OFFSET
1,2
COMMENTS
Each Brillouin zone has the same area, and they are all equivalent mod the periodicity of the lattice, but the higher-order ones consist of a set of polygons. This is the number of polygons in that set.
EXAMPLE
The 2nd Brillouin zone consists of 6 equilateral triangles, so a(2) = 6. The 5th Brillouin zone consists of 12 30-60-90 triangles, so a(5) = 12.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Keenan Pepper, Sep 11 2010
STATUS
approved