# Math Monday: Try a Torus

By Glen Whitney for the Museum of Mathematics

Just suppose you wanted to make your own model of the Ungar-Leech map on the surface of a torus, like this one created by Norton Starr in 1972:

You’d probably want to start by making a torus. And just suppose you didn’t happen to have access to a Fab Lab or a Modela MDX milling machine, so that you couldn’t follow these instructions to produce a wooden torus like this one:

What could you do? Well, you could try making a plaster mold of a torus, as in the following detailed video showing the entire process from start to finish:

And if you do make a torus in this way, you really might want to paint it with the Ungar-Leech map shown above. Why? Because that map shows that unlike on a sphere, where any map can be colored with four colors, it takes at least seven colors to color certain maps on a torus. In particular, the Ungar-Leech map divides the torus into seven congruent regions, each of which touches all of the other six. So there’s no way to color it with fewer than seven colors.

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### BY Gareth Branwyn

Gareth Branwyn is a freelancer writer and the former Editorial Director of Maker Media. He is the author or editor of a dozen books on technology, DIY, and geek culture, including the first book about the web (Mosaic Quick Tour) and the Absolute Beginner’s Guide to Building Robots. He is currently working on a best-of collection of his writing, called Borg Like Me.

### 4 Responses to Math Monday: Try a Torus

1. Henry on said:

Sliceform is a really easy and “123d make” makes it easy all you need is a printer because a torus is one of the sample models you can play with and make.
is you want a more permanent torus you can make a sliceform torus with cardboard and cover it with clay.

2. Why did he make a plaster mold sitting freeform on a board? Why not put it in a box? Seems a LOT easier.

3. Reblogged this on ilripassinodimatematica and commented:
belle cose da fare…