Genus
Genus is defined by the following formula discovered by
Leonhard Euler:
1+(E-V-F)/2
where
V is the number of vertices,
E is the number of edges,
F is the number of faces.
Genus is a measure of the complexity of a polyhedron.
Since these polyhedra can be infinite in size V, E and F
are calculated for the smallest repeating unit that preserves the internal/external property.
This unit will be a parallelepiped.
The internal/external property is necessary as otherwise the translation unit for some polyhedra
would be half the size but would give an erroneous genus value.