## November 22, 2022

### Inner Automorphisms of the Octonions

#### Posted by John Baez

What are the inner automorphisms of the octonions?

## November 16, 2022

### The Icosidodecahedron

#### Posted by John Baez

The icosidodecahedron can be built by truncating either a regular icosahedron or a regular dodecahedron. It has 30 vertices, one at the center of each edge of the icosahedron—or equivalently, one at the center of each edge of a dodecahedron. It is a beautiful, highly symmetrical shape. But it is just a shadow of a more symmetrical shape with twice as many vertices, which lives in a space with twice as many dimensions! Namely, it is a projection down to 3d space of a 6-dimensional polytope with 60 vertices.

Even better, it is also a slice of a more symmetrical 4d polytope with 120 vertices, which in turn is the projection down to 4d space of an even more symmetrical 8-dimensional polytope with 240 vertices: the so-called ‘E_{8} root polytope’.

Note how the numbers keep doubling: 30, 60, 120 and 240.

## November 1, 2022

### Categories and Epidemiology

#### Posted by John Baez

I gave a talk about my work using category theory to help design software for epidemic modeling:

• Category theory and epidemiology, African Mathematics Seminar, Wednesday November 2, 2022, 3 pm Nairobi time or noon UTC. Organized by Layla Sorkatti and Jared Ongaro.

This talk is a lot less technical than previous ones I’ve given on this subject, which were aimed mainly at category theorists. You can watch it on YouTube.