see on page 7-15 I should inspect the wires every 100k, so there's my answer.
Comment: Anytime. I have a ton of manuals in my database =). Glad to help.
Answer: spark plug wires should last a very long time, so just check them when you get the plugs done, and replace if they're worn out.
wires don't really wear with distance, it's time and heat cycles that cause the insulation to eventually degrade. usually there good for several years
look for cracking of insulation, particularly at joints.
check for corrosion of terminals
and if you're really motivated, check resistance (lower is better)


Is there any benefit to running CiviCRM on a non-profts website server rather than its own server?

Question: When I have implemented CiviCRM for various clients I have always set it up on its own server.  One of my clients saw some benefits in having it run on their website server - partly economics - and so it was moved.  And then the problems started...one problem was conflicts with the many Drupal modules on the website which broke various bits of Civi.  So my conclusion is that it's better to run Civi on a dedicate server where the environment can be controlled & suited to Civi.  What do others think? What are the pros and cons?
Comment: It might not be directly related but see this: http://civicrm.stackexchange.com/a/6470/872 Allen suggests the viewing of this video https://youtu.be/k1ov2feFrCU from the CiviCON Denver, highly recommended
Answer: I have worked with many clients that maintain their website (usually Drupal, but WordPress and Joomla as well) and CiviCRM running on the same server, usually in the same domain instance.  I seldom have seen issues.
Now, of course, with a mis-sized or misconfigured server, you will have problems.  And if the website gets a huge amount of traffic, you will have problems.  And if your CiviCRM database has many hundreds of thousands of contacts, you will have problems. But these type of installs are few and far between.
There are definite benefits running a Drupal website and CiviCRM together.  You are easily able to embed contribution pages and event registration pages into a Drupal site. With Views Integration to CiviCRM you can pull information out of CiviCRM for display on the Drupal site.  With Webform Integration with CiviCRM you can build robust webforms in Drupal updating CiviCRM.
Ultimately, the key is properly sizing and configuring the LAMP stack, Drupal, and CiviCRM. (Additionally, try to install on a VM rather than a VPS. You want full access (ala root/ssh) to tune things up.)
Answer: It depends what you mean by "server" and "website". A single server can have multiple websites, with (mostly) their own environments. It sounds like it was not only put on the same server but installed as a module into the same website, i.e. into the same set of folders as their "main" website.
If the hosting plan allows for subdomains, you could install the civi "website" into a subdomain on the same "server", and then they would be separate. You will likely have to share the RAM, php version, mysql version, and apache (or nginx or whatever) version, but otherwise you can then set up the environment as you like.
So the main factor is whether php/mysql/RAM on the desired server meet the minimums required for Civi.
Comment: Thanks  - sorry my question wasn't more clear.  Yes, it was sharing the same domain name, same set of folders etc.  Which had some advantages for theming, but significant disadvantages.
Answer: I agree it depends, How active is the website and how active is Civicrm.  I am running fine on a Cpanel with 2 additional test systems and a few other accounts. We are using Drupal and everything plays nice.
Civicrm is very database usage heavy. Depending on your server 1 transaction a second may be a light load.
Do a little math to understand your load and ask the question, somebody should have some benchmarks which can give more meaningful answers.


What is the importance of eigenvalues/eigenvectors?

Question: What is the importance of eigenvalues/eigenvectors?
Comment: Huh. I am extremely surprised this question hasn't already come up.
Comment: I realize this isn't my question, but I would *love* to see answers addressing the specific question, "How do you motivate eigenvalues and eigenvectors to a group of students who are only familiar with very basic matrix theory and who don't know anything about vector spaces or linear transformations?"
Comment: http://en.wikipedia.org/wiki/Eigenvalues_and_eigenvectors have you looked at this? It offers a pretty complete answer to the question.
Comment: @Jason: Then you should post *that* as a question!
Comment: [This video by 3b1b](https://www.youtube.com/watch?v=PFDu9oVAE-g&list=PLZHQObOWTQDPD3MizzM2xVFitgF8hE_ab&index=14) is by far the best explanation I have seen for its size.
Comment: Here is a nice explanation: http://hubpages.com/hub/What-the-Heck-are-Eigenvalues-and-Eigenvectors
Comment: These are the invariants of the important transformations...
Comment: if you are interested in the general application of eigenvealues in the real world then:-https://youtu.be/DwJbHrdj3EU
Comment: Really surprised this question has not yet been linked: https://math.stackexchange.com/questions/243533/how-to-intuitively-understand-eigenvalue-and-eigenvector. User "EuYu" has a beautiful analogy, in that eigenpairs and roots of polynomials are both difficult to give intuition for, "not because there are few applications but because there are too many"
Answer: Short Answer
Eigenvectors make understanding linear transformations easy. They are the "axes" (directions) along which a linear transformation acts simply by "stretching/compressing" and/or "flipping"; eigenvalues give you the factors by which this compression occurs.
The more directions you have along which you understand the behavior of a linear transformation, the easier it is to understand the linear transformation; so you want to have as many linearly independent eigenvectors as possible associated to a single linear transformation.

Slightly Longer Answer
There are a lot of problems that can be modeled with linear transformations, and the eigenvectors give very simply solutions. For example, consider the system of linear differential equations
\begin{align*}
\frac{dx}{dt} &= ax + by\\\
\frac{dy}{dt} &= cx + dy.
\end{align*}
This kind of system arises when you describe, for example, the growth of population of two species that affect one another. For example, you might have that species $x$ is a predator on species $y$; the more $x$ you have, the fewer $y$ will be around to reproduce; but the fewer $y$ that are around, the less food there is for $x$, so fewer $x$s will reproduce; but then fewer $x$s are around so that takes pressure off $y$, which increases; but then there is more food for $x$, so $x$ increases; and so on and so forth. It also arises when you have certain physical phenomena, such a particle on a moving fluid, where the velocity vector depends on the position along the fluid.
Solving this system directly is complicated. But suppose that you could do a change of variable so that instead of working with $x$ and $y$, you could work with $z$ and $w$ (which depend linearly on $x$ and also $y$; that is, $z=\alpha x+\beta y$ for some constants $\alpha$ and $\beta$, and $w=\gamma x + \delta y$, for some constants $\gamma$ and $\delta$) and the system transformed into something like
\begin{align*}
\frac{dz}{dt} &= \kappa z\\\
\frac{dw}{dt} &= \lambda w
\end{align*}
that is, you can "decouple" the system, so that now you are dealing with two independent functions. Then solving this problem becomes rather easy: $z=Ae^{\kappa t}$, and $w=Be^{\lambda t}$. Then you can use the formulas for $z$ and $w$ to find expressions for $x$ and $y$..
Can this be done? Well, it amounts precisely to finding two linearly independent eigenvectors for the matrix $\left(\begin{array}{cc}a & b\\c & d\end{array}\right)$! $z$ and $w$ correspond to the eigenvectors, and $\kappa$ and $\lambda$ to the eigenvalues.  By taking an expression that "mixes" $x$ and $y$, and "decoupling it" into one that acts independently on two different functions, the problem becomes a lot easier.
That is the essence of what one hopes to do with the eigenvectors and eigenvalues: "decouple" the ways in which the linear transformation acts into a number of independent actions along separate "directions", that can be dealt with independently. A lot of problems come down to figuring out these "lines of independent action", and understanding them can really help you figure out what the matrix/linear transformation is "really" doing.
Comment: +1: This is the clearest explanation I have seen so far about the connection between systems of differential equations and eigenvectors - impressive!
Comment: I was a bit puzzled after reading the justification that the simplified system was not dz/dt=kappa*w ; dw/dt=lambda*z.
Comment: See also https://bookstore.ams.org/mbk-50
Comment: Very insightful and rich. Chapeau bas!
Answer: A short explanation
Consider a matrix $A$, for an example one representing a physical transformation (e.g rotation). When this matrix is used to transform a given vector $x$ the result is $y = A x$.
Now an interesting question is

Are there any vectors $x$ which do not change their direction under this transformation, but allow the vector magnitude to vary by scalar $ \lambda $?

Such a question is of the form $$A x = \lambda x $$
So, such special $x$ are called eigenvector(s) and the change in magnitude depends on the eigenvalue $ \lambda $.
Comment: Direction of eigen-vector may change, but only by $180^{\circ}$.
Comment: A useless regurgitation of the eigenvector/eigenvalue relation is helpful how exactly? Why bother? Jeezus.
Comment: Good short answer!!
Comment: So what? that's the question!
Comment: @kaka: and perpendicular ones to which the projection is null, I believe. I would like just to say that this short explanation was great! I find this good simple example very precious to serve as a motivation for eigenvalues, matrizes, etc. Thank you!
Comment: Concise and simple for anyone. Great answer
Comment: This is just the definition of eigen values/vectors. The question is : WHY ARE THEY IMPORTANT?
Answer: The behaviour of a linear transformation can be obscured by the choice of basis. For some transformations, this behaviour can be made clear by choosing a basis of eigenvectors: the linear transformation is then a (non-uniform in general) scaling along the directions of the eigenvectors. The eigenvalues are the scale factors.
Comment: For someone just beginning linear algebra, this doesn't really make sense or motivate eigenvectors. For example, it could make the student naively ask, "why does the basis matter at all? a rotation by 90 degrees is the same rotation no matter what basis I use to look at it; it's never going to become a scaling in any basis..." and then you have a lot more explanation to do. It would be nice to be able to address this without assuming they already know a lot of linear algebra.
Comment: By throwing in non-uniform, this is the only answer here that makes me appreciate Jordan Blocks. As the existence of a Jordan Block signals that some transformations act on certain combinations of axes that are inherently non-decomposable.
Answer: I think if you want a better answer, you need to tell us more precisely what you
  may have in mind: are you interested in theoretical aspects of eigenvalues; do
  you have a specific application in mind? Matrices by themselves are just arrays of
  numbers, which take meaning once you set up a context. Without the context, it seems
  difficult to give you a good answer. If you use matrices to describe adjacency relations,
  then eigenvalues/vectors may mean one thing; if you use them to represent linear maps
  something else, etc.
One possible application: In some cases, you may be able to diagonalize your
  matrix $M$ using the eigenvalues, which gives you a nice expression for $M^k$.
  Specifically, you may be able to decompose your matrix into a product
  $SDS^{-1}$ , where $D$ is diagonal, with entries the eigenvalues, and $S$ is the
  matrix with the associated respective eigenvectors.
    I hope it is not a problem to post this as a comment. I got a couple of Courics
  here last time for posting a comment in the answer site.
Mr. Arturo:
   Interesting approach!. This seems to connect with the theory of characteristic
   curves in PDE's(who knows if it can be generalized to dimensions higher than 1), which
   are curves along which a PDE becomes an ODE, i.e., as you so brilliantly said,
   curves along which the PDE decouples.
Comment: yes, the method of characteristics can be generalized to dimensions higher than 1. In general the method of characteristics for partial differential equations can be had for arbitrary first-order quasilinear scalar PDEs defined on any smooth manifold. It however will not necessarily work for systems of PDEs or higher order PDEs.
Comment: (It can in fact also be extended to fully nonlinear first order scalar PDEs, but changing the point of view from vector fields to Monge cones is a bit hard to visualize at first.)
Answer: When you apply transformations to the systems/objects represented by matrices, and you need some characteristics of these matrices you have to calculate eigenvectors (eigenvalues).

"Having an eigenvalue is an accidental property of a real matrix (since it may fail to have an eigenvalue), but every complex matrix has an eigenvalue."(Wikipedia)

Eigenvalues characterize important properties of linear transformations, such as whether a system of linear equations has a unique solution or not. In many applications eigenvalues also describe physical properties of a mathematical model.
Some important applications -

Principal Components Analysis (PCA) in object/image recognition;
Physics - stability analysis, the physics of rotating bodies;
Market risk analysis - to define if a matrix is positive definite;
PageRank from Google.
Comment: thanks for your answer,  but aren't eigenvectors/eigenvalues calculated by iteratively multiplying the matrix by a random vector?   https://en.wikipedia.org/wiki/Power_iteration

In other words, it's just as fast to start with a random distribution and look at where all the traffic ends up after several iterations as it is to use a sophisticated algorithm using eigenvalues?
Answer: In data analysis, the eigenvectors of a covariance (or correlation matrix) are usually calculated.

Eigenvectors are the set of basis functions that are the most efficient set to describe data variability. They are also the coordinate system that the covariance matrix becomes diagonal allowing the new variables referenced to this coordinate system to be uncorrelated. The eigenvalues is a measure of the data variance explained by each of the new coordinate axis.

They are used to reduce the dimension of large data sets by selecting only a few modes with significant eigenvalues and to find new variables that are uncorrelated; very helpful for least-square regressions of badly conditioned systems. It should be noted that the link between these statistical modes and the true dynamical modes of a system is not always straightforward because of sampling problems.
Answer: Lets Go Back to the Historical Background to get the motivation!
Consider The Linear Transformation T:V->V.
Given a Basis of V, T is characterized beautifully by a Matrix A,which tells everything about T.
The Bad part about A is that " A changes with the change of Basis of V ".
Why it is bad?
Because the Same Linear Transformation due to two different basis selection are given by two distinct matrices. Believe me! You cannot relate between the two matrices by looking at their entries. Really Not Interesting!
Intuitively, there exist some strong relation between two such Matrices.
Our Aim is to capture THE THING in common(Mathematically).
Now Eigen Values  are a necessary condition to check so but not sufficient though!
Let make my statement clear.
"The Invariance of a Subspace of V under a Linear Transformation of V" is such a property.
That is if A and B represent the same T, then they must have all the eigenvalues equal. But The Converse is Not True! Hence not Sufficient but NECESSARY!
And The Relation Is " A is Similar to B ". i.e. $ A=PBP^{-1} $  where P is a non-Singular Matrix.
Comment: Interesting explanation!!
Answer: Eigenvalues and eigenvectors are central to the definition of measurement in quantum mechanics
Measurements are what you do during experiments, so this is obviously of central importance to a Physics subject.
The state of a system is a vector in Hilbert space, an infinite dimensional space square integrable functions.
Then, the definition of "doing a measurement" is to apply a self-adjoint operator to the state, and after a measurement is done:

the state collapses to an eigenvalue of the self adjoint operator (this is the formal description of the observer effect)

the result of the measurement is the eigenvalue of the self adjoint operator

Self adjoint operators have the following two key properties that allows them to make sense as measurements as a consequence of infinite dimensional generalizations of the spectral theorem:

their eigenvectors form an orthonormal basis of the Hilbert space, therefore if there is any component in one direction, the state has a probability of collapsing to any of those directions
the eigenvalues are real: our instruments tend to give real numbers are results :-)

As a more concrete and super important example, we can take the explicit solution of the Schrodinger equation for the hydrogen atom. In that case, the eigenvalues of the energy operator are proportional to spherical harmonics:

Therefore, if we were to measure the energy of the electron, we are certain that:

the measurement would have one of the energy eigenvalues
The energy difference between two energy levels matches experimental observations of the hydrogen spectral series and is one of the great triumphs of the Schrodinger equation

the wave function would collapse to one of those functions after the measurement, which is one of the eigenvalues of the energy operator

Bibliography: https://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics
The time-independent Schrodinger equation is an eigenvalue equation
The general Schrodinger equation can be simplified by separation of variables to the time independent Schrodinger equation, without any loss of generality:
$$
\left[ \frac{-\hbar^2}{2m}\nabla^2 + V(\mathbf{r}) \right] \Psi(\mathbf{r}) = E \Psi(\mathbf{r})
$$
The left side of that equation is a linear operator (infinite dimensional matrix acting on vectors of a Hilbert space) acting on the vector $\Psi$ (a function, i.e. a vector of a Hilbert space). And since E is a constant (the energy), this is just an eigenvalue equation.
Have a look at: Real world application of Fourier series to get a feeling for separation of variables works for a simpler equation like the heat equation.
Heuristic argument of why Google PageRank comes down to a diagonalization problem
PageRank was mentioned at: at https://math.stackexchange.com/a/263154/53203 but I wanted to add one cute handy wave intuition.
PageRank is designed to have the following properties:

the more links a page has incoming, the greater its score
the greater its score, the more the page boosts the rank of other pages

The difficulty then becomes that pages can affect each other circularly, for example suppose:

A links to B
B links to C
C links to A

Therefore, in such a case

the score of B depends on the score A
which in turn depends on the score of A
which in turn depends on C
which depends on B
so the score of B depends on itself!

Therefore, one can feel that theoretically, an "iterative approach" cannot work: we need to somehow solve the entire system in one go.
And one may hope, that once we assign the correct importance to all nodes, and if the transition probabilities are linear, an equilibrium may be reached:
 Transition matrix * Importance vector = 1 * Importance vector

which is an eigenvalue equation with eigenvalue 1.
Markov chain convergence
https://en.wikipedia.org/wiki/Markov_chain
This is closely related to the above Google PageRank use-case.
The equilibrium also happens on the vector with eigenvalue 1, and convergence speed is dominated by the ratio of the two largest eigenvalues.
See also: https://www.stat.auckland.ac.nz/~fewster/325/notes/ch9.pdf
Answer: I would like to direct you to an answer that I posted here: Importance of eigenvalues
I feel it is a nice example to motivate students who ask this question, in fact I wish it were asked more often.  Personally, I hold such students in very high regard.
Comment: The linked answer really makes no sense at all.
Comment: I would like to point out the useless nature of your "natural frequency" and Tacoma Narrows Bridge regurgitation.   I'm going to venture a guess you have no grasp of this topic.
Answer: This made it clear for me https://www.youtube.com/watch?v=PFDu9oVAE-g
There's a lot of other linear algebra videos as well from 3 blue 1 brown
Answer: An eigenvector $v$ of a matrix $A$ is a directions unchanged by the linear transformation: $Av=\lambda v$.
An eigenvalue of a matrix is unchanged by a change of coordinates: $\lambda v =Av \Rightarrow \lambda (Bu) = A(Bu)$. These are important invariants of linear transformations.


What address does a US Citizen living abroad and employed as a consultant by a foreign company put on a W-9 FORM?

Question: I am a US citizen who has lived in Thailand on a yearly-renewable non-immigrant business visa for the past 5 years. I have been religiously paying my dues and filing my Thai income tax returns but was not aware until recently that I have to continue filing in the US as well. I have also received a couple of W-9s from Thai establishments which I am at a loss as to how to fill out. I no longer have a US address and am working on getting permanent residency in Thailand. On my Thai returns my address is of course my home in Bangkok, but what address should I enter into the pertinent fields in the W-9s and the US returns (past and future) I intend to file? At this point, because of my ignorance, the address on file with the IRS would be my last US address.
Answer: You should fill your current address.
On the default W9 form from the IRS web site, it says "City, State and Zip code", but nothing prevents you from writing "Bangkok, Thailand, WhateverPostal-Code", right?
Same for the tax returns, the IRS form 1040 actually does have designated spaces for foreign addresses.


Is this a valid structure for the nitrate ion?

Question: Why is the below structure not considered a valid structure for the nitrate ion? Is it because adding a double bond reduces the formal charges present on the nitrogen? Does the above form exist anywhere?
Answer: Nitrate actually exists as a superposition of these three resonance forms:

(Source: Wikipedia)
Essentially, a lone pair from an oxygen with a negative formal change forms a double bond to nitrogen. However, as nitrogen is a period II element, and hence must obey the octet rule, it can not have 5 bonds to other atoms. The already-present double bond then turns into a single bond and the pair of electrons moves onto the oxygen. This "movement" can happen with all three of the oxygen atoms and all three of the bonds. This is called "resonance".
Due to resonance, the energy of the system is minimised making it more stable.
Your Lewis structure is not far off. The "true" structure of nitrate is:

(Source: Wikipedia)
Approximately, each oxygen atom contributes one third of a bond with the nitrogen, lowering the formal charge on each atom from what you have written, which does not exist in nature.
The true structure is sort of an average of the properties of each possibility for where the electrons could be located, including the three resonance forms above, your structure, and any other configuration. However, these other forms are of minuscule probability to be formed (perhaps read up on wavefunctions and electron distributions if you're interested), and so the main contributors to the "real" structure are the three resonance forms.
Note that none of these structures ever exist in isolation for more than the most infinitesimal of time periods.
Comment: Technically, the asker's suggested canonical structure (and indeed many others) also plays some role in describing the nature of the nitrate ion, but its importance is significantly smaller than the three main forms with one double bond.
Comment: None of the resonance structures exist, not even in the most infinitesimal time frame. That's why it is a superposition. The OP suggested a valid Lewis structure, which I think has an important (non-negligible) contribution to the overall electronic structure. Probability should not be taken as an indicator that this structure exists, the probability expresses the contribution of this hypothetical state to overall state. [This Q&A](http://chemistry.stackexchange.com/q/21856/4945) is related and explains the structure in the molecular orbital picture.
Comment: Downvoting because this answer suggests that a small, finite number of resonance structures can be drawn and that were it.
Comment: Edited to address that. Hopefully this is more accurate and still clear now.
Comment: I'm a bit confused about resonance structures. Does the electron "circulate" between the oxygen atoms so that the ion constantly switches between the resonance structures, or do all three resonance structures exist for a single ion at the same time? (Because I don't think 1/3 of an electron can exist)
Comment: @user3932000 no. Nothing move if not in our drawings.
Answer: Your resonance structure is valid. There are many more valid resonance structures, the most common ones being the three which feature one $\ce{N=O}$ double bond and two $\ce{N-O}$ single bonds with a single formal positive charge on nitrogen and two single formal negative charges one on each of the single-bonded oxygens.
These four resonance structures are by no means the end! A multitude of further, more minor contributors can also be written. Each of these can be considered to contribute to the overall picture which cannot really be displayed accurately in the Lewis formalism. While the structure you propose contributes less than the three with a single double bond, it still contributes more than some others.
Comment: Apart from the structure with two positive charges on the nitrogen as in Martin's answer (which he linked to in his comment above), I don't get how more structures can be drawn. Could you show me an example or two?


Difference between "broma" and "chiste"

Question: Both words broma and chiste translate to the English word joke.  What's the difference between these two Spanish words, and how do I know when to use each one?
Answer: The main difference that comes to my mind is that chiste can be something witty that you tell, like a joke. For example, the "three guys walk into a bar..." thing.
While broma is a joke that you play on someone, such as putting a sign at the wrong place.
A broma inocente is a joke played on someone that makes the "victim" laugh together with the ones who did the joke, something light; while a broma pesada is a joke that went a little too far.
If we go worse than that, then we're talking about a burla.
Comment: Yes, "estoy bromeando" or "es broma". *To pull someone's leg* is, instead, *tomar el pelo a alguien*.
Comment: And when I just make fun of someone, and want to say "I'm just just kidding," that's a *broma*, too, right?
Comment: Generally, a *broma* is closer to a [practical joke](https://en.wikipedia.org/wiki/Practical_joke) than a *chiste* is.
Comment: When you say "Sólo estoy bromeando" (="I'm just kidding), "bromear" is not the action of playing a prank/practical joke on someone, it usually refers  to something *you said* ironically/joking. So, I assume that "broma" could mean either "prank/practical joke" or "joke" when it means "something said in a playful or ironic way"
Comment: When you say "Sólo estoy bromeando" (= I'm just kidding), "bromear" is not the action of playing a prank/practical joke on someone, it usually refers  to something *you said* ironically/joking. So, I assume that "broma" could mean either (1) "prank/practical joke" or (2) "joke" with the meaning "something said in a playful or ironic way". On the other hand, "chiste" only means "joke", when it means "amusing and short story, usually fictional".
Answer:
broma is like "prank" in English.

Example:

Le hicimos una broma a Carlos.
Aquellos niños le hicieron una broma pesada a Juan.

chiste is like "to tell a joke".

Example

El profesor nos contó un chiste que nos hizo reir.
Comment: Yes, but 'broma' is also used in a widest sense that 'prank'
Answer: Chiste is a relatively simple concept as explained in the other answers. The thing is that broma is a complex word that has many meanings:

Bulla, algazara, diversión.
Chanza, burla.
Persona o cosa pesada y molesta.

You need many words in english to explain what broma really is: to make fun of, to mock, to kid around, to have fun.
Comment: I have never heard "broma" used as "persona pesada y molesta", so I assume it must be a rare meaning. It'd be nice to mention that in your answer and give examples of each meaning.
Answer: A "broma" can be an innocent practical joke that you pull on a person or something less innocent, but not as malicious as a "burla." In any case, it may not be funny to the other person and might cause embarrassment.
"Chiste," on the other hand refers to a joke in the sense of a humorous story, not told a someone's expense.

Difference between "broma" and "chiste"


Why is it legit to evaluate $\lim_{x\rightarrow 1} \frac{(x-1)(x+1)}{x-1}$ by cancelling common factors?

Question: I haven't ever taken an analysis course, so maybe that's where I would really learn this, but I've always wondered why it's okay to do this when evaluating a limit. I guess it's the case that there is a theorem which says that the limit of a rational function as $x\rightarrow a$ is equal to the limit of that function in lowest terms as $x\rightarrow a$, regardless of division by zero. Is there a name for that theorem? Or does it follow from some other property of limits or rational functions?
Comment: even if you haven't learned limit yet, you may try changing that x into 0.999 and 1.001... lim(x->1) means x is soooooooooo near to 1 but x is not equal to 1
Answer: The important thing to remember about the limit process in something like this is that whenever we evaluate something like $\lim_{x\rightarrow 1} f(x)$, we don't require the function $f(x)$ to be defined at $x=1$, so we look at the values of $f(x)$ as $x$ approaches 1, but taking values which are not equal to 1.
In your example, it means that as $x$ approaches 1 without actually equalling 1, the factor may be cancelled because it is definitely not zero.
I don't know of any general result, though, and have always done things like this by examining each case on its merits, by asking if we can be sure that cancellation is allowable in a particular case.
Answer: The limit is about letting $x$ "approach" the value of $1$, not setting $x=1$.
For this reason you are allowed to cancel since the denominator will never be $0$ for these values of $x$.
Answer: One relatively subtle reason that this is true for rational functions is that polynomial functions (so in particular, the denominator of your rational function) have what are known as isolated zeroes: because a polynomial has only finitely many zeroes (this is a form of the Fundamental Theorem of Algebra), no two zeroes can be too close to each other.  In particular, around each zero there's a disc of finite radius in which no other zeroes can occur.  This means that a rational function is well-defined in some neighborhood of each of the zeroes of its denominator.
For an example of what can go wrong when you step away from rational functions, consider instead the function $\displaystyle f(x) =\frac{x\sin(\frac{1}{x})}{\sin(\frac{1}{x})}$: then can you see why $\displaystyle\lim_{x\rightarrow 0} f(x)$ is undefined?
Comment: I like this answer - explaining why it works for rational functions ($p(x)/q(x)$ for $p,q$ polynomials), but not in general.
Answer: Your expression $f(x)$ is not definied at $x=1$, therefore the limit represents the value of $f(x_n)$ for each sequence $x_n\rightarrow1$ with the property that $x_n \neq 1$ for all $n$. But when $x_n \neq 1$ you can of course cancel the two terms as they are not zero.
Answer: It is a theorem that if the limits below are exist and finite,
$$
\lim_{x\to x_{_{0}}} f(x)=A
$$
and
$$
\lim_{x\to x_{_{0}}} g(x)=B,
$$
then
$$
\lim_{x\to x_{_{0}}} f(x)g(x)
$$
exists and equals $AB$.
In your case
$$
\lim_{x\to 1} \frac{x-1}{x-1}=1
$$
and
$$
\lim_{x\to 1} x+1=2,
$$
so your limit exists and equals $2$.
To emphasize the idea
$$
\lim_{x\to 1} \frac{x-1}{x-1}=1=g(x),
$$
where the function $g(x)$ is defined for $x\neq 1$ as the constant $1$.
This is the reason why we can cancel the $\frac{x-1}{x-1}$ to $1$.
Answer: You must understand that there is a difference between $x=a$ and $x \rightarrow a$. For $x \rightarrow a$, x approaches a but never becomes exactly a, so (x-a) is always non-zero for $lim$ $x \rightarrow a$, and hence you can cancel out two same non-zero quantities in numerator and denominator.
Answer: Here's the way I'd say it precisely:
If $f$ and $g$ are functions with $f(x) = g(x)$ for all $x \neq a$, then either both
$\lim_{x \to a} f(x)$ and $\lim_{x \to a} g(x)$ fail to exist, or
$$
\lim_{x \to a} f(x) = \lim_{x \to a} g(x).
$$
(In fact we can weaken the hypothesis to all $x \neq a$ in some neighborhood of $a$). In your example, $f$ and $g$ happen to be rational functions that differ only by multiplying by $\frac{x-a}{x-a}$.
Answer: Well logically speaking no matter what value x takes as it tends to 1(but not equal to 1) the numerator and denominator will cancel out as they are the same (
$(x-1)$ ). So you can cancel it with the limit in place.
Also look at L hospitals rule for some basics.
Answer: To say that for $x \rightarrow a$ "x" never becomes actually "a" is not entirely precise. Think of a constant function where the $a_{n}$ of a sequence are all equal to some constant b and hence to the limit b. It would be more precise to say that it is not $\underline{necessarily}$ the case, since for the definiton of the limit x only has to come arbitrary close to its limit a.


Wearing tzitzis to bed?

Question: I know many frum people wear tzitzis to bed, but a while ago a fellow told me that you should wear the tzitzis that you're going to wear the next day, because of an obligation to wear the same set for 24 hours or something like that. I wonder where it says this (if it does). Also, if you have tzitzis you wear specifically for bed (because they often get tangled or knotted etc.), can you wear these and put on a daytime one in the morning before the bracha?
Comment: As far as I'm aware, it is actually preferable to take it off when sleeping as there is a specific bracha that you make when putting them on that you would not be able to make if you hadn't taken them off. Where did he say this chiyuv is from?
Comment: Closely related: http://judaism.stackexchange.com/q/47265
Answer: I have never heard of a need to wear the same pair of tzizit for 24 hours. The assumption lends itself to a lot of questions and suffice it to say that the question would be enhanced if you can ask this bachor to provide sources for his statement.
As to your question if you can wear a pair of tzizit specifically for bed and change in the morning before the bracha this is actually preferable:

Night Tzitzit
.... many are careful to wear tzitzit even during
  the night, and even while sleeping.
There are several reasons for this practice: a) As mentioned above,
  according to certain opinions, one can fulfill the mitzvah of tzitzit
  during the night if wearing a designated day garment. b) If waking up
  after daylight, one will have fulfilled the mitzvah of tzitzit during
  the morning hours which have elapsed before his wakening. c) According
  to kabbalah, tzitzit afford a measure of protection to its wearer even
  after dark.
If one wears tzitzit while sleeping, it is preferable to have "night tzitzit." Otherwise, if one wears the same pair of tzitzit night and
  day, reciting the blessing on the tzitzit in the morning is
  problematic, for a new mitzvah is not being performed—it is merely a
  continuation of the mitzvah from the day beforehand. However, when
  putting on a different pair of tzitzit in the morning there is no
  problem with saying the blessing on the new pair—new mitzvah.
Comment: Clothing designated for use only at night is not obligated in tzitzit at all. "night tzitzit" is probably the worst idea for this possible.
Comment: http://judaism.stackexchange.com/questions/6030/tztzis-at-night
Comment: eramm, is that a response to me? How does it do so?
Comment: @DoubleAA, if the purpose of separate "night _tzitzis_" is to be able to recite a blessing in the morning, then it's sufficient to change in the morning, that is, to have a set of _tzitzis_ designated for **that** night (as opposed to the next day). But it need not then be designated for night in general: it can be identical to other sets of _tzitzis_, and be worn other days. Then it'd not be a _beged layla_ (I assume; CYLOR).
Comment: @msh210 "it is preferable to have 'night tzitzit'" is a really, really, really strange way of saying: "make sure to change your beged in the morning".
Answer: As far as I know, the preferable way to do this is to put on a fresh pair of Tzitzis in the morning, so that the Bracha can be said without question, and then sleep in that same pair the following night, changing it in the morning.
This avoids any problems of the Tzitzis being dedicated to the night time, as well as any issues with saying a Bracha on them in the morning.
I heard this from a Rabbi over 20 years ago.
Menachem Posner's comment at eramm's link seems to say the same thing.
So although I have never heard of some obligation to wear the same pair for 24 hours, it is possible this is where the idea comes from: Wear the same pair at night, as the previous day.
Comment: You could also wear the same pair as the _following_ day, which also "avoids any problems of the Tzitzis being dedicated to the night time, as well as any issues with saying a Bracha on them in the morning."
Comment: @DoubleAA, I don't think it is as complete and avoidance of the issues (although le-Halacha you are probably right that the effect would be the same).
Comment: In what way is it an incomplete avoidance of those issues? The only 'negative' aspect of that solution is that for the first 12 hours you haven't said a bracha yet.
Comment: @DoubleAA, I believe there are opinions that you should have.
Answer: see arizal who writes one should wear tzitzis even at night, while sleeping.
Comment: -1 I don't see how this addresses the question.
Comment: I don't know what color has to do with it, but the antecedant of "_this_" in your quote is "you should wear the tzitzis that you're going to wear the next day, because of an obligation to wear the same set for 24 hours" as far as I can tell. Does the R Luria address that? If he does, consider editing it into this answer.
Comment: maybe i'm colour blind! "I wonder where it says this (if it does)"!!

Wearing tzitzis to bed?


Does aloe vera fade acne scars?

Question: Can aloe vera gel make scars from acne fade? If so, how is it used for this purpose?
Answer: Yes. It is used as a gel.
WebMd.com

Aloe seems to be able to speed wound healing by improving blood
  circulation through the area and preventing cell death around a wound.
It also appears that aloe gel has properties that are harmful to
  certain types of bacteria and fungi.

Aloe - ncbi.nlh.nih.gov:

anti-inflammatory
skin soothing

Aloe Vera has been shown effective on scars here.
Answer: It can. It depends on your skin type. Anatomically, everyone is different. I've seen it work quite well and not so well other times.
For the most part it does work. It's compound and gentle nature and nurture of your dermis is what allows it to heal the outer layer of your skin.
Vitamin E is what most people refer to in scar healing. Cocoa butter is another over the counter remedy for this as well.
Comment: how long it takes for average scars to heal?i have oily skin.i can see it is doing wonderful job on my acne but iam worried about the scars caused by it.and is it enough if i wash it after 15 minutes because this is winter and iam afraid that it may dry my skin too much.
Comment: Welcome to Health SE! This post has the makings of a good answer, but here on Health, we [*strongly encourage*](http://meta.health.stackexchange.com/q/1) using references. They are the only way in which we can tell if information is reliable or not. If you are struggling to find good sources, check out, [What are reliable sources?](http://meta.health.stackexchange.com/q/112) If you want to learn more about our site's stance on answers without references, check out, [Should answers without references be immediately deleted?](http://meta.health.stackexchange.com/q/283) Thanks :)

Does aloe vera fade acne scars?


DAM, Digital asset management for Linux

Question: I am using Dark Table for proceesing my photos and am very happy with it, but I lack some catalog software to be able to quickly find relevant photos. Any reccomendation for software available on Linux?
BR
Bo
Answer: digiKam is something I came across as well.
Comment: When I was in the linux world, I used digikam.  It has been in more or less continuous development for over 20 years.  Good documentation.  Now it runs under windows (WINE, I think) and theres a port for Mac. Recommended.
Answer: After a long wandering for several years in searching of the perfect DAM solution, I didn't find it.
However the program which comes closer (IMHO of course) and fits best for our company's needs is XnView MP.
It is a cross-platform solution under active development with rather often updates (see the forums there - you can ask everything you want) with very good viewer, good editor and more than 500 file formats supported.
What differentiates it tough is:

his powerful file management
best in class cataloging (AFAIK of course, but I looked at almost any DAM solution I could find)
search features (both on disk as well as in database)
its scalability

I can say many more about this (I have enough years of experience both with this program and with other DAMs) but in order to not transform the answer in a promotional material I'll mention that we also use it in conjunction with other RAW editing back-ends (fpr reference, Photoshop CC and Photivo). It is very easy to configure XnView MP for this task: Go to Tools | Open With... | Configure programs...
Answer: Have you looked at Shotwell? It doesn't look very fancy, but it provides some cataloging/searching functionality.
Edit: Just now I found F-Spot, which also may meet your requirements.

DAM, Digital asset management for Linux


Phonegap WatchPosition stops updating. EBUSY?

Question: i have a phonegap project that uses WatchPosition in background of the app. It works sometime but sometimes when the app starts i get Location Timeout error 3.
And the log for my iPhone says that the location is (EBUSY).
Is this a phonegap bug?
Code:
<code>  function onDeviceReady() {
                    var watchID = navigator.geolocation.watchPosition(onSuccess, onError, { timeout: 10000, frequency: 3000,enableHighAccuracy: true });
                }
function onSuccess(position) {
            var deviceID = device.uuid;
            $.post("http://server.com/project/savelocation.php", { lat: position.coords.longitude, lng: position.coords.latitude, device: deviceID },
               function(data) {
               console.log("Location updated.");
               });
               }
</code>
It does work sometimes as i said, but the errors i get are :
<code>[INFO] code: 3
    message: Geolocation Error: Timeout.

locationd[41] <Notice>: received EBUSY, try kScanTypeChannel, channel, 11, rssi -60, delayed 0.000s
</code>
Can this be iOS 6 errors?
I also have Phonegap 1.2.0 so i dont use the latest Cordova project yet.
Answer: Turns out i needed to upgrade to latest Cordova. iOS 6 support was horrible on Phonegap 1.2.0!


What is this vine-like weed and how do I kill it?

Question: I have a viney weed growing in my yard. Weed and Feed doesn't seem to affect it. It's slowly spreading to more and more of my yard every year. What is it, and how do I kill it?
Answer: Looks like Ground Ivy or Creeping Charlie.
Control requires persistence via Michigan State University Turfgrass Science
Click below images to enlarge...
Answer: For killing weeds in an established lawn, a Weed Spray is usually the easiest way.  A weed spray kills weeds without harming grass (provided the grass is healthy).
I've had good luck with Spectracide, but the Ortho product mentioned in here works well too.  The Spectracide label says it kills Ground Ivy, so it should work on your weed.
Some tips:

Note that a targeted weed spray is a
different than a general purpose
herbicide like RoundUp.  Don't get
them confused, or you will have a
dead lawn.  Read the label!  If you
are really cautious, look up the
active ingredients and see what they
do (Google them).

If you have a lot of spraying to do,
consider buying a Pump Sprayer.
The squeeze trigger bottles will
cramp your hand if you have a lot of
spraying to do.  You can also re-use
the pump sprayer for other things
like bug spray and deck cleaner.

Big established weeds will take a long time to die.  It will probably also require multiple applications of the spray (spaced several weeks apart, read the label).  Try plucking/cutting the Ivy so the spray doesn't have as much work to do.

If you get "holes" in the weeds (e.g. by ripping pieces off), you get an entry point for the spray into the plant's "bloodstream".

After the weeds are dead, you may have bare spots in your yard where they were growing (or just big chunks of dead ivy.)  You may need to re-seed those spots, but wait a while.  The weed spray can interfere with new grass.  It's also best to plant grass in the fall anyway.

Weed and Feed products and weed preventers like Preen are usually pre-emergent, meaning they don't kill stuff that is already growing (check the label).  That's why you usually only apply them early in the year, before the weeds can germinate.  The Sprectracide spray I suggested also contains a pre-emergent weed preventer (not sure about Ortho).
To make life easier in the future, try to stop the weeds before they get too big.  Use a weed preventer, and spray/pluck them early.
Answer: I suspect most weed and feed products will be pre-emergents, meaning you need to get it in your lawn before the weed seeds have a chance to germinate, which is a limited window.  Once something like this is already growing, you're usually left with the round-up type of products, which kill everything they touch, or hand pulling.
After a long day in the office, sitting outside on a nice day, pulling weeds, can be therapeutic.  Just make sure you get as many of the roots as possible.  And with viney or runner spreading weeds, try to get the entire runner.
Comment: I have also had a lot of success against weeds by just going outside and pulling one or two a day.  Before you know it, they're all gone.
Comment: +1 For the effectiveness & power of hand-weeding (a little bit each week will eventually reap huge! rewards).
Answer: I have had some pretty good success with Ortho Weed-B-Gon Max.  I have used it on crabgrass, clover, and wild onion and it does a pretty good job.  Pretty much I have used it for spot treatment in small areas but you can also use it with a sprayer to spray a larger area.
Answer: Don't kill it, just learn to live with it and look how beautiful it is. If ever it takes too much space, reduce it mechanically by hand. Remember that plants are very strong and that products which are able to kill them hurt you also...
You will save money, and learn more interesting things by tolerating wild flora in your garden! In fact you are lucky that they are willing to come!
Comment: Since I got honeybees I am significantly less inclined to kill plants, especially those with small, shallow flowers. The clover and ground ivy in my yard has made a comeback, and I love it.
Answer: Round up! Careful with it though - it kills anything it touches. You can put it on with a foam brush to be more selective than spraying.
Answer: This Henbit loves moisture.  Although prolific it would not stand a chance if you would train your grass to have deep roots.  You've  been watering too little too often.  This technique makes for shallow rooted grass.  These cool season grasses have huge root systems, genetically.  They appreciate being trained by allowing the soil to dry out before watering again.  Water deeply and do not water again until you see your footprints stay down in the grass.
Watering deeply and allowing the soil to dry saves water, your lawn will be healthier and drought tolerant and Henbit will not be happy.  You need a broadleaf herbicide designed for lawns.  Weed and feed is always one of this type of weedkiller.
The trick is to wet your lawn down first and apply the herbicide (granular form) using a rotary spreader.  The water will help the broadleafed weeds to hold onto the herbicide.  You can purchase just the broadleaf weed lawn killer without the fertilizer.  Don't water or mow for a couple of days.
Mow your grass on high at the very least 3" no shorter.  Have you aerated this year?  Look up questions on lawn care maintenance for cool season grasses on this site.  There are some hard and fast rules for cool season lawns that if followed you'll never have to use an herbicide again.  Also check the pH of your lawn.  Ideally 6.5 to 7.0.  Henbit likes more acidic soil.  Again, just the management of your lawn will help the grass itself to outcompete, shade out germination of weed seeds, and you will be selecting for your grass and discouraging weeds.


Imported contacts are empty

Question: I have an odd problem. I got an iphone5 and of course, I want all my contacts from my nokia into my iphone, I imported the contacts into outlook, then exported to an csv file, then imported to gmail and it worked perfectly until that point. When I imported the gmail contacts to my iphone, all the names were there, but barely any mobile numbers. However, in my gmail profile everythings seems to be ok, so I doubt that there is a field organisation problem.
If anyone has an idea, please help me, that thing is driving me crazy!
Thanks!
Answer: I found out what was wrong. It seems the phone number types were the problem. They were classified as "Other". I changed them all to "Mobile" and it worked.
I hope this helps somebody, because I looked the problem up for hours and didn't find a solution.
Answer: Instead of passing through Outlook and Gmail to transfer nokia contacts to your iphone - i'd simply use the Nokia Desktop Sync sofware to export the nokia contacts to your pc then import the contact files from the PC to the iPhone using an app to import contacts to iPhone.

Imported contacts are empty


Voili-voilou : Is this a common saying?

Question: I received an email with attachments. The text opened with "Voili-voilou les notes de..." Is voili-voilou known argot for voilà? Or something else?
Answer: Yes, it is kind of a jocular alternative for voilà, or voilà voilà !
It is not very used and has no inherent particular meaning, but is definitely understood.
The tone used will tell how to interpret it, but the rare times I heard it, that was to express the happiness to have completed something and to deliver it like "Yeah, here you go!" or to mark that you have finished to tell something like disclosing some story, and soften the end with a "That's all, folks!".
Comment: This form is quite possibly issued, as a popular distortion, from the "voilà-voilà" or "voilà-voilà-voilà" that one could find in the Loire region or thereabouts at least thirty years back; as far as I know it wasn't used with a complement and its use didn't aim so much at jocularity as to communicating a certain finality relative to a state of affairs just discussed or exposed and to which it was a conclusion;   supposing the process of formation suggested is verified is it still correct to classify this form as "slang"?
Comment: @user168676 *Slang* might be excessive indeed, the reason why I wrote *kind of*. It is neither offensive to people, nor restricted to a particular social group. May be jocular is also not the best adjective, I just wanted to mean that *voili-voilou* is often said with a smile.
Comment: On the contrary, jocular is the proper adjective insofar as the form "voili-voilou" is concerned; I was trying to draw the difference between this latter and what I think might be the precursor, that one being used (I think) on another basis (not so jocular and quite possibly sarcastic); the problem remains for "slang"; for that matter it suggested  a full post to me.
Comment: What are its characteristics outside denotation? Sarcastic? Familiar? Imitating child speech?
Answer: "Voili Voilou" is quite common and used much more frequently verbally rather than in writing. It is only used in very informal situations such as at home or with close colleagues. Compared with just "Voilà" there is a note of finality. "Voilà" when handing a report written by someone else a close colleague and you are just passing it on, "Voili Voilou" when you hand a report that you wrote yourself over the last week. As with all subtleties, there is no golden rule
Comment: Je n'ai jamais entendu « Voil**à** voilou », l'expression est « Voil**i** voilou. »
Comment: @Toto I've heard both. I think it's a regional thing. I think *voili* is more common but this may be my own bias.
Comment: @Gilles: Il est bien possible que cela se dise. Dans quel coin as-tu entendu cette formulation ?
Comment: @Toto Paris, so it could have been people from anywhere.
Comment: My typing mistake, "Voili Voilou" it is
Comment: Not uncommon, for sure, but "quite common", I doubt it...

Voili-voilou : Is this a common saying?


Chainring and cassette configuration

Question: How does the 52/36 chainring coupled with a 11/30 cassette match up.
Will it be good for a 70 year old recreational rider? Or what combo would you suggest?
Answer: Highly unlikely this is the gearing for you. Standard chainrings on most drop bar road bikes are 50/34 'compact'. 52/36 would be considered a higher gearing choice these days.
It's likely you will need a lower gearing, especially if you live in a hilly area, so consider a 48/32 'sub compact' on a road bike. You may also want to consider bike that can take a wider range cassette. 11-36 cassettes are available on road groupsets.
You may even want to consider a flat-bar hybrid with even lower gearing and more upright riding positions. Find a good local bike shop for advice and to try out some bikes.
Answer: Generally speaking, it seems too high.  I am rather fit, a few years younger, run a 50/34 compact, don't have an 11, and don't miss it.  Whether you use 52/36 or 50/34 doesn't matter too much but I would prefer the smaller.  For the low end, it depends on the hills you want to ride.  My low is 34/30 which does what I want, but each area and each rider is different.  It seems to have become easier to find 32 or 34 cogs on road bikes.  You need a medium cage derailleur for them and the spacings get a bit larger, but you can't use gears you don't have.  If you can borrow a bike, go climb some hills and see what gears you use.
Answer: The reality is that this is impossible for anyone to answer for a specific rider, without knowing the rider.
When I read "70 year old recreational rider", my inclination is to assume relatively short/easy rides, and a fitness level which is not ideal and may include medical restrictions. However, 2 of my favorite shop customers are people that would describe themselves as "recreational riders" but put in more miles in a year than I do by far. Not least because retirement allows them to ride consistently. They are both well over 70 at this point, also.
That said, if you are looking at a comfortable, stable road bike, and your goal is not to race your bike, then I would consider a 50/34 (compact) front end, and a 12-28 rear cassette. To my way of thinking, that provides a good balance of top end speed, and reasonable climbing gears.
Things that will affect your decision:

Fitness level/body weight: The strength to weight ratio is critically important in cycling. If you are 125 lbs. and strong enough to push your weight up hill without losing your ability to continue riding, then erring on the side of high torque/high speed gears makes sense. If you are 250 lbs, and any hill is the one you die on, then erring on the side of low torque/low speed gears makes sense.
Terrain:  If you are retired and and living in the Arizona desert, and an overpass is the biggest hill you see, then high torque/high speed. If you live in San Francisco, and every direction away from your home is uphill and beyond category, then low torque/low speed.
Medical restrictions: There are a million things that can go on this list, but the big thing is, if you have medical restrictions, don't push them. Err on the side of easier pedaling, rather than creating issues for yourself.

As for how to decide what specific gearing fits your needs? Well, use a gear calculator to compare the different combinations and find one that fits your needs, with the things I mentioned in mind.
There are lots of choices for gear calculators. Bikecalc.com is my favorite.

Chainring and cassette configuration


Should you change all of your transmission fluid?

Question: I took my Lexus RX 300 to the shop, to change the transmission fluid. While talking to the tech, he said he sucked out half the fluid and left the other old half there. Is this proper? Shouldn't they change it all out?
Comment: You can call Lexus and ask them. They have a phone line on their web site somewhere for tech questions like this.
Comment: I second xpda's comment here. I was ready to post about why automatic's fluid should be changed in such a manner, however, no one is going to know better than those who designed it. I know from experience that Honda automatics - at least mid 2000s - just call for a drain and fill of the pan. This will never get all of the old fluid out as much is stuck in the valve body, torque converter, etc. Regardless, call lexus directly and ask. While this seems appropriate to me it may not be what lexus intended. Anyways, best of luck.
Answer: That sounds pretty strange to me. I'll admit I don't know a lot about automatic transmissions, but with a manual gearbox I would always drain all the fluid when changing it. As gearboxes wear, small metal filings accumulate, and one of the reasons for changing the oil is to get rid of these filings which, if left to build up unchecked, cause even more wear. Most gearboxes I've dealt with have also had a drain plug on the bottom (so you can't only drain half if it anyway), with a magnetic element to collect the aforementioned filings
Comment: Historically I've always dropped the entire pan on auto transmissions because there are filters and such to replace. Never have worked on a RX300 though.
Answer: The old lore goes that if you change it all out, the shock of all the new fluid and detergents will knock crud loose and cause the transmission to fail. I've heard it many times but never actually seen any evidence that it's true.
Given that no Haynes or factory service manual mentions anything about it, I shelve it up as false.
Answer: Sounds like "Cleatus" the slack-jawed yokel changed your fluid.
As stated above, there are lots of tiny little metal filings floating around in that stuff. The more that stuff moves around the more it scratches and wears out your transmission. Then you have half of your new transmission fluid filter life because, instead of all new CLEAN fluid, half of it is already full of filings that will now get trapped in the filter.
Do not go to that mechanic again!
Comment: Filings floating around should have already (mostly) been trapped by the filter, or the magnetic drain plug which any modern transmission should have.
Answer: I have a Toyota Land Cruiser with an automatic transmission. If I drain the transmission with the drain plug, I only drain 40% of my transmission fluid. The rest of the fluid is, in my case, circulating through the transmission fluid cooler, transfer case and gearbox. I only drain what is in the pan. The Toyota Factory Manual does not state any full fluid flush for my vehicle. (Here is a "Rodney Flush" that does change all the fluid). Here's a DIY for the RX300 that flushes all the transmission fluid.
Bottom line: if the Lexus technician left 50% of the oil intentionally, I'll be worried. But, I bet he has done a "drain & refill" that left some old oil in the system, and I'm sure that's what the Lexus Factory Manual has stated to do.
If unsure, call the dealer and ask for more information.
Answer: That is completely wrong. The automatic transmission fluid is designed with a special viscosity to ease the changing of gears. This fluid starts to lose its standard properties and should be changed to keep the transmission parts and its performance at its peak. Mixing old and news fluids wont give you the ideal viscosity and the reduces the performance of the transmission system.


Is there a way to mimic Windows 7's WinKey+Number shortcuts?

Question: One of the biggest things I miss from Windows 7 when I'm on Linux is the ability to push WinKey and a number to switch to the program in that place on the task bar. (e.g. You push WinKey+1, and it switches to the first program in the taskbar.) Is there a way to replicate that on Ubuntu 12.10 with Cinnamon? I tried to add custom shortcuts to do it, but I don't know any commands to switch to windows by their place in the taskbar. Is this possible?
Answer: In Unity Desktop Environment this option is there by default.
Just hold the Windows key (also called super) and the numbers for the shortcut will appear on each program's icon.
I don't think Cinnamon has this option.
Answer: In Unity that's how it works. If I hold Super (WinKey) it shows me the number to hit for each app in the launcher bar.
Comment: Yeah, I was wondering if it was possible in Cinnamon though.
Comment: Sorry, I missed the reference to Cinnamon.
Answer: This list of Cinnamon shortcuts suggests you can't switch among windows with Super+1..9.
However, see the solutions for Gnome in this other question. Maybe one of them works?


What is the name of the form of the line equation $y = m(x-x0)+y0$

Question: I have been looking everywhere for the name of this form of equation of a line $y=m(x-x_0)+y_0$. It's not quite point-slope. It's the same form you would write in the linear approximation of a function, $L(x) = f'(x_0)(x-x_0)+f(x_0)$. It seems important enough to warrant its own name. Does anyone have a clue ?
Comment: As all of the answers agree, there may not be another name for this form.
Answer: Wolfram calls it the "point slope form", you just have $y_0$ on the rhs rather than lhs.
Answer: Several Pearson calculus textbooks, such as Calculus: Graphical, Numerical, Algebraic for high school and Calculus and Analytic Geometry for university use what you show for the point-slope form. The latter puts the $y_0$ term first, but I believe the former puts it last as you do.
Answer: Definition of slope of a line, given two known points on the line, say $(x_2,y_2), \;(x_1, y_1)$, where $m$ is the slope of the line:

$\dfrac {y_2 - y_1}{x_2-x_1} = m$.
Note that this is equivalent to the equation: $(y_2-y_1) = m(x_2-x_1)$, which bears close resemblance to the next form of an equation.

Once one calculates slope, or it is given, one can use either of the following point-slope forms of the equation of a line, and one of the points on the line, call it $(x_0, y_0)$:

$(y-y_0) = m(x-x_0)$
$y= m(x-x_0) - y_0$ (This is algebraically equivalent to the former, in that one merely subtracts $y_0$ from each side of the point-slope form of a line.)

If one knows the slope of a line, $m$, and $b$ (the y-intercept of the line given by the $y$-value of the line when $x=0$, i.e., $(0, b)$), one can use the slope-intercept form of an equation, given by:

$y= mx+b$
This is particularly helpful when the slope of the line is determined, and one point on the line is given by $(0, b)$ is known.

Finally, what is often called the standard form of the equation of a line, is given by:

$ax + by=c$.
This equation is equivalent to the point-slope form of the equation given by $$y = m(x-x_0) - y_0 \\ \\ \iff (\frac 1m)y= x - x_0 - (\frac 1m)y_0\\ \\ \iff x+ (-\frac ym) = \frac{y_0}m-x_0\\ \\ \iff a=1, b =-\frac 1m, c= \frac{y_0}m - x_0.$$ More generally, $(a,b,c)$ can be any ordered triple of the form $\Big(d(1), d\left(-\frac 1m\right), d\left(\frac{y_0}{m}-x_0\right)\Big),$ where $d$ is any constant.

One of the reasons I catalogue each of the above forms of equations of a line is to make evident, that given any of the equations given above, including the equation for calculating slope, if needed, a student should be able to obtain any other form of the equation of a line.
Comment: Interestingly whilst ax+by=c is called the 'standard form', my experiance is the most commonly taught and therefore probably what many people would consider the normal or standard form is y=mx+c


How does the rate of getwork requests correlate with the hashrate of a miner?

Question: In a setup where a standard bitcoind serves full difficulty targets to some mining nodes, how can I calculate the hashrate of a mining node from looking at the rate of getwork requests? Does this depend on the used mining program or the used mining extensions?
Comment: Why would you want to do something like that? If you want to minimize the amount of shares sent, you can just increase the difficulty by some factor, rather than giving full difficulty and using getworks as a measurement. Your solution could potentially lead to exploits where the miner would send more getwork requests while not processing them all.
Comment: Thank you for pointing that out. I understand that this question is of little importance for practical implementations. It does however help to better understand the getwork mechanism. I was experimenting with a local setup and was wondering about the exact relationship.
Answer: The standard getwork protocol forces the miner to send a new request every second. This means that if a miner takes longer than 1 second to check all nonces in a given getwork (2^32), it will be making a new request before it checks all values in a given request, thus giving you 1 getwork request per second regardless of the hashrate.
If the miner can solve the getwork faster than that, it would be making requests more often. In theory, you could use this formula to calculate the hashrate:
Hashrate = noncemax/time
Where time is the interval between the getwork requests and noncemax is the size of nonce field (2^32). However, this assumes no time delay due to the network lag and so forth, so it's an ideal scenario. Moreover, some miners discard unfinished work after submitting a share, so it would appear they are doing more work in your configuration than they actually are.
If you intend on using getwork extensions, specifically noncerange and rollntime, you could calculate the hashrate more accurately. Noncerange would change your noncemax to whatever size you set, and rollntime would allow you to measure time intervals greater than one second. However, not all miners support the extensions.
Comment: Thank you for this explanation. In case anybody wonders about the same thing: in my local setup your formula results in hashrates which are consistently about 36% above the values I get displayed by cgminer. I am looking at various hashrates, ranging from (according to cgminer) about 600Mh/s at 5.23 seconds between getworks to 1700Mh/s at 1.86 seconds between getworks. I'm assuming this is due to the effects you mentioned. Thanks again!
Comment: Getwork is old. Could you discuss [getblocktemplate](https://en.bitcoin.it/wiki/Getblocktemplate)? thanks
Comment: @Geremia Create a separate question and someone might elaborate.


ratio of flour to liquid in bread

Question: I am new to baking bread, and I have a recipe that calls for 2 1/4 cups total liquid to 2 3/4 cup whole wheat flour and 2 3/4 cup unbleached flour. I have made this several times, and can never mix in all of the flour, which will cause the bread to fall when baking. I do not have a mixer or bread machine, and this recipe is not for a bread machine. Help! Thank you!
Comment: Welcome to the site! Where did you get this recipe? It's way, way too dry. What kind of bread are you trying to make.
Comment: The liquid/flour ratio doesn't look too off to me...
Comment: This recipe was on the back of a Bob's Red Mill flour bag. I have tried this recipe several times, and the amount of liquid is never enough for the amount of flour. Does it matter if the flours are mixed together, or as the recipe says, mix in the unbleached flour first and then the whole wheat?
Answer: This works out to a ratio of 77% hydration (see this answer for an explanation of the term), which is a very good choice for this flour mixture and should not be dry at all. So the problem is most likely in your measurement. The best you can do is to use a scale, because volume is very unreliable. In  If you absolutely insist on measuring by volume, you will have to invest time in getting to do it properly - not packing, levelling well, etc.
Also, you should not be mixing the flour into the dough, you should bne kneading it. If by mixing you mean you are stirring it with a spoon, this is only an early step, you will have to continue kneading the soft dough in the bed of flour until the flour has been absorbed, and then continue for enough time to get a nice elastic dough (about 15 min of hand kneading). I am sorry if this sounds patronizing and you are already kneading properly, you did not describe it in detail so I am trying to exclude the possible culprits here.
Answer: I agree with rumtscho that the hydration (77%) is very consistent with other breads I've made, so I don't think the amounts are wrong.
Flour measurements are very prone to inaccuracy when using volume measurements.  Different flours will measure differently by as much as 30% in my experience.
If you cannot use by-weight measurement, you should use proper technique in measuring flour.  Spoon (with a tablespoon or similar) the flour into the measuring cup, not pressing down or packing between spoons, and when the cup is slightly over-ful take a knife or similar flat edge and scrape horizontally across the top of the measuring cup.
Also, ensure you are using a dry measuring cup, and not a liquid measuring cup; liquid measuring cups are very inaccurate for dry measurements.
Finally, be cognizant of the humidity of your environment.  If you are in a very dry environment (and if you have A/C on full blast you probably are, and winter is similarly more dry than summer), you may need slightly more water, and in a very wet environment slightly less water.  This is only a few tablespoons, though, it's unlikely you'd need more than that much more in a very dry environment, and in my experience most recipes are written for dry environments (as most of us cook in air-conditioned rooms) and so it's more likely you'd have too much water than too little (I usually reserve a few tablespoons from the total water if it's a new recipe and/or the humidity indoors seems high).

ratio of flour to liquid in bread


Does a low acceptance rate imply quality?

Question: I am presenting a paper in a conference with a 20% acceptance rate. However the conference is just 3 years old and does not figure in the tiered rating system for conferences in my field Computer Science (that some universities use) . Recently I found out that another reputed international conference (that figures in the conference rankings) has almost double the acceptance rate. Did I make the right choice. How should I evaluate conferences in the future??
Comment: I think, as it seems your question is with regard to your situation more than the quality of conference, and you care and need to publish in tiered rating conference, you will be better off avoiding this conference for now and give it more years until it becomes more mature.
Comment: Thank you. This was my first conference and most of the decisions were taken by my TA. I did not have much of a say in it. I guess it is too late now and I will keep this in mind the next time.
Answer: Low acceptance rate does not necessarily imply quality, but it is correlated.  Crappy conferences often have high acceptance rates because they don't care.  Broad-community high quality conferences often have low acceptance rates because lots of people are trying to get their material into the same conference.
These are just correlations, however, and not causal in nature.  For example, some very good conference have high acceptance rates because they serve a highly specific community; thus they tend to draw high-quality submissions from within that community and few submissions from outside.
Complementarily, I know of at least one conference that games its acceptance rate by artificially regulating the number of papers accepted.  They decide in advance how selective the conference will be, and set the cutoff not in terms of quality but in terms of their judgement of "good conference" acceptance rate.
Finally, having too high a selectivity may be bad for a conference.  A colleague of mine who is an insider at one of the extremely selective CS conference told me that they were worried that their acceptance rate was too low, and that therefore they weren't getting very interesting work any more: a single dubious reviewer was enough to sink a paper, so only conservative and incremental work was making it in.
Thus, acceptance rate is not a very good metric.  It's much better to look at who the people are who go to the conference, what they've been publishing there, and whether you want to be associated with those people and that type of work.
Comment: I agree with your last paragraph: I always look at the list of invited speakers before deciding whether to go to a conference or not
Comment: Thank you. I don't have much knowledge about the kind of researchers who go to this conference. But I have a fairly good idea about the organizers, and if the website is to be believed, they are among the most reputed in my country. Also, the previous edition of this conference was held in highest ranked science institute in my country (India).
Comment: _but it is correlated_ — [citation needed]
Comment: @JeffE Consider the extremes: do you know of any conferences with <15% acceptance rates that are not good conferences?  Complementarily, many crap conferences reject almost nothing.  There's your correlation; it's just very weak, since the range of practices of good conferences is so broad.
Comment: _do you know of any conferences with <15% acceptance rates that are not good conferences?_ — Yes.
Comment: @JeffE I'm intrigued... would you care to expand, or not in public on the internet?  That certainly is very different than my CS experiences...
Answer: Absolutely not.
In my field (theoretical computer science), the acceptance rate in top-tier conferences is around 30%, the acceptance rate in second-tier conferences is around 30%, the acceptance rate in third-tier conferences is around 30%, and the acceptance rate in bottom-tier conferences is around 30%.

How should I evaluate conferences in the future??

By reading the proceedings, of course.
Comment: I think my notion of "conference" is well above the true bottom.
Comment: I think your "bottom" is well above the true bottom...
Comment: Alas, as we see here quite often, I fear that many do not find it quite so clear---as [even a large organization like the IEEE seems to have recently learned the hard way](http://retractionwatch.com/2015/06/25/one-publisher-appears-to-have-retracted-thousands-of-meeting-abstracts-yes-thousands/).
Answer: Here is my two cents:

I judge the conferences based on the chair members and people
who would attend it and I would like to meet and talk or see their
state of art. Conferences are first and foremost a place to know
other researchers in your field, share thoughts off the record and
outside of the boundaries of email and web and learn and network
with others who share interests. Possible collaboration and sharing
resources or experiences and collecting feedback on your work should
be the number one outcome of your conference experience.
Ranks are 80% meaning, 20% noise. If a lower rank conference of a
sub-field is in winter in a horrible place, but I know for sure the
top researchers of my subfield although small group of people would go to,
I should rank it higher for myself than the generally higher rank and
biggest conference that everyone would stop by for a day and there are too
many people there to be able to establish a meaningful event.
Publishing the acceptance rate is a good sign, 20% also shows they are
as selective as possible. however, you should consider for the selection
bias among other people who submit their papers there.
Answer: To add, single track conferences, e.g., ACM SIGCOMM, tend to have very low acceptance rates.  On other hand, big conferences such as IEEE ICC, have around 40% acceptance rate, but for some community, that's their top conference!


Measuring scalability of the web app hosted on cloud

Question: I have developed a social networking site using the elgg framework and I am hosting it on amazon cloud (Amazon EC2, the free tier micro instance service) and thus develop a benchmark for it.I am creating around 200 columns describing each user (most of them dummy) and after that I should create around a million users with each users profile updated with some data.This is done to reflect the image of big data.When hosted on cloud we should measure the cloud's performance based on a query and an update action for all users. The problem is how to create so many users? Which tool would be optimal to choose? Done this, I should also consider storage on a file system(HDFS) and do the same with some modifications (The output should be a row and the input should be an unstructured data).
For elgg framework we are using mysql as backend. I have no idea how to start with it. Any suggestions would be really helpful.
Thank you.
Answer: I had to perform a similar task recently and came up with this script.. maybe it can help you


Paying off a higher interest loan using a lesser interest loan

Question: I took a loan for 3,000,000 at 9.2% from a bank for a period of 4 years (I need this much for my purpose). EMIs (equated monthly installments) are calculated based on a reduced balancing basis and works out to be around 75000 (EMIs have not started as of now). Now, my loan paying capacity per month is 95000. Also, I have access to an amount of 600,000 flat-rate loan at 4% for a year which also have the nice condition that there are no EMIs and I can pay it back in parts or full in any time in the year (4% of the total amount has to be paid back . Given my paying capacity, is it a good idea to take this lesser loan of 600K, prepay the original loan for by that much. In details, finish the lesser loan in 6 months (7*95K), then retake it again and so on.
Comment: If you can refinance to a lower interest rate (which seems to be what you are asking about) without having to pay an unreasonable amount in fees, you can generally save money by doing so. But to get the lower rate you may need to shorten the term of the loan and wind up paying more each month. -- assuming you can get that lower-rate loan at all, and that your existing loan allows paying it off early. Run the specific numbers -- most banks have a tool on their website which can help you determine whether refinancing makes sense.
Comment: It's not clear to me what happens with the EMIs if you take out the small loan and pay them.  Do you still have to pay monthly?  Do you skip that amount in payments (i.e. if you pay off as much as eight EMIs in a lump sum, do you skip eight EMIs)?  Do your EMIs get smaller?  I.e. if you pay off a fifth of the loan, are your EMIs reduced by a fifth?  When you say this is a flat-rate loan, what does that mean?  Do you pay 4% of the loan amount regardless of when you pay it back?  Writing up examples might help you as well as us.  Which country often matters.
Comment: @Brythan The smaller loan is flat-rate. I have to pay back 24000 as (4% of 600K) whenever I wish to pay it back in a year in addition to the principal 600K. For the larger loan, I make a lump sum payment of 600k. Then, this payment goes towards the principal. The emi now remains same. However, 3 years now get reduced.
Comment: The 3 years gets reduced at the end?  So you only have 20,000 (20k) to pay off the new 624k loan?  Or you can skip the next eight EMI payments (600k/75k = 8)?  Plus a little extra at the end for reduced interest.  Then you'd have twenty-eight months of payments left on the big loan.  Possibly a little less.  I'm also having trouble making the numbers add up.  The 75k payment doesn't seem to pay off the loan in three years at 9.2% interest.  It does pay off a four year loan that is otherwise the same.
Comment: Can you double check your numbers? I agree with Brythan, 3000000 at 9.2% for 3 years would be a payment of 95679/month. At 4 years it would be 74940/month.
Comment: sorry for the late reply, yes it is 4 years.
Answer: TL;DR:  NO, you should not use it the way that you suggest using it.
I threw this into a spreadsheet.  The result that I got by following your method (borrow 600000 and pay the next eight monthly payments with it; then pay 95000 per month against the flat-rate loan until the 600000 is paid) is paying off the loan after thirty-nine months rather than the forty-eight with monthly payments of 75000.
This seems to be due to paying 95000 a month rather than 75000.  If I just change to 95000 without playing with the smaller loan, the loan gets paid off in just thirty-six months.  That's actually cheaper than the other loan, presumably because you would normally only pay half of the interest on that 600000 (interest payments normally reduce during the loan).  Also, the flat 4% is paid on only eight months borrowing, so it annualizes to something like 12% interest.
I tried an alternative that delayed paying the 624000 as long as possible to maximize the interest advantage.  However, this still took thirty-six months.  It might have produced a small advantage over just paying the 95000, but not much of one.
Borrowing 600000 and paying off 695000 of the larger loan in the first month is the most effective.  Then pay 95000 a month on the larger loan, except every twelfth month pay 71000 on the larger loan and the 24000 interest on the flat-rate loan.  Take out a new flat-rate loan to pay off the principal on the old one.  That's the most effective.  It reduces you from thirty-six payments to thirty-five.  You pay 72000 in interest on the three 600000 loans and 287427 interest on the 3000000 loan.
In summary, you can pay off the loan in thirty-six months by paying 95000 every month.  You can cut one month off that by taking out a 600000 loan in the first month and waiting as long as possible to pay it back.  To repay the flat-rate loan, you take out a new one every twelve months.  Once the large loan is paid off (after twenty-eight months), pay off the flat-rate loan.  That will take seven months.
Note that if they refuse to extend a new loan to you to replace the old one, there's nothing that you can do in that case.  You'd have to take the consequences of defaulting on payment of the loan, which might be worse than the original loan.  Of course, perhaps you can get them to loan you the same amount for three years at 12%.
I'll leave it up to you if cutting a month off the repayment period is worth enough to justify the headache of the flat-rate loan.
The limited savings primarily caused by the flat-rate weirdness.  The 4% rate is comparable to an 8% APR the way that the other loan is calculated.  And that's only if you wait as long as possible to pay it back.  The quicker you pay it, the less advantage there is from the flat rate.  That's why you should pay off the flat-rate loan last if you get it.