I am trying to understand  paper. It uses PCA and further uses LDA for dimensionality reduction.   I have read about eigenfaces and PCA approach but I am not familiar with LDA and I am unable to understand the mathematics behind the scatter matrices : why to minimize the determinant of projected scatter matrices and how we perform it.   Can anyone explain the maths behind LDA and how the generalized eigenvalues come into picture or point to some related tutorial where I can understand the approach?
Apparently,     the Fisher analysis aims at simultaneously maximising the   between-class separation, while minimising the within-class   dispersion. A useful measure of the discrimination power of a variable   is hence given by the diagonal quantity: $B_{ii}/W_{ii}$.     I understand that the size (p x p) of the Between (B) and Within-Class (W) matrices are given by the number of input variables, p. Given this, how can $B_{ii}/W_{ii}$ be a "useful measure of the discrimination power" of a single variable? At least two variables are required to construct the matrices B and W, so the respective traces would represent more than one variable.  Update: Am I right in thinking that $B_{ii}/W_{ii}$ is not a trace over a trace, where the sum is implied, but the matrix element $B_{ii}$ divided by $W_{ii}$? Currently that is the only way I can reconcile the expression with the concept.
using a:hover to change the color of an image How can I find hard links on Windows? I've created some hard links on my Windows 7 file system using mklink. It was some time ago and I can't remember for sure where, or which files. When I use Explorer, all files look the same. When I use the command line and type "dir", they all look the same.  How do I find hard links? Or how do I determine whether a specific file IS a hard link? Why does my regular expression work in X but not in Y? No homomorphism from $Z_{16}\oplus Z_{2}$ onto $Z_{4}\oplus Z_{4}$. For some reasons, homomorphism is a very hard area for me to make improvements. I've been hitting the brick wall for almost 2 hours.   Prove that no homomorphism exists from $Z_{16}\oplus Z_2$ onto $Z_4\oplus Z_4$.  Assume a homomorphism $\phi$ exists from $Z_{16}\oplus Z_2$ onto $Z_4\oplus Z_4$. Then $\phi$ is an isomorphism.  Note that the $\left | \ker \phi \right |=2$ The $\ker \phi$ is also a normal subgroup for $Z_{16}\oplus Z_{2}$. We want possible normal subgroup of order 2. I.e., 2 elements in each normal subgroups.  By Lagrange's theorem, the order of each element in a group divides the order of a group so the possible order of the elements are 1 or 2. If the elements has order 1, then the Ker \phi cannot have 2 elements.  Hence, $\ker\phi$ = $\left \{ (0,0),(8,0) \right \}$ ,$\left \{ (0,0),(0,8) \right \}$,$\left \{ (0,0),(8,1) \right \}$, possibly.  By the First isomorphism theorem:  $\phi: Z_{16}\oplus Z_2 \rightarrow Z_2\oplus Z_2$  $\Psi: G/\ker \phi \rightarrow \phi\left ( Z_16\oplus Z_2 \right )=Z_2\oplus Z_2$  $g\ker \phi  \mapsto \phi(g)=\Psi(g\ker \phi)$  Any help is appreciated.  Edit: I know that $(2,0)$ has order $8$ in $Z_{16}\oplus Z_{2}$ but any elements in $Z_{4}\oplus Z_{4}$ does not have order $8$ which would have solved the question at the outset. But I would like a different route. siunitx tabular numbers with separated uncertainty I would like to format the space reserved for a number shown in a table with separated uncertainty. It seems that when I use the table-format option the brackets around the mantissa disappear. This is illustrated in the example below:  \documentclass{article}     \usepackage{siunitx}  \begin{document}     \begin{table}   \begin{tabular}{                   S[separate-uncertainty = true]                   S[table-format=1.1(1)e1,                   separate-uncertainty = true]                   }     {Header} &amp; {Header}\\     1.0(2)e2 &amp; 1.0(2)e2\\   \end{tabular} \end{table}     \end{document}     I have also tried to use the bracket-numbers and multi-part-units = brackets without solving this issue. Is there any workaround to center the number and keep the brackets like in the first column of the example? The email for getting notifications can be changed by clicking the "preferences" tab in your profile. After entering a new email address, the "Save email settings" button becomes clickable:    Clicking the button turns it grey:    So all looks well. But if you don't confirm the email by clicking on the link sent to your email inbox, the next time you go back to the preferences, you will see your old email address with no indication that it changed.  You can subscribe to the weekly newsletter from the "edit" tab of your profile. If you click the option and save, the next time you visit the edit page before confirming your email you get this message:    Once you confirm the email, the resend link disappears so you know the subscription is set up:    Ideally, the two systems would be unified so that all email subscriptions happen the same way. But short of that, it would be very helpful to indicate that an email address is unconfirmed for notification emails just like newsletters. Making the bibliography appear in the table of contents Why aren't logarithms defined for negative $x$? Given a logarithm is true, if and only if, $y = \log_b{x}$ and $b^y = x$ (and $x$ and $b$ are positive, and $b$ is not equal to $1$), are true, why aren't logarithms defined for negative numbers?  Why can't $b$ be negative? Take $(-2)^3 = -8$ for example. Turning that into a logarithm, we get $3 = \log_{(-2)}{(-8)}$ which is an invalid equation as $b$ and $x$ are not positive! Why is this? What does the |= operator do in Java? How do I solve the following second order partial differential equation?  $4u_{xx}+5u_{xy}+u_{yy}+u_x+u_y=2$  I have classified the equation to be hyperbolic and changed variables to obtain the canonical form as $u_{\epsilon\nu}={1\over{3u_\nu}}-{8\over9}$ which I believe is correct but I am struggling to find the general solution? A step by step solution would be appreciated. MicroSD card is set to Read-only state. How can I write data on it? Assuming $T_1$ is the force that acts on box $1$ and $T_2$ is the force that acts on box $2$.  Exactly what causes the Tension? Why does $T_1 = T_2$?  The problem is we are told to memorize that $T_1 = T_2$ for mass less ropes, but I do not understand why, especially in an Atwoods machine.  When I think about it, I know it has something to do with the force $m_2g$ which causes the box with mass $m_1$ to rise, and vice versa.  But I cannot apply physics terminology to this, or really understand whats going on. Java obfuscators I'm looking for a good Java obfuscator.  I've done initial research into the following Java obfuscators: proguard, yguard, retroguard, dasho, allatori, jshrink, smokescreen, jobfuscate, marvin, jbco, jode, javaguard, jarg, joga, cafebabe, donquixote, mwobfu, bbmug, zelix klassmaster, sandmark, jcloak, thicket, blufuscator, and java code protector.  I tried proguard and it has a really nice GUI, seems really stable, and seems to be the most popular, but it seemed to not like some enumeration on a referenced jar file (not within the code I was trying to obfuscate) which was weird.  Yguard seems to require some interaction with ant, which I didn't know too much about.  What is a good java obfuscator?  It doesn't need to be free, it just needs to work well and be easy to use. I have a edge loop that has vertices on it. Is there a way I can separate the vertices on the the edge evenly without moving the end points? I would also like for the vertices to maintain the current shape. How to call child component controller function/helper from Parent Component Controller Story about astronauts who set off an alien warning beacon on the moon (or Mars?) In my game, XP needed to reach the next level is Current Level × Level Threshold.  Given that, how can I get my current level from the total XP ever earned?    For example:  Level Threshold = 50 Current Level = 1   Starting at level 1, I would need (1 × 50) = 50 XP to get to level 2, and so on.  Level 1: 50 XP needed to reach level 2 Level 2: 100 more XP needed to reach level 3 Level 3: 150 more XP needed to reach level 4   In other words, the progression table is as follows:  Level 1: 0 XP to 49 XP Level 2: 50 XP to 149 XP  Level 3: 150 XP to 299 XP Level 4: 300 XP to 499 XP   If I have 300 XP, I'll just reached level 4. How can I calculate that generally? How do i convert a binary int into a string? Greek pdf bookmarks I am puzzled with sections titled with Greek characters. They do appear in the pdf's Content list, though not correctly.  For example, in my thesis--written in English with LyX, based on the KOMA-script--I choose to include a translation of the summary in Greek (=Περίληψη). The "raw" code for the (unnumbered) chapter title reads: \addchap{\textgreek{Per\char208lhyh}}. In the final .pdf file (using pdflatex) the respective entry appears as Per208lhyh. Using the unicode option for the hyperref package does not fix the problem.  Update detail: the document (should) not only contain Chapter or Section titles in Greek, but normal text as well.  I have found some relevant references within tex.stackexchange and elsewhere (e.g., , the manual of the textgreek package, , ). However, I cannot extract a concrete answer.  How can Greek characters appear correctly as bookmarks in the .pdf's Content list?  Note, the information of this Q&amp;A applies to/under LyX as well. How do I check whether a file exists without exceptions?