Linear Algebra December 16, 2009

# MIT Linear Algebra, Lecture 3: Matrix Multiplication and Inverse Matrices

<- previous article next article ->

This is the third post in an article series about MIT's course "Linear Algebra". In this post I will review lecture three on five ways to multiply matrices, inverse matrices and an algorithm for finding inverse matrices called Gauss-Jordan elimination.

The first lecture covered the geometry of linear equations and the second lecture covered the matrix elimination.

Here is lecture three.

## Lecture 3: Matrix Multiplication and Inverse Matrices

Lecture three starts with five ways to multiply matrices.

The first way is the classical way. Suppose we are given a matrix A of size mxn with elements aij and a matrix B of size nxp with elements bjk, and we want to find the product A·B. Multiplying matrices A and B will produce matrix C of size mxp with elements .

Here is how this sum works. To find the first element c11 of matrix C, we sum over the 1st row of A and the 1st column of B. The sum expands to c11 = a11·b11 + a12·b21 + a13·b31 + ... + a1n·bn1. Here is a visualization of the summation:

We continue this way until we find all the elements of matrix C. Here is another visualization of finding c23:

The second way is to take each column in B, multiply it by the whole matrix A and put the resulting column in the matrix C. The columns of C are combinations of columns of A. (Remember from previous lecture that a matrix times a column is a column.)

For example, to get column 1 of matrix C, we multiply A·(column 1 of matrix B):

The third way is to take each row in A, multiply it by the whole matrix B and put the resulting row in the matrix C. The rows of C are combinations of rows of B. (Again, remember from previous lecture that a row times a matrix is a row.)

For example, to get row 1 of matrix C, we multiply row 1 of matrix A with the whole matrix B:

The fourth way is to look at the product of A·B as a sum of (columns of A) times (rows of B).

Here is an example:

The fifth way is to chop matrices in blocks and multiply blocks by any of the previous methods.

Here is an example. Matrix A gets subdivided in four submatrices A1 A2 A3 A4, matrix B gets divided in four submatrices B1 B2 B3 B4 and the blocks get treated like simple matrix elements.

Here is the visualization:

Element C1, for example, is obtained by multiplying A1·B1 + A2·B3.

Next the lecture proceeds to finding the inverse matrices. An inverse of a matrix A is another matrix, such that A-1·A = I, where I is the identity matrix. In fact if A-1 is the inverse matrix of a square matrix A, then it's both the left-inverse and the right inverse, i.e., A-1·A = A·A-1 = I.

If a matrix A has an inverse then it is said to be invertible or non-singular.

Matrix A is singular if we can find a non-zero vector x such that A·x = 0. The proof is easy. Suppose A is not singular, i.e., there exists matrix A-1. Then A-1·A·x = 0·A-1, which leads to a false statement that x = 0. Therefore A must be singular.

Another way of saying that matrix A is singular is to say that columns of matrix A are linearly dependent (one ore more columns can be expressed as a linear combination of others).

Finally, the lecture shows a deterministic method for finding the inverse matrix. This method is called the Gauss-Jordan elimination. In short, Gauss-Jordan elimination transforms augmented matrix (A|I) into (I|A-1) by using only row eliminations.

Please watch the lecture to find out how it works in all the details:

Topics covered in lecture three:

• [00:51] The first way to multiply matrices.
• [04:50] When are we allowed to multiply matrices?
• [06:45] The second way to multiply matrices.
• [10:10] The third way to multiply matrices.
• [12:30] What is the result of multiplying a column of A and a row of B?
• [15:30] The fourth way to multiply matrices.
• [18:35] The fifth way to multiply matrices by blocks.
• [21:30] Inverses for square matrices.
• [24:55] Singular matrices (no inverse matrix exists).
• [30:00] Why singular matrices can't have inverse?
• [36:20] Gauss-Jordan elimination.
• [41:20] Gauss-Jordan idea A·I -> I·A-1.

Here are my notes of lecture three:

My notes of linear algebra lecture 3 on the matrix multiplication and inverses.

Have fun with this lecture! The next post is going to be about the A=LU matrix decomposition (also known as factorization).

PS. This course is taught from Introduction to Linear Algebra textbook. Get it here:

Computer Science December 14, 2009

# Recursive Regular Expressions

The regular expressions we use in our daily lives are actually not that "regular." Most of the languages support some kind of extended regular expressions that are computationally more powerful than the "regular" regular expressions as defined by the formal language theory.

For instance, the so often used capture buffers add auxiliary storage to the regular expressions that allow them to match an arbitrary pattern repeatedly. Or look-ahead assertions that allow the regular expression engine to peek ahead before it making a decision. These extensions make regular expressions powerful enough to describe some context-free grammars.

The Perl programming language has an especially rich with regex engine. One of the engine's features is the lazy regular subexpressions. The lazy regular subexpressions are expressed as (??{ code }), where the "code" is arbitrary Perl code that gets executed when the moment this subexpression may match.

This allows us to construct something really interesting - we can define a regular expression that has itself in the "code" part. The result is a recursive regular expression!

One of the classical problems that a regular expression can't match is the language 0n1n, i.e., a string with a number of zeroes followed by an equal number of ones. Surprisingly, using the lazy regular subexpressions this problem becomes tractable!

Here is a Perl regular expression that matches 0n1n:

$regex = qr/0(??{$regex})?1/;


This regular expression matches a 0 followed by itself zero or one time, followed by a one. If the itself part doesn't match, then the string this regular expression matches is 01. If the itself part matches, the string this regular expression matches is 00($regex)?11, which is 0011 if $regex doesn't match or it's 000($regex)?111 if it matches, ..., etc. Here is a Perl program that matches 050000150000: #!/usr/bin/perl$str = "0"x50000 . "1"x50000;
$regex = qr/0(??{$regex})*1/;

if ($str =~ /^$regex$/) { print "yes, it matches" } else { print "no, it doesn't match" }  Now let's look at the Yo Dawg regular expression in the picture above. Can you guess what it does? It matches a fully parenthesized expression such as (foo(bar())baz) or balanced parentheses ((()()())()). $regex = qr/
$$# (1) match an open paren ( ( # followed by [^()]+ # (3) one or more non-paren character | # OR (??{regex}) # (5) the regex itself )* # (6) repeated zero or more times$$                 # (7) followed by a close paren )
/x;


Here is how to think about this regular expression. For an expression to be fully parenthesized, it has to start with an open paren, so we match it (point (1) in the regex). It also has to end with close paren, so we match a close paren at the end (point (7)). Now we have to think what can be in-between the parens? Well, we can either have some text that is neither an open paren or closed paren (point (3)) OR we can have another fully parenthesized expression! (point (5)). And all this may be repeated either zero times (point (6)) to match the smallest fully parenthesized expression () or more times to match a more complex expression.

Without the /x flag (that allows multiline regexes), it can be written more compactly:

$regex = qr/$$([^()]+|(??{regex}))*$$/;  But please don't use these regular expressions in production as they are too cryptic. Use Text::Balanced or Regexp::Common Perl modules. And finally, in Perl 5.10 you can use recursive capture buffers instead of lazy code subexpressions to achieve the same result. Here is a regular expression that matches 0n1n and uses the recursive capture buffer syntax (?N): my$rx = qr/(0(?1)*1)/


The (?1)* says "match the first group zero or more times," where the first group is the whole regular expression.

You can try to rewrite the regular expression that matches balanced parens as an exercise.

Have fun!

The New Catonmat December 10, 2009

# 50 ideas for the new catonmat.net website

I have been working on the new catonmat.net website for quite some time now and I have gathered a very long list ideas of what a modern website should have. They include ideas from psychology, search engine optimization, social media, programming, and the best web design practices.

Since I started to use github last week, all the new catonmat code will be there. I already created a catonmat repository and I am going to be pushing code there daily.

I love to share my ideas, so here are the first 50 of them. I am going to share more later in the upcoming article series "Designing the new catonmat.net together with me." You should subscribe to my blog here, if you are interested in this topic and haven't subscribed yet.

The ideas are in no particular order. If some of them seem fuzzy or unclear, please ask me to clarify in the comments.

## 1. 301 Redirect Table.

The idea is to maintain a table that will be checked against the request URLs. If the request URL is in the table, it gets redirected to a new destination URL with the 301 HTTP header, which forwards the link juice from one URL to another. This is necessary because some forums break URLs in multiple lines and the resulting clickable URL is 404. If I didn't do a 301 to the new location, this link would be lost. Anyone who clicked it, would end up on the 404 error page. Another example is if someone links to a mistyped URL. Anyone who clicks it ends up being unsatisfied, at 404 page. With a 301 table I can quickly fix these problems. Here is a concrete example: Someone links to www.catonmat.net/artikle when they wanted to link to www.catonmat/article. I'd simply insert an entry to 301 redirect /artikle to /article and everyone's happy.

## 2. 404 Error Log.

I absolutely have to know what pages trigger 404 errors so that I can fix them ASAP with the 301 redirect table. There may be many false alarms, therefore I have to add filters to the 404 error log to ignore some spammy patterns.

## 3. A Great 404 Page.

If someone ends up on a non-existing page, log it as described in #2 and then explain to the visitor why the page he visited was not found. Suggest him or her or it to view 5 latest post titles, or 5 most popular posts, or most popular downloads, or something else (still have to think about it when I implement it).

Tracebacks are usually displayed among comments. This is very messy. There is no place for them among comments. The idea is to have them on a separate page like www.catonmat.net/post-title/trackbacks. Also add bulk moderation as spammers love to exploit trackback. (Perhaps drop trackbacks altogether as they are of very little value.)

## 5. A Page With Last Comments.

Currently I am displaying only the 10 last comments on the right sidebar. This is insufficient. I want to see all the comments (like www.reddit.com/comments) on a single page, so that even if I am away from computer for several days, I can easily navigate through them and reply/hide/edit/delete in-place. Perhaps add a RSS feed for comments page.

## 6. Comment Statistics.

I need to tell my blog readers who are the most active people on my website. This will stimulate people to be more active. Therefore I should add a comment statistics with most popular commentators, link to their pages without nofollow to give them link juice. Also add statistics of the most commented articles.

## 7. Add Incoming Search Term Statistics On All Pages.

When people come from Google or Yahoo, save the query they used and display it on the page. This way I will always know what terms were searched for on each of the pages, without having to write complex queries.

Currently I have sloppy download statistics. I want nice graphs and want to see the most popular downloads by day, month, etc. This should be written as a statistics framework as traffic statistics, article statistics, delicious statistics and download statistics will all have more or less same graphs.

## 9. Public Statistics Section For del.icious.com Bookmarks.

I want to see how often my posts get bookmarked on del.icious.com. I also want nice graphs for them by day, week, month, etc. I also want a list of people who bookmarked my posts the most often, and tags they used. Make this public and also very modular so other people can reuse the code and put it on their sites. Reward the most frequent bookmarkers with links to their sites.

## 10. Insert Beautiful Images To Give Rest To Eyes.

Insert some beautiful landscape images after serveral paragraphs of text to give rest to eyes and give the reader positive emotions.

An image from ginnerobot's photostream on Flickr.

## 11. Public Traffic Statistics.

I want the super statistics for my website. And make them public. I want to see which is the most popular article today, which was yesterday. I want nice traffic graphs and trends.

Find who's tweeting about catonmat and put all these tweets on a separate page. Find who's the most active catonmat tweeter and make this person stand out. Link to twitter profiles.

## 13. Integrate GitHub Directly In Catonmat

Integrate GitHub with status updates, friend count, directly in catonmat. Would have to write some kind of a scraper or use their API, if it's usable for this purpose.

Currently I have linear comments, which are no good. To find whom someone replied to I have to scroll through the comments. This doesn't make any sense. I have to add threaded comments. This will also engage users is more conversations.

I don't like links to comments in a form www.catonmat.net/article/#comment-55. I want them to be in form www.catonmat.net/article/comment-55 so that each comment (or thread of comments) is on its own page. This way, if linked to a comment, person will precisely have that comment loaded. Browsers can misbehave in case of anchor links like #comment-55 and I don't like that.

## 16. Lightweight Syntax For Comments.

There are just a few things comments need:

• Quote someone.
• Emphesize part of a comment in bold or italic.
• Share code fragment (auto syntax highlight it).

That's it. Nothing else is necessary, no stupid HTML comments.

## 17. Filter Comments By Language.

My website is getting a lot of spam from Russia with comments in Russian. I am sure no one would write comments in Russian on my site. Add a filter to leave just the English comments. All other are spam.

Gravatar is at www.gravatar.com. It's a map from emails to jpegs of user icons. This will make people stand out. Also display the gravatar user icons on comment statistics (point above). Having gravatars will emotionally associates you with commentators and the next time you see a gravatar you can predict the nature of the comment (depending if you had positive emotions or negative before).

## 19. Add More "Contact Me" Options Near "About me" on The Right Sidebar.

You want to get yourself out. If you don't do it, no one will come after you. Add links to Facebook, Linked In, Twitter, Plurk, GitHub, FriendFeed, perhaps some other sites. Show the email as an image and add a link to my IRC channel #catonmat on irc.freenode.net. Initially show only Twitter, Facebook and GitHub. Add an arrow down clickable image. After clicked displays all other contact options.

## 20. Snippets Page.

I have been writing and collecting various programming snippets. I want to have them in a central database on my site. Instead of putting them on some foreign service like GitHub's Gist or Pastebin of some kind, I want to keep them on my website in my database so that I can easily modify them in a single place and integrate within posts.

An image from digital cat's photostream on Flickr.

## 21. Add Revision Control For All The Pages.

Currently if I edit a page, the previous page is lost and I can't see the changes. It's crucial to keep the changes as sometimes I need to get something from a year ago. I have to add revision control like wikipedia does. The URL scheme could be www.catonmat.net/article/revisions - displays all available revisions, and www.catonmat.net/article/revisions/r1/r2 displays changes between r1 and r2, but I have to think about it a bit more.

## 22. Create Tiny URLs For Articles On My Own Site

I don't want to depend on some service that may go down. Make short urls like http://catonmat.net/abc, where abc is [a-zA-Z0-9] this will give me 238328 URLs, more than enough. I could even go for something shorter.

## 23. Optimize Catonmat Load Speed To Maximum.

Use the page-speed FireBug plugin to optimize site loading speed. Page-speed is at http://code.google.com/speed/page-speed/. Things to be optimized include minified javascript, minified html, gzip compressed content, maximizing caching, use asynchronous js loading of google analytics and others.

## 24. Make The Posts More Available

Currently the posts are only available as HTML documents. I should try to convert them to PDF and put them to Scribd. I have to think about consequences as Scribd may show up on search engines at a higher ranking position than catonmat iself, which would have drastic impact on the traffic. Saving to PDF has a benefit that it's a single file. If saved as HTML, the browser creates a folder with tons of images. Can't be shared easily and is clutter. Perhaps offer a PDF download for all articles.

## 25. Make Posts Printer Friendly.

Create a nice CSS template for printing articles. At the end of the article include URLs to all the mentioned resources. Add an option to choose whether to print comments or not. URL structure could be www.catonmat.net/article/print

Add "Share this" widget and perhaps "Reddit this", "Digg this", "Stumble this", etc., buttons. This should be based on referrer as I don't want to show "Reddit this" to a Digg visitor as there is a holy war between Reddit and Digg. Also add "Tweet this" button somewhere.

## 27. Utilize The New Google Feature Of Displaying Named Anchors In Search Results.

See this post: http://googleblog.blogspot.com/2009/09/jump-to-information-you-want-right-from.html Some of my posts utilize this (10 Awk Tips, Tricks and Pitfalls), but I need to utilize it more.

## 28. Highlight The Python Code As In SQLalchemy Documentation.

I like how the code is highlighted in SQLalchemy documentation. I am not sure if they are using Pygments or not, but I'll try to make mine exactly the same. Example: http://www.sqlalchemy.org/docs/05/ormtutorial.html

## 29. Create Pagination For Posts/Categories/Tags As In Flickr.com

I like the style of Flickr's pagination. Got to implement the same on catonmat. Example: http://www.flickr.com/photos/frijole/

## 30. Have Pages Open In A New Window By Default.

I feel that opening links in a new window would keep visitors on the website longer. I haven't tested but I will A/B this. Update: This is not a good idea, won't implement it.

An image from paraflyer's photostream on Flickr.

## 31. Investigate What Do Various <a rel="..."> Do.

There are a bunch of different relations like rel="bookmark", rel="prev", rel="next". This could improve the website navigation greatly. More info here: http://www.w3schools.com/TAGS/att_a_rel.asp

## 32. Perhaps Remove The Article Date Altogether.

I have noticed myself that if I search for something and I find an article from 2004, I want to look for something fresher. Got to A/B test this and see how long do people stay on the site.

## 33. Add An IP Ban List.

Sometimes spammers use the same IPs. Rather than iptables -I INPUT --src IP -j REJECT them, just block them at application level, 404 all pages, or redirect them elsewhere.

## 34. Automatically Translate All Pages To All Languages Via Google Translate

Sometimes people search for something in their own language and can't find it. Perhaps they don't know English term and therefore can't find what they wanted. If I automatically translate all pages to all languages, people would end up on my website and find what they were looking for. The URL scheme for this could be: www.catonmat.net/xx/..., where xx is two letter international language code.

## 35. Try Out Dustin Curtis' Advice On Best Performing Link Texts.

Dustin Curtis did an experiment where he tried various link texts to invite readers to follow him on Twitter. The first version was "I'm on Twitter", this got 4.70% click through rate. The last version was "You Should follow me on Twitter <here>". This had 12.81% CTR, which is a massive improvement.

People ask me a lot of questions over email. I could answer all the questions on my website instead of email. This way everyone could always find all my answers.

## 37. Add A "FAQ" section.

I get asked the same questions over and over again over email. For example,

• What do I have to know for Google interview?
• What books do I read?
• How to learn C++/C/Python/Algorithms?
• etc.

Instead of sending the answer over email, I could send these people to FAQ page where they could find my latest answers (as they change over time).

This idea has the highest priority. A knowledge database is a section on my website where I can write everything that I learned each day. This should be accompanied with a desktop application that has a hotkey that instantly brings up input dialog and I can type what I just learned. I had a database like this in 2002-2004 and my knowledge literally went exponential. I wrote out key facts that I learned each day and could easily locate as necessary.

## 39. Add A Miniblog For Quick Articles.

Sometimes I have some cool idea or quick hack that I want to share, but as I am used to writing large and well thought out articles, I can't post the quick hacks and my thoughts don't get shared. A miniblog would allow me to share even the smallest thoughts that I have.

## 40. Add More Programming Quotes.

I love various smart programming quotes. I should make them more accessible, make them searchable by author/text.

An image from paraflyer's photostream on Flickr.

## 41. Integrate LaTeX In My Posts.

As I am sometimes writing about maths, I need to integrate LaTeX directly in my posts. I should not forget to do SEO on images it generates - instead of having some ridiculous <img src="latex-generator?q=$\begin{bmatrix}1&2\\3&4\end{bmatrix}$>" I should have it generate an image "<img src="matrix.jpg" title="Matrix">". This way people will be able to find my posts via image search if they search for some mathematical terms like "Matrix".

## 42. Try Out How The Articles Look With Text-Align: Justify.

I currently have the default left-aligned text. Books and journals have it justified. Not sure how it would look on my website. Have to try it out. A/B. I read somewhere that this may feel confusing to dyslexic users.

## 43. Have In-Line Code Snippets And Variables Stand Out From Rest Of The Text.

A nice example of this is Github's blog. They have gray background for constant-width things. See this: http://github.com/blog/530-how-we-made-github-fast

## 44. Have <a> Links Change Background Color On Hover.

I love how mattt.me has done it: http://mattt.me/articles/ I want this.

## 45. Have Only One Category Per Post But Multiple Tags.

A post should have only one single category. The category must strictly define the main theme of the article. A post can have multiple tags. Tags define topics discussed in the post.

## 46. Add Crazyegg Tracking For The First Month.

Must add crazyegg to track how the users navigate the site, where they click and what they visit. Optimize based on the results. http://crazyegg.com/

## 47. Add A Job Board.

As my site is getting more popular and popular among programmers, it may be a good idea to add a job board. Joel Spolsky made a million \$ in a year with job boards. As the popularity of my site increases, I might make a few dollars out of it as well.

## 48. Use Statcounter and Google Analytics

This is obvious. Statcounter is for real-time data. The free version is limited to statistics of last 500 visits. Google Analytics is for keeping long term statistics with a day of delay in updates. Load them asynchronously: http://googlecode.blogspot.com/2009/12/google-analytics-launches-asynchronous.html

## 49. Form Input Fields And Text Fields Should Change Border Color On Focus.

Users should know what field they are focused to without trying to find the cursor.

## 50. Mandatory Alt Attributes For Images, Title Attributes For Links.

The rationale of this feature is that if images don't get loaded or if blind people are listening to the content of my posts via text-to-speech engine, they should know what the image displays. Alt tags also help the search engines to classify images. The same goes for title attributes for links.

## 51. Optimize Meta Description For Categories And Tags

Category and tag pages usually have meta description as "Posts in <category>". This is unsatisfactory. I want a description of category and if it's missing or is too short, I want it to have some post titles in it, to make it unique. The same for tags, make meta tags "Posts in <tag>: post title1, post title2, ..." not exceeding 20 words or so.

An image from dsevilla's photostream on Flickr.

The next post in this series will be about Python libraries that I use for the new catonmat and the structure of the site. You can also follow the development on GitHub (I started importing code only yesterday so there is not much yet).

Projects December 08, 2009

# I pushed 30 of my projects to GitHub

Hey everyone, I just pushed 30 of my projects to GitHub. I realized that all the projects were scattered across my blog and there was no central repository. So I took time to organize them, write documentation, and uploaded them to GitHub.

I did all of these projects for fun and to learn better programming. You can't become a great programmer if you don't program a lot. The more you program, the more language idioms and constructs you'll learn. You'll learn common patterns that occur frequently in programming and it will greatly improve your problem solving skills.

These were all relatively small projects and I think I am ready to move to the next level. I have several larger ideas in mind that need to be turned into code. I will post the updates to catonmat.

If you find any of my projects interesting, clone and start hacking. You can also follow my profile at github. :-)

Here they all are. Enjoy!

## 1. Busy Beaver

Busy beaver is a computer science problem to finding the smallest Turing Machine that outputs the most data and eventually halts. This project is an implementation of a Turing Machine in Python and C++ that runs the busy beavers. It also comes with Turing Machine's tape visualization tool written in Perl.

## 2. Feedburner Graph Generator

Current Feedburner statistics graphs do not look nice. I wrote this Perl program to generate the nice graphs they had in 2008.

## 3. CodingHorror Keyword Analyzer

This is a Perl program that parses public statcounter data for codinghorror.com blog and stores the search keywords in an SQLite database.

This is a tiny Python module that does asynchronous DNS resolution with adns library. My benchmarks were 20,000 resolutions per minute on a 512kbit/s connection.

## 5. Winamp Music Reporter Plugin

This is a Winamp plugin that reports to digitalpoint forums the tracks you are listening to. Written in C and uses Winamp SDK. The code can be modified to make it report to Facebook or Twitter, or anywhere you wish.

## 6. Bithacks.h

Bithacks.h is a C header file that implements various bit operations.

## 7. Set Operations in Unix Shell

This is an implementation of 14 set operations by using only Unix utilities such as sort, uniq, diff, comm, cat, head, tail, awk, and others.

## 8. Hacker Top

Hacker top is a Unix top-like program written in Python for monitoring Hacker News from the console.

## 9. Reddit Top

Reddit top is a Unix top-like program written in Python for monitoring Reddit from the console.

This is a program written in GNU Awk that downloads YouTube videos. It works well but please don't take it too seriously. It's a proof of concept code, written to see what can be done with GNU Awk's networking interface.

This is a program written in VBScript that downloads YouTube videos. I wrote it because when I was a child, I did a lot of programming in Visual Basic and I wanted to remember what it was like.

This is a Perl one-liner that downloads YouTube videos. I wrote it because I love Perl golf.

This is a YouTube video uploader that works without any APIs. It just simulates what a browser would do and takes all the steps to post the video and set the video info. Written in Perl.

## 14. Plurk Translation Tool

This is a GreaseMonkey script that translates Plurks to English.

Plurk is like Twitter but more fun and organized. Come be my friend on Plurk, my profile name is pkrumins. Written in JavaScript.

## 15. Command Line Plurker

Plurk from your command line. Written in Perl.

## 16. Find Plurks on Google

This program searches for plurks that were indexed by Google. It outputs URLs to indexed pages. It's written in Python and uses my xgoogle library.

## 17. Delete Plurks

This is a GreaseMonkey script that adds a "delete" button on individual plurk pages. This way you can delete your or other people plurks (if it's your thread) directly from the plurk page. Written in JavaScript.

## 19. Reddit Media

The old Reddit Media website that I created in 2007. No longer maintained. It was written in Perl.

## 20. Reddit River

The old Reddit River website for mobile devices. No longer maintained. It was written in Python.

## 21. Digpicz

This is the digpicz.com website that I created back in 2007. It got massive attention back then because Digg didn't have picture section then. It was written in Perl.

## 22. Picurls

Picurls.com is a picture aggregator much like popurls.com but for pics. Currently down for maintenance, will be soon up again. Written in PHP.

## 23. Bash Vi Editing Mode Cheat Sheet

Bash has two input modes - emacs and vi. This is vi input/editing mode keyboard shortcut cheat sheet.

## 24. Bash Emacs Editing Mode Cheat Sheet

Bash has two input modes - emacs and vi. This is emacs input/editing mode keyboard shortcut cheat sheet.

## 25. Bash History Cheat Sheet

This is the bash history cheat sheet. It summarizes everything there is to know about working efficiently with command line history in bash.

## 26. Screen Cheat Sheet

This is the screen terminal emulator cheat sheet. It lists the default keyboard shortcuts for working with screen.

## 27. Perl Special Variable Cheat Sheet

This is Perl predefined variable cheat sheet. It lists all the variables from perldoc perlvar with a concise description and some example usages. I created it when I was mastering Perl. I enjoy Perl golf and I wanted to know all of the variables.

## 28. Perl pack/unpack and printf/sprintf Cheat Sheet

This is Perl pack/unpack/printf/sprintf cheat sheet. The pack/unpack cheat sheet is on page one, and it lists the pack/unpack template parameters and what they do. The printf/sprintf cheat sheet is on page two, and it lists the printf/sprintf format specifiers and format attributes.

I created this when I was mastering what could be done with pack/unpack. I added printf/sprintf as I could never remember all the format specifiers.

## 29. Awk Cheat Sheet

AWK programming language cheat sheet.

## 30. Sed Cheat Sheet

This is sed cheat sheet. Sed is the Unix stream editor. If you don't know it, you don't know Unix.

## 31. Ed Cheat Sheet

This is ed cheat sheet. Ed is The Unix Text Editor.

One day when I was learning sed, I got interested if it originated from ed, which got me interested in ed itself. I find that cheat sheets are a great way to learn new topics and therefore I created this cheat sheet.

## 32. The New catonmat.net Website

I just started pushing code to the new catonmat.net repository. It's going to be a state of the art personal website from now on.

I have around 100 ideas for it, and the next big article series on catonmat is going to be "Designing the new catonmat.net together with me." You should subscribe to my blog here if you are interested!

If you have any questions, don't hesitate to ask in the comments!

Linear Algebra December 03, 2009

# MIT Linear Algebra, Lecture 2: Elimination with Matrices

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This is the second post in an article series about MIT's course "Linear Algebra". In this post I will review lecture two on solving systems of linear equations by elimination and back-substitution. The other topics in the lecture are elimination matrices (also known as elementary matrices) and permutation matrices.

The first post covered the geometry of linear equations.

One of my blog readers, Seyed M. Mottaghinejad, had also watched this course and sent me his lecture notes. They are awesome. Grab them here: lecture notes by Seyed M. Mottaghinejad (includes .pdf, .tex and his document class).

Okay, here is the second lecture.

## Lecture 2: Elimination with Matrices

Elimination is the way every software package solves equations. If the elimination succeeds it gets the answer. If the matrix A in Ax=b is a "good" matrix (we'll see what a good matrix is later) then the elimination will work and we'll get the answer in an efficient way. It's also always good to ask how can it fail. We'll see in this lecture how elimination decides if the matrix A is good or bad. After the elimination there is a step called back-substitution to complete the answer.

Okay, here is a system of equations. Three equations in three unknowns.

Remember from lecture one, that every such system can be written in the matrix form Ax=b, where A is the matrix of coefficients, x is a column vector of unknowns and b is the column vector of solutions (the right hand side). Therefore the matrix form of this example is the following:

For the elimination process we need the matrix A and the column vector b. The idea is very simple, first we write them down in the augmented matrix form A|b:

Next we subtract rows from one another in such a way that the final result is an upper triangular matrix (a matrix with all the elements below the diagonal being zero).

So the first step is to subtract the first row multiplied by 3 from the second row. This gives us the following matrix:

The next step is to subtract the second row multiplied by 2 from the third row. This is the final step and produces an upper triangular matrix that we needed:

Now let's write down the equations that resulted from the elimination:

Working from the bottom up we can immediately find the solutions z, y, and x. From the last equation, z = -10/5 = -2. Now we put z in the middle equation and solve for y. 2y = 6 + 2z = 6 + 2(-2) = 6 - 4 = 2 => y = 1. And finally, we can substitute y and z in the first equation and solve for x. x = 2 - 2y - z = 2 - 2(1) - (-2) = 2.

We have found the solution, it's (x=2, y=1, z=-2). The process we used to find it is called the back-substitution.

The elimination would fail if taking a multiple of one row and adding to the next would produce a zero on the diagonal (and there would be no other row to try to exchange the failing row with).

The lecture continues with figuring out how to do the elimination by using matrices. In the first lecture we learned that a matrix times a column vector gave us a combination of the columns of the matrix. Similarly, a row times a matrix gives us a combination of the rows of the matrix.

Let's look at our first step of elimination again. It was to subtract 3 times the first row from the second row. This can be expressed as matrix multiplication (forget the column b for a while):

Let's call the matrix on the right E as elimination matrix (or elementary matrix), and give it subscript E21 for making a zero in the resulting matrix at row 2, column 1.

The next step was twice the second row minus the third row:

The matrix on the right is again an elimination matrix. Let's call it E32 for giving a zero at row 3, column 2.

But notice that these two operations can be combined:

And we can write E32(E21A) = U. Now remember that matrix operations are associative, therefore we can change the parenthesis (E32E21)A = U. If we multiply (E32E21) we get a single matrix E that we will call the elimination matrix. What we have done is expressed the whole elimination process in matrix language!

Next, the lecture continues takes a step back and looks at permutation matrices. The question asked is "what matrix would exchange two rows of a matrix?" and "what matrix would exchange two columns of a matrix?"

Watch the lecture to find the answer to these questions!

Topics covered in lecture two:

• [00:25] Main topic for today: elimination.
• [02:35] A system with three equations and three unknowns:
• [03:30] Elimination process. Taking matrix A to U.
• [08:35] Three pivots of matrix U.
• [10:15] Relation of pivots to determinant of a matrix.
• [10:40] How can elimination fail?
• [14:40] Back substitution. Solution (x=2, y=1, z=-2).
• [19:45] Elimination with matrices.
• [21:10] Matrix times a column vector is a linear combination of columns the matrix.
• [22:15] A row vector times a matrix is a linear combination of rows of the matrix.
• [23:40] Matrix x column = column.
• [24:10] Row x matrix = row.
• [24:20] Elimination matrix for subtracting three times row one from row two.
• [26:55] The identity matrix.
• [30:00] Elimination matrix for subtracting two times row two from row three.
• [32:40] E32E21A = U.
• [37:20] Permutation matrices.
• [37:30] How to exchange rows of a 2x2 matrix?
• [37:55] Permutation matrix P to exchange rows of a 2x2 matrix.
• [38:40] How to exchange columns of a 2x2 matrix?
• [39:40] Permutation matrix P to exchange columns of a 2x2 matrix.
• [42:00] Commutative law does not hold for matrices.
• [44:25] Introduction to inverse matrices.
• [47:10] E-1E = I.

Here are my notes of lecture two:

My notes of linear algebra lecture 2 on elimination with matrices.

Have fun with this lecture! The next post is going to be either on lectures three and four together or just lecture three. Lecture three will touch a bit more on matrix multiplication and then dive into the inverse matrices. Lecture four will cover A=LU matrix decomposition (also called factorization).

PS. This course is taught from Introduction to Linear Algebra textbook. Get it here: