The words "antipodes" and "antipodal" are usually used in the context of a sphere. Their meaning there is clear: two points in a sphere are antipodal if the distance between them is as great as possible.

This works because there is an obvious distance measure to use in a sphere: great circle distance. It is possible to devise at least two different sensible distance measures in a torus (or on "different toruses", if you prefer). For surfaces of genus greater than 1, it is even less clear what distance measure should be used.

In these pages I use these words "antipodes" and "antipodal" in a way
not based on a distance measure. My definition applies to all finite
2-manifolds including the sphere. It is

If a symmetry operation on a regular map (which I
prefer to regard as a polyhedron) fixes some structure (*e.g.*
a vertex, edge, or face) of it, and also fixes some other structure,
these two structures are said to be antipodal.

This relation is transitive, so sets of two or more structures are
mutually antipodal.

For the sphere, this definition is consistent with the usual one. We find that the five Platonic solids have the following sets of antipodes

- tetrahedron: (vertex, face), (edge, edge).
- octahedron: (vertex, vertex), (face, face), (edge, edge).
- cube: (vertex, vertex), (face, face), (edge, edge).
- icosahedron: (vertex, vertex), (face, face), (edge, edge).
- dodecahedron: (vertex, vertex), (face, face), (edge, edge).

The definition covers structures other than vertices, edges, and
faces; *e.g.* holes and
Petrie polygons. Here is how
this applies to the Platonic solids.

- tetrahedron: (vertex, face), (edge, edge, Petrie polygon).
- octahedron: (vertex, vertex, hole), (face, face, Petrie polygon), (edge, edge).
- cube: (vertex, vertex, Petrie polygon), (face, face), (edge, edge).
- icosahedron: (vertex, vertex, hole, hole, Petrie polygon), (face, face, 2nd-order Petrie polygon), (edge, edge).
- dodecahedron: (vertex, vertex), (face, face, Petrie polygon), (edge, edge).

One of the more interesting regular maps on the torus consists of
seven hexagons, each bordering the other
six. It has 14 vertices and 21 edges. Its sets of antipodes are

(vertex, vertex), (face), (edge, edge, edge).

Its rotational symmetry group is C7⋊C6, the Frobenius group of order 42.
It has as its Sylow-subgroups

1 | normal | C7, | so its | 3 | 14-sided Petrie polygons | form 1 antipodal | threesome |

7 | conjugate | C3s, | so its | 14 | vertices | form 7 antipodal | pairs |

7 | conjugate | C2s, | so its | 21 | edges | form 7 antipodal | threesomes |

The genus-2 regular map G2:{8,3} has
six octagonal faces, 16 3-valent vertices, and 24 edges. Its sets
of antipodes are

(vertex, vertex, vertex, vertex, Petrie polygon), (face, face), (edge, edge).

Its rotational symmetry group is GL(2,3). GL(2,3) has eight elements of order 3, so its
Sylow-3-subgroups are

4 | conjugate | C3s, | so its | 16 | vertices | form 4 antipodal | foursomes |

The genus-3 regular map G3:{7,3} has
24 heptagonal faces, 56 3-valent vertices, and 84 edges. Its sets of antipodes are

(vertex, vertex), (face, face, face), (edge, edge, edge, edge, Petrie polygon).

Its rotational symmetry group is PSL(2,7). Its
Sylow-subgroups are

8 | conjugate | C7s, | so its | 24 | faces | form 8 antipodal | threesomes |

28 | conjugate | C3s, | so its | 56 | vertices | form 28 antipodal | pairs |

21 | conjugate | D8s, | so its | 84 | edges | form 21 antipodal | foursomes |

Some regular maps drawn on orientable 2-manifolds

Some pages on groups

Copyright N.S.Wedd 2009