Conway symmetry for polygons

Polytile notation only concerns itself with cyclic/rotational symmetry. We may also be interested in reflectional symmetry. John H. Conway presented a system that applies to all planar polygons.

It uses a letter class followed by the symmetry order.

Conway defines 5 symmetry classes:

  • a=asymmetric (no symmetry),
  • g=gyrosymmetric (rotational symmetry only),
  • r=regular (reflections on vertices and edges),
  • p=persymmetric (reflections passing through edges),
  • d=diasymmetric (reflections pass through vertices), and
  • i=isosymmetry (reflections pass through both vertices and edges).

Reflectional symmetry orders are doubled from a gyration symmetry. We may add these names to qualify a given polytile: a1, gn, p2n, i2n, d2n symmetric.

We can inspect polytile notation to determine the symmetry. A p-tile, p:a1.a2…am^n, explicitly expresses the gyrosymmetric as gn, representing repeated sequences of angle indices.

A p-tile with reflectional symmetry will have the form: p:A.b1.A^-1.b2, where A is a chain of zero or more vertices, A^-1 is the same chain in reverse order, and b1 and b2 are zero or one vertices. A diasymmetric form will include both b1 and b2, an isosymmetric form only have one of them, and persymmetric form has neither.

© 2020 Created by Tom Ruen