# Monthly Archives: June 2016

# A Tetrahedral Exploration of the Icosahedron

Mathematicians have discovered more than one set of rules for polyhedral stellation. The software I use for rapidly manipulating polyhedra (*Stella 4d*, available here, including as a free trial download) lets the user choose between different sets of stellation criteria, but I generally favor what are called the “fully supported” stellation rules.

For this exercise, I still used the fully supported stellation rules, but set Stella to view these polyhedra as having only tetrahedral symmetry, rather than icosidodecahedral (or “icosahedral”) symmetry. For the icosahedron, this tetrahedral symmetry can be seen in this coloring-pattern.

The next image shows what the icosahedron looks like after a single stellation, when performed through the “lens” of tetrahedral symmetry. This stellation extends the red triangles as kites, and hides the yellow triangles from view in the process.

The second such stellation produces this polyhedron — a pyritohedral dodecahedron — by further-extending the red faces, and obscuring the blue triangles in the process.

The third tetrahedral stellation of the icosahedron produces another pyritohedral figure, which further demonstrates that pyritohedral symmetry is related to both icosidodecahedral and tetrahedral symmetry.

The fourth such stellation produces a Platonic octahedron, but one where the coloring-scheme makes it plain that *Stella* is still viewing this figure as having tetrahedral symmetry. Given that the octahedron itself has cuboctahedral (or “octahedral”) symmetry, this is an increase in the number of polyhedral symmetry-types which have appeared, so far, in this brief survey.

Next, I looked at the fifth tetrahedral stellation of the icosahedron, and was surprised at what I found.

While I was curious about what would happen if I continued stellating this polyhedron, I also wanted to see this fifth stellation’s convex hull, since I could already tell it would have only hexagons and triangles as faces. Here is that convex hull:

For the last step in this survey, I performed one more tetrahedral stellation, this time on the convex hull I had just produced.

# Variations of the Snub Dodecahedron

To make the first of these variations, above, I augmented each triangular face of a snub dodecahedron with an antiprism 2.618 times as tall as the triangles’ edge length, and then took the convex hull of the result. The other polyhedra shown, below, were obtained by various other manipulations of the snub dodecahedron, all performed using a program called *Stella 4d: Polyhedron Navigator*, which you can try right here.

The variant above looked like it needed a name, so I called it an expanded snub truncated dodecahedron. As for the one below, it is one of many facetings of the snub dodecahedron.

Finally, the last figure shown (stumbled upon during a “random walk” with *Stella*) is one of many possible figures which are non-convex relatives of the snub dodecahedron.

# “What’s Your Favorite Color?”

It’s a mystery to me why this happens, but the parallels between different conversations which start with this question are simply amazing. First, I don’t get asked this question unless talking to a teenager . . . and then, nearly every time this happens, the rest of the conversation follows the same pattern.

First, I answer the question honestly, with a single word, by simply naming my favorite color.

After telling this one-word, five-letter truth, I then get a response which has become utterly predictable: “Black’s not a color!”

Even stranger: such inquisitions only seem to come from teenagers who are dressed in such a way as to let the following response work: “What color is your t-shirt?”

Sometimes they even look down at that point, presumably to check, which lets them see the answer to my question for themselves:

After that one question from me, for some reason, they tend not to say much more.

# Fractiles’ Mandala, Based on Angles of Pi/7 Radians

Although this was based on something I constructed using the *Fractiles-7* magnetic tiling toy, I did not have enough magnetic pieces to finish this. The idea was, therefore, converted into a (non-Euclidean) construction using *Geometer’s Sketchpad*, and then refined using *MS-Paint*. The reason I describe this as a non-Euclidean construction is that an angle of pi/7 radians, such as the acute angles in the red rhombi, cannot be constructed using compass and unmarked straight edge: antiquity’s Euclidean tools. The other angles used are whole-number multiples of pi/7 radians, up to and including 6pi/7 radians for the obtuse angles of the red rhombi.

The yellow rhombi have angles measuring 2pi/7 and 5pi/7 radians, while the blue rhombi’s angles measures 3pi/7 and 4pi/7 radians. None of these angles have degree measures which are whole numbers. It is no coincidence that 7 is not found among the numerous factors of 360. It is, in fact, the smallest whole number for which this is true.

I have a conjecture that this aperiodic radial tiling-pattern could be continued, using these same three rhombi, indefinitely, but this has not yet been tested beyond the point shown.