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.

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 an arc than a tower. Here is the first picture, and you can look forward to an instructable!

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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 what this emerging not-for-profit is up to!

I believe in the intrinsic value of mathematics, and most especially geometry and symmetry. Learning to appreciate math for what it is will reduce our aversion to it when we actually need to use some.

**Learning objective:** Students will learn about two of the 5 platonic solids,
the icosahedron and the dodecahedron. They are both visible in this structure
because their dual relationship is highlighted here. The tensegrity ball is
neither the icosahedron nor the dodecahedron, yet it is also both. Students will
also learn about the principle of tensegrity, and this activity can be partnered
with the building of a smaller tensegrity such as the ones on GeoBurst.
Students will also learn about knots.

**Learning Keywords:** 3D geometry, Icosahedron, dodecahedron, platonic solids,
dual solids, tensegrity, symmetry (and symmetry groups), triangles, pentagons,
engineering, structures, building materials, knots… etc :)

A little background: I first built a small model last year after seeing George Hart’s post on Make. I also had these 1/2-1 inch diameter 8-foot bamboo poles, lying around for a project like this, but the giant tensegrity ball idea went into limbo because I didn’t know how to reliably attach the ropes to the ends of the poles. You can’t just notch bamboo, because it will split to pieces, and I didn’t want to buy fastenersfor 60 ends of poles.

The answer lay in knots, and this is what my instructable is about. Good, non-slipping, easy and fast knots. Tying the knots for this project took nearly 35 man hours, for which I had help from three friends. Veronika Irvine, Eric Davies, and Charles Campbell. The rest of the time was spent rolling the ball around at the park!

Let us start with a video of the end result.

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 with $r$ rows and $c$ columns satisfies the $tatami$ condition if no four tiles meet at any point. (or you can think of it as the removal of a matching from a grid graph that breaks all $4$-cycles).</p>
```

This simple restriction, brought to my attention by Don Knuth in Vol 4 of TAOCP, became my PhD thesis topic, when my research group and I discovered that it imposes a aesthetically pleasing structure with a nice description, and opened up lots of fun questions.

Here is my favourite example, which has all of the possible "features". First it is shown uncoloured

And here it is coloured

The magenta tiles show the types of features it can have, and here they are on their own:

I'll introduce my question here, and then summarize some more results later. **I want to find more and better ties between tatami tilings and other less esoteric math problems.** If you think of a paper or subject I might want to look into, don't hesitate to answer.

In our first paper, we proved the above structure and showed that:

- A tiling is described by the tiles on its boundary, and hence has a description that is linear in the dimensions of the grid.
- The maximum number of monomers is at most $\max(r+1,c+1)$, and this is achievable.
- We found an algorithm for finding the rational generating polynomial of the numbers of tilings of height r (which I think can also be calculated with the transfer matrix method).

We posed a couple of complexity questions (which I am working on), for example, is it NP-hard to reconstruct a tatami tiling given its row and column projections?

Or tile a given orthogonal region with no monomers?

Next we focused on enumerating tilings of the $n\times n$ grid, and found a partition of $n\times n$ tiles with the maximum number of monomers into $n$ parts of size $2^{n-1}$. We also counted the number tilings with $k$ monomers, and this curious consequence:

The number of $n \times n$ tatami tilings is equal to the sum of the squares of all parts in all compositions of $n$. That is, $2^{n-1}(3n-4)+2$.

We also found an algorithm to generate the ones with $n$ monomers, and a Gray code of sorts.

Nice numbers, and a cute problem, but another paper that is self contained with elementary (albeit, somewhat complicated) reasoning.

Our third paper in this story (in preparation), looks at the generating polynomial for $n\times n$ tilings with $n$ monomers whose coefficients are the number of tilings with exactly $v$ vertical dimers (or $h$ horizontal dimers). It turns out this generating polynomials is a product of cyclotomic polynomials, and a somewhat mysterious and seemingly irreducible polynomial, who's complex roots look like this:

We've found a bunch of neat stuff about it, for example the evaluation of this polynomial at $-1$ is $\binom{2n}{n}$, for $2(n+1)\times 2(n+1)$ tilings, and we found our generating polynomial gives an algorithm to generate the tilings in constant amortized time. Here is some output of the implementation:

That's the most of the published (and almost published) story. There is a loose connection with other monomer-dimer problems, and things I can look into, like Aztec tatami tilings, but **I'm looking for direct applications of other results to these, or vice versa, especially with this last paper in preparation. I'm not asking you to do research for me, but just your thoughts as they are now, so I can go learn new stuff.**

Feel free to comment about what you think is interesting, or not, about tatami tilings too!

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