# Introduction to polytopes

We can try to define a polytope by saying "its elements must be polytopes of lower dimension."[1] This means that 2-dimensional polygons, as well as 3-dimensional polyhedra, are polytopes. This in turn tells us that line segments (formally referred to as "dyads") and points must be polytopes too.

(The "elements" you'll need to worry about are the vertex (which is a point), the edge (which is a dyad), and the face (which is a polygon).[2])

The lowest-dimensional polytope, the one at the base of the definition of all others, is the –1-dimensional nullitope. Visualizing it is not important, but it serves as a placeholder in advanced operations.

## The Platonic solids

Interest in polytopes often begins with the Platonic solids, shapes commonly found in dice. They are special because for each of them, each element is the same as each other element of that type. This property is known as regularity.

For instance, each of the square faces of the cube connects to four other identical faces and four other identical vertices, each edge lies between two identical faces and two identical vertices, and each vertex connects to three identical faces and three identical edges.

On a side note, there are other possible regular polyhedra, but they are not convex - that is, they are spiky or they have holes, and wrapping a hypothetical rubber sheet around them would give you a different shape than them. (This is not true of the Platonic solids.) They may be made of "star" polygons instead of convex polygons, but each one still has only one type of vertex, one type of edge, and one type of face

Even more regular polyhedra are possible, but they all have infinite amounts of faces. The mucube shown here is a relatively simple example: each vertex has six square faces, each edge is between two identical vertices, and each face is surrounded by a saddle-like arrangement of other faces. Even if it goes on forever, it still qualifies as regular.

Other infinite regular polyhedra are possible, that further stretch the bounds of what is possible by having infinite polygons as faces. All the polygons in the polyhedron are the same, and all the vertices in the polygon are the same, so why can't they be regular too? This video is a good place to learn about those.

## The Archimedean solids

The Platonic solids can be modified in many ways. When these modifications produce a convex polyhedron that has identical vertices (like the restriction of regularity, but only applying to the vertices - it is formally called vertex-transitivity) and a single edge length, the result is called an Archimedean solid. The modifications (often called operations) that produce Archimedeans typically involve cutting off vertices or edges, or pulling faces away from one another.

### Some Archimedean solids

The Archimedean solids are a small subset of the uniform polyhedra, and there are 13 of them in total.

## The Johnson solids

Pieces of Platonic and Archimedean solids can be cut-and-pasted to form Johnson solids, although some are "elementary" and are not related to any other shape. There are 92 of these in total.

## Higher dimensions

What if we were to make a 4-dimensional polytope out of 3-dimensional elements, just like the 3-dimensional polytopes (polyhedra) are made of 2-dimensional elements?

The 3-dimensional element is called a "cell," and in polychora, two cells join together at a shared face just like two faces join at an edge in a polyhedron.

In this view of the tesseract (which is regular, just like the Platonic solids), we see eight cubes (one in the middle, six around the sides, and one encompassing the whole thing.) Three cubic cells meet at each edge, and four cells meet at each vertex.

All of the previously discussed elaborations and modifications are possible in 4 dimensions[3] and above, as well as some that weren't discussed, and ones that aren't even possible in dimensions as low as 3. This is the scope of the study of polytopes.

## Footnotes

1. This is NOT the entire definition, but it is one important part of it.
2. There are also higher-dimensional elements than these, like the "cell" that is a polyhedron, but we'll get to those later.
3. Although we don't know as much about infinite regular polytopes in 4D as we do in 3D