Alejandro Erickson
Postdoctoral Research Associate at the University of Victoria
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A little randomness can yield remarkably organic images. The JavaScript snippet below randomly assigns colours to pixels, row-by-row starting at the top, such that the colour is similar to that of other pixels close to it. Try the Random Canvas generator yourself. Remember: Each new image you generate has NEVER...

It’s sugar cane season! Chewing on stalks of raw sugar cane was a wonderful part of my childhood in Vancouver, and while visiting my home town today, I came across this delight once again. This time, however, I’ve made a delicious drink for grown-ups. Simply mix sugar cane with rye...

Datacenters provide low-cost, efficient and flexible computing resources, so that users and applications (e.g., Google Search, Pokemon Go) share a pool of resources, and can expand into that pool as needed. Being such a critical building block of our digital world, every aspect of datacenters undergoes constant research and development...

In 2012-2013 I travelled to several remote First Nations communities around British Columbia to do hands-on mathematical activities with elementary school kids and teachers. Math Catcher, which provided the free math presentations, has also born several mathematical stories about a young boy, named Small Number, that have been translated into...

“Cracking the Coding Interview 6e” question 4.10: T1 and T2 are binary trees with m and n nodes, respectively. Test whether T1 contains T2 as a subtree. Here is my solution, which performs much better than the book’s solution. The problem asks, more specifically, if there is a node in...

I saw a nice question on leetcode today, apparently asked recently at a Google on-site interview. Given N rectangles with integer coordinates, check whether they are an exact cover of a rectangular region If you don’t know already, Google’s on-site interview process involves 5 independent interviews in one day. In...

Suppose you have two unsigned 16-bit integers, X=0000 0000 0111 1111 1000 1010 0010 0100 and Y=0000 0000 0000 0000 0011 0000 1010 1111. How can you tell if Y is more than half as long as X? I came across this because I wanted some code to branch on...

In doing a simple programming exercise while preparing for a job interview, I decided to sort the input. I wrote a solution for the exercise in 10 minutes, and then spent the next two days implementing “all of the basic sorting algorithms”. When I finally had bubble sort, insertion sort,...

Postdoc at Durham University, built INRFlow analysis framework to optimise datacenter networks using graph theory. Seeking R & D career. I am writing various bios, About Me pages, and LinkedIn summaries for my upcoming job search and I find it borders on the unbearable sometimes. So instead of using the...

A few years ago I made my PhD in to a pen-and-paper puzzle book. Here it is, in full. Contact me if you want it in pdf format. Complete the puzzles by deciding which grid-squares to cover with each For each puzzle you are given some tiles and a grid....

As I prepare for the most important job-search of my career thus far, I am faced with the problem of organising all the directories and files that will factor into it. There are versioned copies of my CV and cover letters, old applications, reference letters and student evaluations, advice and...

My most popular Youtube video, from 2011, now has over half a million views. It is a how-to video about how to dismantle a pallet without splitting it, without special tools, and recover the nails, and as far as I know, no one else’s method does all of these things....

As I prepared to move to the UK three years ago, there was one thing I knew for certain: It is not (yet) a coffee culture. Much less so is the post-industrial wasteland that I have moved to. So, being the coffee enthusiast that I am, I bought a nice...

Try Spacemacs. If you only take one thing away from this post, that’s it. No matter whether you are Emacs, Vim, or you hate these old fashioned editors with terrible defaults, you must try Spacemacs. Spacemacs is a curated initialisation directory for Emacs. Get Emacs at least version 24.4, and...

Last week I gave an interactive presentation on tatami tilings to senior high school students. It was very well received, and a long-time attendee raved about it. I paraphase the keen student’s words, “I’ve been to almost every A Taste of Pi presentation, and I think this has been my...

I transformed my PhD work into a 5-day mini course for Grade 10s through The Brilliant Club. I am making the course material available here. Contact me or write in the comments for more resources and help using them! Below is an excerpt of a case study that I wrote...

Unsuspecting, I picked this classic baby toy up at a local charity sale. As it turned out, it harbours a little combinatorial challenge that I never suspected! The toy is a set of five stacking cubes with a different face on each face. Save for size, the cubes are all...

Having grown up around SLR cameras, I have always had an appreciation for the superior image quality that a “proper” camera is capable of. I have owned severalof them, and I am quite familiar with the technical aspects of exposure and depth of field. A tipping point occured, however, when...

I am a Canadian citizen, resident in England, and I have recently learnt some things while getting a UK Driving Licence. First and foremost, the following website is a gold mine: HOW TO EXCHANGE A CANADIAN DRIVER’S LICENCE FOR UK LICENCE Read that whole webpage. There were a few details...

If you have ever traveled to give a mathematical talk at a conference, you may have encountered the following situation: Your slides are in LaTeX (beamer). You would like to leave your laptop at home. You have a tablet or smart phone. You would like to be able to make...

I created a BibDesk template which exports some code executable in Sage. The output is a graphic with authors on the left connected by lines to their publications on the right. It is good for familiarising yourself with the authors of your bibliography. To use it, execute the template on...

“You just want to give the impression of bushes,” said my painting instructor. What is an “impression” of the Petersen graph? The tension between art and mathematical concepts often sets artistic expression against details, similar to my instructor’s trees. A “mathematical impressionism” emerges in which the scene is a mathematical...

This is a humourous humus recipe that appeared in a cookbook that was compiled at Simon Fraser University, for a United Way fundraiser in 2002. It can still be found at www.sfu.ca/cookbook/humus.htm The recording is by Alejandro Erickson, and is posted here with permission from Tom Brown. Please be sure...

I have the pleasure of presenting my PhD research on tatami coverings at the ACiD seminar. Click to download my slides. I recorded the talk:

This is on my Master’s thesis. A paper I published later has the following abstract: “We provide new evidence that spanning forests of graphs satisfy the same negative correlation properties as spanning trees, derived from Lord Rayleigh’s monotonicity property for electrical networks. The main result of this paper is that...

Fenwick Lawson’s sculpture, “Cry for Justice”, is installed outdoors in the middle of a wide throughway at Durham University. Thousands of students and staff walk past it every day. The characters express despair and anguish, and when I first encountered “Cry for Justice” I was immediately struck with a deep...

This collection features man-made objects that have become unintentional foci. Layers placed upon layers of walls, and posters, and signs and sidewalks, which have weathered, cracked, left the faintest traces of their existence, and then been reinstalled, looking like a stack of insurance stickers on a car’s license plate. They...

Here is a 16-page long emacs pdf reference card from Aaron Hawley’s list. It is my first elisp program and undoubtedly full of hacks and such, but the result is useful to me. I have converted the source of Aaron Hawley’s reference sheet from http://www.emacswiki.org/emacs/Reference_Sheet_by_Aaron_Hawley_source and used it in the...

I am very excited to present my work at Bridges 2013 today, in Enschede. Alejandro Erickson. TatamiMaker: A combinatorially rich mechanical game board. Proc. of the international conference Bridges: Mathematics, Music, Art, Architecture, Culture, 63–70, 2013. http://archive.bridgesmathart.org/2013/bridges2013-63.html Download the slides HERE. TatamiMaker: A Combinatorially Rich Mechanical Game Board from Alejandro...

It is my pleasure to give the presentation for my paper with Frank Ruskey, Domino Tatami Covering is NP-complete, at the International Workshop on Combinatorial Algorithms (IWOCA) 2013.

I have built tensegrities with nearly 1000 kids this year, and it keeps me quite busy. I finally found a moment to work out how to make a (approximate) module of the Needle Tower out of my materials. The result is fairly wobbly, of course, so it works better as...

16 foot tall tensegrity ball Here is the introduction to my latest instructable, which was featured on instructables.com. I made this giant tensegrity for GeoBurst at the Vancouver Island Mini MakerFaire 2012. GeoBurst creates ZEST for MATHEMATICS by doing hands-on mathematical activities with kids. Check out our website and see...

I recently asked a question on Math Overflow, to see if other mathematicians and students could think of possible connections to tatami tilings. I have reposted the text of it here, but if you like the question, please go and see it here. <p>A monomer-dimer tiling of a rectangular grid...

I was browsing my google analytics page to see how people are getting to my website, when I came across a delightful forum post at 99chan.org. A young working-class lad of perhaps 23 sets himself up at a table and typewriter in my city of residence and, dressed in gentlemanly...

That's right!  Hexastix Puzzle Kits are now available on ebay Canada.  Click here to order one! I have pasted the listing below. Hexastix Puzzle Kits are based on a beautiful piece called "72 Pencils", by Mathematical Sculptor, George Hart. Each kits is guaranteed to contain 148 brightly coloured 3 inch...

  18th Century French mathematician, Buffon, asked "What is the probability that a needle which falls on the floor, crosses one of the lines between the floor boards?".  It turns out that this number can be used in a Monte Carlo method to approximate π.   We dropped our "needle"...

I recently saw a picture of some spaghettis stuck in pieces of wieners, and the results were these cute little lumps with floppy tails sticking out.  "So much potential," I thought.  Here are the results of my first geometric crafts using spaghetti and wieners.  This is definitely something I can...

I'm pleased to announce that GeoBurst has provided materials to the local girl guide chapter, for the girls to make tensegrities.  They will be on sale Saturday, November 19, at their Fall Tea and craft faire. The craft fair/bake sale are free to get in to, and the tea costs...

I have been practicing a brazilian martial art, called capoeira, fairly regularly for about 1 year.  I have been with two groups, Axé Capoeira and Grupo Cativeiro Capoeira, both in Victoria BC, and previously I trained a few months with Ache Brasil, in Vancouver.  Each group has its merits, and...

</embed>I am teaching these at Iconocraft, Thursday, November 10th at the Victoria Event Centre, in Victoria BC: http://iconocraft.com/ Contact me to arrange workshops in the area.I make materials for these, and I look forward to your requests for kits!Enjoy Geometry.

This beautiful and solid sculpture feels nice and fairly heavy in the hand. Keep it where guests can touch and inspect it. There are 91 sticks of each colour: Blue, Green, Deep Red, and Deep Brown (Black in dim light).   This video shows how I made it: </embed> I...

I am preparing a workshop where we will make tensegrities.  These beautiful little mathematical decorations require (the way I make them) dowels with notches in the ends.  I made a special saw to notch the dowels on the spot, instead of notching a bunch of them in advance.  I just...

I got hopelessly lost the first time I tried to make this, but with a bit of patience and some images from the article on Wikipedia, I have come out of it one Compound of 5 Tetrahedra richer. Here is an animated GIF of the model.  It will be available to buy...

These vividly colored icosahedrons look great when hung from the ceiling, or a wall, but the fun doesn't stop there. You'll notice that the colors seem to follow a pattern which is quite amazing. I'll skip over the cool math that is involved, but try holding two opposite nodes still, and...

</embed>   I will sell this by dutch auction, starting at $150, and reducing by $5 each week. You never know who's waiting for it to come down, so buy it quick! http://www.etsy.com/listing/84895025/beautiful-geometric-sculpture-with This is a "Hexastix", shaped into a "Stellated Rhombic Dodecahedron". It casts a beautiful shadow, and belongs...

I am pleased to announce the publication Japanese tatami mat tilings: No four tiles meet, in the Notes from the Margin, Volume II, 2011, from the Student Committee of the Canadian Mathematical Society. The title of the newsletter comes from the story behind Fermat's last theorem.  He had written in the margins...

  You'll no longer be sitting beside that sexy marine biologist dressed up as a sea anemone wishing that you could have a research themed costume too. </embed> Finally, there is a combinatorics themed halloween costume!  Sport this beautiful Arduino and LED based rendition of Cool-lex combinations (research by Aaron...

This week I had the pleasure of teaching a Hexastix workshop to a group of volunteer mathematics educators, at the invitation of the 2011 PIMS Education Prize winner, Veselin Jungic. As with my previous experience teaching Hexastix at Math Camp, these beautiful mathematical objects will be the take-home project for...

I invited some friends over to make icosahedrons in my workshop!  Here are the pictures. After eating some delicious apple butter bars, cookies, and drinking chinese tea, we headed out to the studio where I helped my friends make these awesome cool geometric shapes.  The colouring of these icosahedrons is...

Do you like fog machines?  Do you like fog machines that can go in your pocket? That is what an e-cigarette is.  A fog machine that produces fog when you suck on it.  The "e-liquid" is actually the same, or a very similar chemical to the one found in fog...

How to make a Hexastix, like George Hart’s “72 pencils”. Shoutout to Matt Parker’s standupmaths fans! Say hi in the comments if you came here from Matt’s video! About Hexastix: If you haven’t seen George Hart’s sculptures before, you need to go check him out: http://www.georgehart.com/sculpture/pencils.html .  And while you...

In my sculpting endeavours I have needed many coloured sticks of all shapes and sizes. It was immediately apparent that I would need to source skewers in bulk, cut them quickly without fraying them, and colour them cheaply. The useful answer to the first item, is that skewers are available...

I have wanted to busk for many years, but lacking the skills of a musician and the bravado of a circus performer, I would need something different.  Doing things differently is my specialty, however, and with the good fortune of finding a free typewriter on the street, I began to...

I have completed a project that I conceived at CanaDam, the Canadian Discrete Math Conference at the University of Victoria.  After getting the geometry wrong once, and making several smaller prototypes, I have finally made one as large as possible, given the dimensions of the laser cutter at the MakerSpace...

Last weekend I saw a mini glass flameworking (lampworking) workshop at Mini Makerfaire in Vancouver.  I didn't get a chance to try it, but it looks like fun, and something that is perfect for a place like the MakerSpace, here in Victoria BC.  The workshop and materials were provided by...

I was at Mini Makerfaire in Vancouver last weekend, and I came across a booth presenting a very interesting vest.  The Point Locus Wearable Tactile Pathfinder is a vest which reads GPS data, and vibrates on the wearer's shoulders to indicate the direction they should take along a path to a...

Over seven months after building my first Hexastix, a geometrical sculpture introduced to me by Matt DeVos, and originally by George Hart, I was finally graced with the opportunity to teach it to a group of keen teenagers. My students, who were participants in Math Camp at the University of...

Here is a video I posted on how to dismantle a pallet without splitting the wood (video).  I made a set of shelves with it and some recovered 2x4s. </embed>  

  Walking past the theatre yesterday, I saw they were showing THOR 3D I laughed out loud, and continued walking.  But thinking back over the last year or two, I can make the obvious  observation that every movie titled <insert title here> 3D is on the same intellectual plane as...

Recently I caught the geometric bug from Matt DeVos, when he showed an open faced icosahedron made of sticks and elastic bands.  Since then most of my free time has has been occupied by attempts to improve the icosahedron (the model of it, rather).  What began with a little model...

Last fall I made Tomoku into a book with 80 pen-and-paper puzzles based on the web game at http://tomokupuzzle.com.  Four months later I have not found a market for it, and I am looking for the lessons I need to learn. The book is professionally printed, and the instructions are...

Enjoy these nice and peaceful sounds of seaside camp fire and gentle waves.  I recorded 10 minutes of sound from the ocean and fire during a February camping trip on the Juan de Fuca trail, on Vancouver Island, BC, Canada. It was beautifully peaceful and a wonderful sound to sleep...

I just discovered commandlinefu.com, and it is very helpful for learning command line stuff. I wanted a command that would show me duplicate files on my computer and found this: http://www.commandlinefu.com/commands/view/3555/find-duplicate-files-based-on-size-first-then-md5-hash For the lazy, the command given is: find -not -empty -type f -printf "%s\n" | sort -rn | uniq...

I have an HTC Desire Z, Android phone, running Froyo 2.2, with my contacts synced with google contacts.  When I entered a new contact on my phone, it would disappear when it synced with google contacts.  I could not find a solution online, but discovered this by accident. My google...

This is a very simple drink using fresh fruit and vodka.  Perfect for halloween, zombie parties, and any other situation where the highlight is morbidity. For one drink, blend half a grapefruit ice vodka simple sugar syrup into a slushy.  Pour into a wide mouthed glass such as a martini...

I don't know if this happens to everyone, but when I shop for a new computer, it goes more or less like this.  

In 2010 we had the "Like" button.  Perhaps the most significant examples were facebook and youtube like buttons, but there were many variations, some of them invented earlier, like the tweet button, the digg button and the "insert social web service"-button.  The central idea is that you can share your...

Recently, I was introduced to a wonderful object constructed of sticks and elastics, called the hexastix. These mind boggling structures were last seen in a book called The Symmetries of Things by John H. Conway, Heidi Burgiel and Chaim Goodman-Strauss. Naturally, I wanted to build some of my own, so...

This poster appeared at the 2010 CMS Winter Meeting.  Download the pdf for a larger version.

This page will be updated as new publications appear. A careful search reveals the following publications and mentions of tatami tilings in mathematics: Mitsuyoshi Yoshida, Japan 1641, Jinkōki ( translated as “Inalterable Treatise”, first published in 1627 according to britannica.com). Knuth reprints a 6 x 5 dimer (1 x 2)...

Quite often when I discuss math problems with other mathies, they make a connection I had not seen or they have an approach to solving it that I had not thought of. Although I'm sure this happens to most of us, I find myself wondering why I hadn't thought of...

I had a word with Ron Graham at the 2010 CMS Winter Meeting and he mentioned a short paper of his in which he talks about fault-free dimer tilings.  A fault-free tiling of a rectangular grid is one in which every interior grid line intersects at least one tile.  Here...

Tutorial video below! {swf;http://www.alejandroerickson.com/joomla/tatami/tomoku_game.swf;} The Emperor needs you to prepare an auspicious floor for his tea ceremony, but the Ikea instructions only give the row and column projections of the tiling!  You must race the clock to reconstruct it before the Emperor has your head!A tatami mat is a 1x1...

My tatami work at Moka House was briefly interrupted by a very enjoyable coffee with my friend Gwen Temmel today, during which she suggested that tatami tilings looked a little like the game Dots and Boxes.  A light went on inside me and I said, LET’S PLAY DOTS AND BOXES...

What better way to start this tatami blog than with a bit of tatami art!  Here is a calligraphic illumination I created for the letter F.

This is an extension of Conway’s Game of Life in which there is more than one player (colour) and a fifth rule.  To win the game (or be winning) you must have more of your own cells on the board than your opponent does. From Wikipedia, here are the rules...

Starting with a thesaurus, here is a list of words synonymous with “walk”. stroll, saunter, amble, trudge, plod, dawdle, hike, tramp, tromp, slog, stomp, trek, march, stride, sashay, glide, troop, patrol, wander, ramble, tread, prowl, promenade, roam, traipse, stretch one’s legs, mosey, hoof it, perambulate. Wanting a more general list,...

FINALLY! Not much to say about this moment except that I’ve been dreaming about it for a year now and it’s finally happened. I’m adding some text to make this more searchable, but the fun is my giddiness in the video.  I bought the Pygmy Coho kayak with the pontoons...

My dad, Ken Erickson, travels frequently to South and Central America to explore small towns and other places mostly untouched by tourists.  He brought me an ocelot skillfully and ornately carved out of a nut and painted from his 2008-2009 trip to Panama.  I was so impressed by the thing...

Tatamibari is a game published by Japanese game company, Nikoli.  We are given a grid and some of the squares contain a symbol; either + or - or |.  The player must partition the grid with line segments drawn on grid lines so that Each partition contains exactly one symbol....

Rachel Peters is an artist, performer and animator and filmmaker.  Raised by bumblebees in an old abandoned mineshaft, Jack Rachel learned all of her trades as a court jester to the Queen Bee.  Now and then the Queen would get tired of a jester (or a couple) and have them...

I bought this boat just over a year ago and I’m nearly ready to bring it to the water.  Here is a video giving a tour of new rigging. The kayak is a Pygmy Coho wood kit which someone built in Campbell River, BC.  I bought it from him on...

I was at a craft fair in Santiago Chile a couple of years ago where there was a foot-treadle floor loom.  Fascinated, I took pictures of it from every angle.  I am not about to make one, however, but the pictures might be of interest to someone else, so here...

Do you collect anything?  Maybe you collect beach stones or shells, or maybe it’s imported beer cans.  Or maybe your collectors items can be found in hobby shops or car dealerships.  Whatever it is, think about the reasons for maintaining this collection.  What are the benefits, what the challenges?  Better...

This is a flash app that I made for my advisor, Frank Ruskey and his former student, Aaron Williams, for their presentation of The Feline Josephus Problem at the FUN With Algorithms Conference 2010, Ischia Island, Italy. From Wikipedia, the history of the original Josephus Problem is the following: The...

I often get ahead of myself and I’m going to do it here.  With no introduction to tatami tilings nor to tomography and not to mention, their intersection, I present this flash application. I appreciate your comments and bug reports (post them here if you like).  Apologies for the jumbled...

A few months back, I promised myself that once I got a real tamper (see my previous video), I would be ready to make a new coffee video.  Last week I visited Reg Barber at his shop in Central Saanich, BC, Canada, and he cut a 51mm C-Flat tamper for...

My recent and fantastic purchase of this musical instrument has made me an unwitting oppressor.  This ocarina is made to play loudly, filling the space you are playing in and matching the volume of other instruments without the need for amplification.  It also fills all four of the units in...

Richard Dawkins is a well known biologist who published a best selling book called The God Delusion and is now essentially the face of atheism.  In his book, he attacks the idea that the universe and everything in it has a creator who is involved in all the details of...

My Master’s thesis was called Negative Correlation Properties for Matroids and I have made the more accessible part into a talk.  I posted a practice run as a slidecast on slideshare.net.  Attached are the slides themselves and when the paper is finished I will link to a preprint. Negative correlation...

The code is in NXC and I used the NXT Camera. To follow the line, the robot uses a simple PID. The robot uses the sonar to find objects by the road. It uses the light sensor to detect whether it found the right object. Source code:

Suppose you have an apple, an orange, a banana, a pineapple a grape and a screwdriver and you choose to take exactly 3 of these things.  This is called a 3-combination of a 6-set, because you chose 3 of the 6 things.  Now suppose you want a different set of...