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## Design note #9: Dragons

Whenever we are showing at art and craft fairs, people of all ages love playing with the Koch tilings. I think the appeal is that like many fractals, they evoke nature. And there is something satisfying about getting the intricate edges to meet, and have a piece slide into place.

So we decided to try another, based on one of the dragon curves. There are several choices, some much more delicate than others. This delicate edge comes from the fact that flat fractals have infinitely long edges, and a fractional dimension between 1 and 2. The closer the dimension is to 2, the more intricate, and therefore more difficult to cut and fragile. We settled on Knuth’s terdragon because the fractal dimension is high enough to be interesting, but still makes for a sturdy tile.

It is pretty easy to learn how to attach Koch tiles to each other. The tiles cannot be the same size. There are a couple of attachment points for two Koch tiles. The Dragon tiles, on the other hand, seem to take a little longer before users readily identify how to connect them. But they are actually more flexible. Any two tiles, regardless of size, can be connected, and there are several ways to attach.

We debuted our Dragon tiles at the 2018 Joint Mathematics Meeting, and as with the Koch tiles, they were a hit with the young and not-so-young. This little guy made our day:

Recently, we’ve added frames as an option for our Dragon puzzles, which is nice if you want to be able to move your work-in-progress.

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## Design note #8: Fibonacci, Sunflowers, Voronoi, and a clock

Fibonacci‘s famous sequence (0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, …) has been linked to many natural phenomena. The arrangement of seeds in sunflowers is one example.

The construction of the sunflower spiral can be described as a series of steps, moving out a certain distance from the center of the circle and rotating a fixed angle for each seed.

To play around with these ideas, I wrote a script in p5js that allows you to vary that angle:

So, you can create some beautiful patterns, and with the laser cutter, you could cut out the circles. But I was also interested in the idea of the dots representing the center of cells. Enter the Voronoi diagram. Given a set of points, it finds cells containing those points, with the property that any border that separates two points is equidistant from the points.

We think this evokes the idea of biological cells, and led to our abstract Cellular Clock:

Eko Hironaka was interested in a design that was more suggestive of 12 hours, and created the design for Eko’s Flower.

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## Design note #7: Inversive geometry

This is a repost of ideas that I used in the classroom.

The design below was created using inversive geometry, combined with a twisting rotation.

Inversive geometry is a nice way to introduce the idea of switching back and forth between different geometries, which can lead to the study of this Poincaré disk. The ideas are accessible with using just geometry.

Inversive geometry requires that we have a circle of a given radius $$r$$. Consider any point in the plane, at distance $$d$$, lying on a ray from the center of the circle. Then that point has an inverse point, at distance $$r^2/d$$ from the center of the circle, on the same ray. Taking the inverse of a point flips points from outside the circle to inside the circle, and vice versa. Imagine the plane being made out of flexible material, and inverting is like puncturing the plane at the center of the circle, and then turning the plane inside out, with the points on the edge of the circle remaining stationary.

You can play with this in desmos by clicking on the graph below. The orange disk is the unit circle. The red dot can be dragged. The green dashed line segment from the red dot towards the origin leads to the purple point, which is at a distance that is the reciprocal of the red dot’s distance from origin.

What shape is traced out by the inverse point as we slide the red dot along the line? Is this always the case?

Now, consider infinite graph paper with integer coordinates. The lines consist of all vertical lines with integer $$x$$ values, and all horizontal lines with integer $$y$$ values. What do you get if you find the inverse of all those lines?

This mapping of the entire plane outside the disk into the unit disk (and vice versa) is a conformal mapping, which means it preserves angles. If we define distance between two points in the disk to be the same as the distance between the inverses of the points, then we are preserving distance. Angles are the same and distances are the same, so we could study the ideas in high school geometry, but staying entirely inside a disk. What project ideas does this give you?

A rectangular grid, inverted, and laser cut on plywood.

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## Design note #6: Penrose P3 tiles

Roger Penrose is well known for his collaborations with Stephen Hawking studying black holes. Perhaps being a cosmologist made him interested in the work of Kepler. And perhaps that led him to thinking about tilings with pentagons. In any case, he ended up discovering a some remarkable things about tiling. In particular, he discovered Penrose tiles. He described three such tilings, called P1, P2, and P3. P3 tiles are shown below.

P3 Penrose tiles could just be two simple rhombuses (thick and thin), but you would have to follow special rules to determine whether two tile edges can be matched. But those rules can just be implemented by altering the edges to limit how you can connect the tiles. Remarkably, the resulting tiling is guaranteed to be aperiodic, which means it is not a typical repeating wallpaper-style pattern.

Amazingly, Penrose’s tilings ended up being useful in explaining physical phenomena that was discovered after Penrose discovered the tiles. You can read more about applications to crystallography, as well as the notion of inflation in the article Penrose Tiles talk across the Miles.

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## Design note #5: Tiling with regular polygons

The Koch snowflake got me thinking about producing other sets of tiles that can be used to tesselate the plane. For the classroom, I created a prototype set of regular polygon tiles, which I used in conjunction with Jacobs’ Mathematics: A Human Endeavor. These happened to be created on a laser with lower power and airflow than mine, so some of the tiles have caramel-colored edges, which I clean up in production. But I kind of like the way it highlights the edges.

Regular tilings of the plane used only regular polygons to completely fill the plane, and this can only be done with triangles, squares, and hexagons.

However, there are many more semi-regular tilings, which allow 2 or more polygon types, always meeting in the same way at the corners. Below are the 3.6.3.6 and 3.4.6.4 tilings, named for the number of sides of the polygons that meet at each vertex in the tiling.

And note that octagons and 12-sided dodecagons can get in on the action:

These are the only regular polygons that can be used to completely tile the plane. However, as noted by Albrecht Dürer and Johannes Kepler, there are some interesting tilings you can create with pentagons. So I created a set of regular pentagons:

As we played with them, we found it a little frustrating that a slight bump tends to move everything out of alignment. So I added tabs. The construction below comes up often in graphics of Kepler’s work.

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## Design note #4: Koch snowflakes

The Koch snowflake is one of the first fractal curves to be described.

Like other fractal curves, it has an infinitely long boundary, and the self-similarity is obvious as you zoom in. One of the cool things about the Koch snowflake is that it can be built from six smaller snowflakes, leaving another snowflake in the middle. That of course can also be decomposed, recursively, giving you this:

So that led to one of our first puzzles, which uses two sizes of snowflakes. I put the box of pieces in the Mathematics Commons at the University of Michigan. Both of the patterns below were created there.

Putting it together, in holiday colors…

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## Design note #3: Patches as wall plaques

Max asked me if I could create a replica of his squadron’s patch in plywood, about 22″ across. I created the vector drawing in Affinity Designer, and cut it out of a combination of 1/2″ and 1/4″ birch ply to get a layered effect. Several of the cadets assembled it, and painted it:

They did a nice job! In cause you’re wondering, Squadron 2’s logo references the F-102 Delta Dagger, a 1960s era fighter-interceptor.

Next thing you know, a couple of his buddies from his water polo team wanted plaques for their CQ desks. Squadron 28 (featuring a stylized SR-71 Blackbird) opted for two tones, alternating stain with a pleasant natural tone.

Squadron 11 went for a subdued look, and selected a dark stain for the entire plaque. I haven’t stained birch before, but this Fine WoodWorking forum has some suggestions for getting the best results.

I must admit that I am partial to the bright colors of the original patch, so I’ve shown a Photoshopped version below.

There was interest in a Squadron 8 patch as well, based on the F-15 Eagle. I prototyped it in Affinity Designer based on a patch from the USAFA website:

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## Design Note #2: Oriented Triangles

This is inspired by the Izzi puzzle, which is composed of squares. I learned about it from Professor Mark Saul of the The Center for Mathematical Talent at NYU, who developed beautiful mathematical content for i2camp.org. The Izzi puzzle consists of squares that have bisected edges that are combinations of black and white.

On my teaching blog, I explored the idea of using equilateral triangles. You need only 24 pieces to have one of each possible triangle, and they can be assembled into a hexagon. The challenge is to match all edges.

Below is the one I created in acrylic. The picture is a hexagon, but not a solution.

I created them initially using etching to create a lighter color. But I’m not a fan of etching unless it is really necessary. For one thing, it is slow. But I’ve also found that there is usually a more aesthetically pleasing alternative using low power cut lines. The new version will be available soon in our store.

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## Design Note #1: the Arbor Circle

An arbor is a small group of trees. I think of it as smaller than a forest, bigger than a grove. Ann Arbor (A2 to locals) loves its trees, and the co-founders of Ann Arbor named their new tree town after their wives, who shared a first name.

Like many of our designs, the Arbor Letter design has mathematical inspiration that has connections to nature. If you look carefully at the trees, you’ll note they are self-similar. Whenever a branch forms, the branch is a copy of the tree, reduced in scale. So the trees are fractals.

At first, we were taken with large (12”) diameter designs that we hung on the wall. And we have a pair of 20″ diameter with our surname initials for our front doors.  But reduced to 3.5″, with a loop added, it makes a beautiful, delicate ornament. We now make the design in a variety of sizes, and have written software for composing variations in the shape, positioning, and number of trees.

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## A shared hobby created

I’ve had a few careers, all of them motivated by curiosity. Flying airplanes, writing software, financial engineering all provided a creative outlet, but as jobs go, that creativity was necessarily narrowly focused. I always enjoyed teaching, and figured I might take it up when I had explored enough other careers. To my surprise, I discovered teaching high school was the job that really indulged my creative side in the most general way. I taught high school math, physics, and computer science. Teaching computer science in particular enabled me to explore a trove of interesting problems to solve with the students. We wrote games in Scratch, constructed enormous structures in Minecraft using Python and Javascript, and sketched dynamic and interactive visualizations with Processing (and later p5.js).

Processing was especially inspiring. I used its pdf library to algorithmically generate drawings. It was this, combined with a visit to Ann Arbor’s MakerWorks, that ultimately led to Cherry Arbor Design.

MakerWorks has an array of maker tools, but I was drawn to the laser cutter, because I could see how the drawings created in Processing could be turned into precise wood or acrylic representations.  When I brought my creations home, Heidi was immediately intrigued with the possibilities. We started having our date nights at MakerWorks (yep, we’re nerds), creating earrings and other small items from thin cherry and maple boards, colorful acrylic, and Baltic birch plywood. Eventually, so that we could have unlimited access, we decided to buy a laser cutter. I still do most of my work with Processing and pf.js, while Heidi prefers Affinity Designer, an alternative to Adobe Illustrator.

Today, Heidi and I spend much of our time together making things, and further developing our design skills. We are all about nonstop learning, so at the moment, we’re taking a PhotoShop class at the local community college.