## Saturday, 11 May 2013

### Mathematical Models

Way back when I was at school, I came across a fascinating little book called Mathematical Models, by Cundy and Rollett.  It had instructions on how to build various mathematical objects, such as stellated polyhedra.  I liked the book so much I actually bought my own copy, new, for £2.95, which was a lot of money back then!

I made a few of the simpler models, but never got much further than the dodecahedron.  I certainly never got as far as making any of the fiddly stellated ones.

So today, when I came across a posting in Google+ about Anselm Levskaya's website polyHédronisme, I was taken right back to those days.  Playing with this interactive web-based systems is much easier than fiddling with card, glue, and scissors, though.  Type in a few commands, and a zoomable, rotatable polyhedron appears!

I've spent my afternoon playing around on this site, and reading up on Conway polyhedron notation that is used to define shapes, and now I can say I have at last "made" some of these polyhedra.

The small stellated dodecahedron is made by raising a pentagonal-based pyramid on every face of a regular dodecahedron. If the pentagons making up the dodecahedron have side length $1$, then the height of each pyramid should be* $\frac{\sqrt{4\sqrt{5}-1}}{2} \approx 1.41$ The Conway notation command in polyHédronisme that achieves this is $k(5,1.41)D$, which means: start with a dodecahedron $D$, then raise a pyramid of height $1.41$ on each $5$-sided face.

The great dodecahedron is made by making a pyramidal dimple in every face of a regular icosahedron. If the triangles making up the icosahedron have side length $1$, then the depth of each pyramid should be* $\sqrt{\frac{1}{2}- \frac{\sqrt{5}}{6}} \approx 0.36$ The Conway notation command in polyHédronisme that achieves this is $k(3,-0.36)I$, which means: start with an icosahedron $I$, then indent a pyramid of height $0.36$ on each $3$-sided face.

The great stellated dodecahedron is made by raising a pyramid on every face of a regular icosahedron. If the triangles making up the icosahedron have side length $1$, then the height of each pyramid should be* $\sqrt{\frac{7+3\sqrt{5}}{6}} \approx1.51$ The Conway notation command in polyHédronisme that achieves this is $k(3,1.51)I$, which means: start with an icosahedron $I$, then raise a pyramid of height $1.51$ on each $3$-sided face.

So much for standard polyhedra.  But polyHédronisme doesn't stop there.  I had great fun playing about with the notation language, producing weird and wonderful forms:

 (i)  $k(20,1)bk(3,2)I$    (ii)  $k(12,1)k(10,2)bk(5,1)D$    (iii)  $k(20,-0.3)k(6,0.3)bk(3,-0.3)I$ (iv)  $k(24,-0.5)k(6,0.2)k(20,-1)bk(3,-0.1)k(5,1)D$

The results of play can only really be appreciated on the site itself, rotating the polyhedra, and getting a real feel for their shapes, with all their dips and bumps.  A marvellous site.

* The book Polyhedron Models has helpful stellation diagrams that allow these heights to be calculated, with a little trigonometry.