## CAD at ICERM’s Illustrating Dynamics and Probability

This is a landing page for our November 2019 visit to ICERM to collaborate with other artists, mathematicians, and craftspeople…

Cherry Arbor Design was founded by Heidi Robb and Pete Benson in 2017. We make pieces that encourage mathematical inquiry by exploiting artistic play as a hook into deeper study. In two dimensions, we use tilings and dissections to learn about geometry, tessellations, fractals, golden ratio themes, and knots. We recently began investigating 3D. Related work:

- Barnsley Fern
- Simple tiles that permit interesting nonperiodic tilings, such as Nessie and Tim Lexen’s tricurve
- Fractal tilings yielding interesting nonperiodic tilings, such as Terdragon, Twin Dragon, and Koch snowflake
- Golden B tiling by Robert Amman. If you have a thing for Fibonacci, have a look.
- Celtic Knot tiling
- Squaring rectangles
- Fractal Penrose tiles, based on Dylan Thurston’s recursive boundary construction of tiles suggested by Bandt and Gummelt
- O’Beirne’s cube

## O’Beirne’s Cube

Much of what we make is two-dimensional. Hugh Montgomery, a mathematician at the University of Michigan, told us about an intriguing 3D dissection puzzle by Tom O’Beirne, a Scottish author and inventor of puzzles. O’Beirne’s cube consists of six pieces that can be packed into a box. The blocks seem irregular at first glance.

However, the pieces are built from four copies of the base component: a 3x4x6 rectangular prism, or cuboid. In our version, each unit in the base cuboid is 0.25 inches, or a bit over 6 mm wide.

O’Beirne combined two of these 3x4x6 cuboids on a matching face, to produce these doubled cuboids:

He then created every possible pair of the doubled cuboids, matching half of a face on each. Below, you can see how a 3x4x12 is paired with a 4x6x6. There are two ways to do this. They are mirror images of each other.

There are three possible pairings of the doubled cuboids. Each pairing produces two mirrored pieces. This generates the six blocks of O’Beirne’s puzzle.

You can combine these six to make a cube.

You can rearrange the six pieces into five other cuboids:

In fact, with any one of the these six cuboids, you can split it into two parts, and recombine to create another cuboid. You can quickly cycle thorugh all six! The original version of the diagram below is described in more detail in Brian Butler’s excellent article on John Rausch’s puzzle site.

You don’t have to limit yourself to cuboids. Here are some other puzzling shapes.

We sell the O’Beirne’s Cube here. But if you have the tools and are up for it, you can make your own. At the San Francisco American Craft Council show, we met Ron Choy, who was captivated by the O’Beirne’s cube. I sent him the link to John Rausch’s site. We were delighted to hear from Ron later, with a picture of the cube he made along with his notes on the process. Now that’s some hands on learning!

## Solve the Golden b puzzle!

Robert Ammann’s Golden b tiling is unlike the tilings we see around us every day. Most tilings are *periodic* (see wallpaper groups). In a periodic tiling, you can find two points where the tiling looks identical. In fact, if there are two, there will be infinitely many such points.

Mathematicians discovered *aperiodic* tilings in the 1960s. Aperiodic tilings must be able to tessellate (i.e. fill in) the entire plane, and use only a fixed number of tile types. Most importantly, any tessellation with the tiles must be non-repeating in this sense: pick any two points in the tessellation, and the tiling must look different at those points.

Robert Ammann was an early researcher in aperiodic tilings. Remarkably, he was not a professional mathematician—he was a postal worker with a passion for discovering new tilings. One of the tilings he discovered is based on a single tile shape, in two different sizes. The ratio of the area of the larger tile to the smaller is the golden ratio, \(\phi \approx 1.618\). Note that the tile is shaped like a chunky letter b, hence the affectionate moniker “Golden b.”

If we take the shortest side of the smaller tile to be one unit long, then we get the other sides by successively multiplying by \(\sqrt{\phi}\). And we create the larger tile by scaling all side lengths by \(\sqrt{\phi}\).

The elliptical markings on the tiles enforce a matching rule. So, for example, you might combine the two tiles above to create this:

And behold! It is another, larger, Golden b. This suggests you can combine it with the larger tile to create a still larger Golden b.

As you build a tessellation with the Golden b, the ratio of the number of large to small tiles converges to—you guessed it, the golden mean.

Now available in our store!

## Create Celtic Knots

To artists and mathematicians, knots can be beautiful, interesting structures. Until recently, I hadn’t explored them much from either perspective. Then my mother-in-law gave me a copy of George Bain’s Celtic Art: The Methods of Construction for my birthday. Bain shows how to create Celtic knots, from simple to elaborate.

Let’s make one, partly based on Bain’s techniques. Celtic knots can be used to fill complex shapes, but many of the motifs fill rectangles, and this is a good place to start. You may want graph paper, pencil, and an eraser. Create a grid with an even length on each side. I’m using 8×6 here, and you’ll notice that I have accented every other line, so that there is a 4×3 grid of 2×2 cells.

Sketch diagonal lines so that each 2×2 cell has a rotated square in it:

Now, we are going to draw the border of our knot. Rotated squares on the corners will have three sides (shown in green) replaced. Rotated squares on the corners have two sides (shown in red) replaced.

Green sides are replaced with a cusp, and red sides with an arc.

It’s starting to look a bit like a Celtic knot. In fact, we could skip this next step, and produce a generic knot. But part of the art of creating a Celtic knot is varying its structure. Pick a number of interior intersections to be erased. I’ve erased two here.

After erasing each intersection, reconnect the strands vertically or horizontally. In my drawing, I’ve chosen to reconnect the broken lower left intersection with vertical lines

Next, we identify where the line passes over and under itself. Pick any intersection to start, and select which line goes under. Erase a little bit on each side of the overpassing knot. Then follow either line to the next intersection, and make sure the line does the opposite of what it did at the previous intersection. In other words, if it went over first, now it goes under. And vice versa.

Here is one of the beautiful results of such diagrams: alternating over and under always works. You cannot find a knot that cannot be completed this way. If you follow the paths, you will see that it is actually two knots that are connected. Mathematicians call this a *link*.

Finally, widen the lines, and you have created a Celtic knot!

We created tiles that allow you to create knots. If you look at each of the small blue squares in the knot above, you notice there are only a few types. These became our tiles. To give them more of a Celtic flavor, we added a rope motif to each tile.

## Squaring rectangles

Consider the following innocent sounding problem. Can you draw a rectangle that can be cut into two or more squares of different sizes?

Give it a try, then come back…

This turns out to be remarkably difficult. It is even more difficult if the rectangle you are cutting up is itself a square.

A solution to the problem is called a *perfect squared rectangle*. Perfect here means that all of the squares are different sizes. If the rectangle is not composed of smaller perfect squared rectangles, then it is a *simple perfect squared rectangle*.

Archimedes wrote about dissecting squares over 2000 years ago. Yet the first perfect squared rectangles were not discovered until 1925. One of those rectangles (pictured below) is 33×32 and uses nine squares, which is the fewest possible. Can you determine the size of the smallest square?

There is an excellent Numberphile video describing this problem and its history, which is known as *Squaring the Square*. Most of the methods used to find perfect squared rectangles were developed at Cambridge University by four students in the Trinity Mathematical Society. They found a useful way to represent the rectangle and its squares as an electrical circuit, and applied Kirchhoff’s circuit laws to help find solutions. For a more detailed description of this and related problems, take a look at squaring.net.

We have created a puzzle which we introduced at the 2019 JMM. It uses squares with side lengths from 1×1 to 50×50. There are 64 ways you can assemble a subset of these squares into simple perfect squared rectangles. To give you a sense of scale, the 50×50 has 10 inch sides.

## Apollonian gaskets

The Apollonian gasket is a fractal. It begins with three circles. Each pair of circles touch each other at a single point, and the three points of contact are distinct.

There are two circles that just touch all three:

Since we have added new tangent circles, we can now take new combinations of circles three at a time that are mutually touching, and find the two circles that touch all three. We continue this, filling in the empty spaces, and creating the Apollonian gasket:

The circles invite decoration with colors or depth.

There are also interesting relationships between the curvature of the circles. A circle’s curvature gets bigger as the circle gets smaller. If it has radius 1, the curvature is 1. If the radius is 1/3, the curvature is 3. And so on. Here is an Apollonian gasket showing the curvature of each circle. If you start at the 2 on the left, and follow the circles around the 3, the numbers are 2, 6, 14,…. Do you see a pattern?

And here is an example of an Apollonian gasket we’ve made, available from our online store.

If you’re curious about the name, Apollonius of Perga, the great Greek geometer, solved the more general problem of finding tangents for three circles that aren’t necessarily touching. In that case, there can be up to eight tangent circles.

## Luca Pacioli and mathematical type design

We’ve been taking typography classes at our nearby community college. Type design necessarily consists of line and curve, but some type faces are explicitly geometric, including the modern Futura fonts you are reading right now.

When movable type appeared in Europe (400 years after it appeared in Asia), humanist type designers such as Nicolas Jensen wanted to improve upon the heavy, difficult-to-read calligraphic Blackletter. Returning from a visit to Italy where he studied the Latin typefaces carved by the Romans, Jensen developed a typeface that was readable, yet retained the look of the human hand wielding a broad nibbed pen.

Luca Pacioli, collaborating with his mathematics student (and housemate) Leonardo da Vinci, studied the stroke widths and curvature of this humanist type, and captured it in his famous series of uppercase letters. Besides being beautiful graphic art, we like how it illustrates geometry at work.

## Nessie, the sea serpent tile

These abstract, fluid shapes first made us think of water and waves:

After we chose the colors, a family member was reminded of sea serpents. So we named this shape *Nessie*.

Nessies allow for a remarkable variety of patterns.

The above tiling patterns have translational symmetry, which means that if you had a tracing of the pattern, you could slide the tracing to other points where it would match the new location. You can also create dramatic radial patterns with rotational symmetry.

The pattern below has 3-fold rotational symmetry. This means you can stick a pin in the center of this pattern, and as you rotate around the pin, the pattern repeats itself every 120°. It is also possible to create 2-fold and 6-fold symmetries.

## Parallelograms and Tetriamonds

One of the kits I used in the classroom was a set of parallelograms. We had a few kits left over, and when we sold them, we decided at the time not to make any more. They didn’t seem as mathematically dramatic as our new fractal tiles, although they are very interesting to play with. Recently, one of our customers commented on how her son spent a lot of time playing with his parallelogram tiles. It was all we needed to hear to bring them back.

A single, simple shape allows for a remarkable amount of exploration.

If you join pieces at the corners, you can put six together to get a star:

Note that pieces can be reflected (i.e. flipped over). By flipping every other piece, the star becomes:

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You can have 3, 4, 5, or 6 tiles meet at a point, and since each tile can have one of two orientations, there are many ways to connect these. Here are three of the several ways you can connect 4 pieces:

All of that flexibility is sufficient to create countless design variations. However, this parallelogram happens to belong to a special set of objects known as polyiamonds, which are polygons that are formed by joining the edges of equilateral triangles.

If a polyiamond is composed of 4 triangles, then it is called a *tetriamond*. Besides our parallelogram, there are two other tetriamonds, the *v* and the triangle:

Here are some designs we’ve created.