(Note: no disrespect whatever to the hard work people that people who run 3rd-party repositories put in.)  I came across a package repository, a ppa, that was suggested for getting php5.5 on Ubuntu 12.04. This has to be added to the list of repositories that Ubuntu knows to download software.  In general, can such 3rd-party repositories be trusted?   In this particular case, the ppa seems to be well regarded - and I would certainly want to support this and recommend it.  Is there a checklist or criteria one can follow to arrive at a decision to trust a 3rd-party repository?  The aim here is to avoid downloading malware or anything else that may damage the computer.
I see a lot of interesting programs out there that can only be obtained by adding a "PPA" to the system but, if I'm understanding correctly, we should stay within the official "repositories" for adding software to our system.   Is there any way for a novice to know if a "PPA" is safe or if it should be avoided? What tips should the user know about when dealing with a PPA?.
What is the purpose of Jordan Canonical Form? Windows 7 USB install error I am trying to install Windows 7 Professional x64 from a bootable USB stick. I have used the Windows 7 USB DVD Download Tool to create the media from an ISO.  The machine boots from USB fine, and enters the Windows installation wizard but then I get an error stating:     A required CD/DVD drive device driver is missing. If you have a driver floppy disk, CD, DVD, or USB flash drive, please insert it now.   I have tried setting the SATA controller in the BIOS to both AHCI and ATA but neither appear make any difference.  I've also tried both the USB 3 and USB 2 ports with no difference.  I've also tried to use the files from an official Dell recovery CD as this is a Dell laptop but again, no dice.  All this because there's no optical drive in the machine!  Dell Vostro V131 / i5-2450M / BIOS A04  Can anyone offer any advice please? supervisor field on the page of the title with Beamer I am editing a presentation of a research project, I am wondering if there is a field for "supervisor" on the page of the title, for instance \supervisor(which actually does not work)... If not, where do people conventionally add it? Converting GeoPDF to vector format like CAD or shapefile? I want to convert a GeoPDF that has vectors entity in it to a vector format and preserve the real coordinates of these vectors. The GeoPDF has been exported from ArcMap and has layers in it.  I have try GDAL, but it rasterize the PDF before exporting it. Which is not good for me. Also I don't want a solution that would convert this image back to vectors, too complicated for the data I have.  Is someone aware of any tool that can do that? The resulting format can be a CAD file or a shapefile or anything that is a vector format. Indefinite integral of $ \int \frac{x^3}{\sqrt{x^2+1}} \,\text{d}x$. Can you please provide any sort of hint or suggestion on how to find the following indefinite integral?  $$\int\frac{x^3}{\sqrt{x^2+1}}\text{d}x$$  I tried substituting everything but it didn't work. I also tried trigonometric substitution but I couldn't find any valid trigonometric identity for $ \sqrt{\cos^2(x)+1} $ or $ \sqrt{\sin^2(x)+1} $. xslt, javascript and unescaped html entities i have a tiny little problem with xslt, js and html entities, eg. within a template:  &lt;script type="text/javascript"&gt;     &lt;xsl:value-of select="/some/node"/&gt;     for (var i = 0; i &amp;lt; 5; i++) {         //            ^^^ js error     } &lt;/script&gt;  &lt;script type="text/javascript"&gt;     &lt;xsl:value-of select="/some/node"/&gt;     for (var i = 0; i &lt; 5; i++) {         //            ^ xslt error     } &lt;/script&gt;  &lt;script type="text/javascript"&gt;     &lt;xsl:value-of select="/some/node"/&gt;     // &lt;![CDATA[     for (var i = 0; i &lt; 5; i++) {         //            ^ becomes &amp;lt;     }     // ]]&gt; &lt;/script&gt;   &lt;script type="text/javascript"&gt;     &lt;xsl:value-of select="/some/node"/&gt;     for (var i = 0; i &lt;xsl:value-of disable-output-escaping="yes" select="string('&amp;lt;')"/&gt; 5; i++) {         // works of course     } &lt;/script&gt;   does anyone have an idea where my problem could come from? i always thought the xslt processor would leave the content of a &lt;script/&gt; element unescaped when using the html output method ...  i run libxslt2 version 1.1.24 on OSX which was installed using macportsports ... I'm trying to understand the derivation of the equations for the logistic regression. I'm following the cs229 notes:    At some point in the derivation, in the notes we need to maximize the log-likelihood of the parameters:  ℓ(θ) = log L(θ) = y log h(x) + (1 - y) log(1-h(x))  In the notes, after the derivation we get:  $\frac{\partial}{\partial \theta_j}$ = (y $\frac 1 {h(x)}$ - (1 - y) $\frac 1 {1 - h(x)}$) $\frac{\partial}{\partial \theta_j}$h(x)  Shouldn't this be:  $\frac{\partial}{\partial \theta_j}$ = (y $\frac 1 {h(x)}$ + (1 - y) $\frac 1 {1 - h(x)}$) $\frac{\partial}{\partial \theta_j}$h(x)  I've been looking at this for a while and I can't see where this minus sign is coming from...  Thanks for your help! What does ‘Sport’ mean when you say ‘the new Apple iPad sports cameras for video conferencing’? Array of Matrices in MATLAB I am looking for a way to store a large variable number of matrixes in an array in MATLAB.  Are there any ways to achieve this?  Example:  for i: 1:unknown   myArray(i) = zeros(500,800); end   Where unknown is the varied length of the array, I can revise with additional info if needed.  Update: Performance is the main reason I am trying to accomplish this.  I had it before where it would grab the data as a single matrix, show it in real time and then proceed to process the next set of data.  I attempted it using multidimensional arrays as suggested below by Rocco, however my data is so large that I ran out of Memory, I might have to look into another alternative for my case.  Will update as I attempt other suggestions.  Update 2:  Thank you all for suggestions, however I should have specified beforehand, precision AND speed are both an integral factor here, I may have to look into going back to my original method before trying 3-d arrays and re-evaluate the method for importing the data. What is object slicing? Relationship between SVD and PCA. How to use SVD to perform PCA? Why can "trans" be replaced with an x? I can't think of an example, so I may be wrong about this, but I think I've seen people replace the prefix "trans" as in transport with an x. "Cross" makes sense, as in "railroad crossing", and I guess "chris" is sort of a punny extension, the extension being like "chris cross" and the example being "xmas", but why trans? What was the first version of Ubuntu and where can I find it? lists Ubuntu 4.10 (Warty Warthog) as the first Ubuntu released by Canonical Ltd.  But I wanted to know whether there ever was an Ubuntu 1.x, as in Windows or Mac OS. If yes, is it available somewhere? Flames with no gravity? I was watching  today, and noticed this: in some moment the space station changed orbit and the people inside experienced zero-gravity. At that moment, a candlestick passed floating in the air, with the candles burning ().   But in our environment the flames go upward because the hot air is less dense, and that effect should disappear when no gravity is present, am I right? How would a candle flame look then? Restore console session after remote desktop disconnects? How do you configure email notifications in Cognito Forms? Google Chrome (Windows) not displaying text/encoding correctly on certain websites I experience a problem similar to    Here's a screenshot on the website     I've tried:   Removing fonts I have installed Setting the encoding to Western, UTF-8, Greek, Automatic Changing the fonts used in Google Chrome Removing Google Chrome, including all application data/user profiles Running CCleaner Reinstalling Google Chrome, twice Installing Chromium (fonts display fine in Chromium) Turning off Windows ClearType Changing Windows/Chrome languages from English (Australia) to US, UK Restarting Windows several times after making changes   All of which did not work. I do not connect over VPN/proxy.   Can anyone give me any tips?  Windows 7 x64 Ultimate How to directly prove that $M$ is maximal ideal of $A$ iff $A/M$ is a field? An ideal $M$ of a commutative ring $A$ (with unity) is maximal iff $A/M$ is a field.  This is easy with the correspondence of ideals of $A/I$ with ideals of $A$ containing $I$, but how can you prove it directly?  Take $x + M \in A/M$.  How can you construct $y + M \in A/M$ such that $xy - 1 \in M$?  All I can deduce from the maximality of $M$ is that $(M,x) = A$. Is an expanding map on a compact metric space continuous? Logistic regression cost surface not convex I am building a simple logistic regression model on 2D data. Here is the input I use.    I built a logistic regression model using this data and it successfully is able to find the discriminating line between these two classes. Here is the separating line found by my program:    Also, just for debugging, I plotted number of iterations vs J(theta), which also looks fine:    Now, confusing part comes into play. I am investigating the cost surface of this model that the logistic regressions is doing the gradient descent on. I am considering three different loss functions. Sum-of-squared loss, hinge-loss, log-loss.  Hinge loss:    Log-loss:    Sum-of-squared loss:    As far as I know, both hinge loss, sum-of-squared loss and log-loss should produce a convex surface. When I look at my surfaces, however, sum-of-squared surface does not look convex to me. Why is that the case?  For convenience, here is the two critical functions from my code:  function retval = J(X, y, theta)     M = size(y, 1);     % sum of squared error     retval = (1/(2*M)) * sum((h(X, theta) - y).^2);  endfunction  function retval = h (X, theta)     % logistic     retval =  1 ./ (1+ exp(-1 * (X * theta))); endfunction