OFFSET
0,1
COMMENTS
Terms are the allowable n-fold rotational symmetries of a crystal (rotation by 360 degrees/n leaves the object unchanged).
The positive values of this sequence {1, 2, 3, 4, 6} are the proper divisors of 12, all having a totient of 1 or 2 (see A000010).
LINKS
W. Scherrer, Die Einlagerung eines regulären Vielecks in ein Gitter, Elemente der Mathematik, 1946, 1(6), p.97-98.
Wikipedia, Crystallographic Restriction Theorem
EXAMPLE
2*cos(2Pi/1) = 2
2*cos(2Pi/2) = -2
2*cos(2Pi/3) = -1
2*cos(2Pi/4) = 0
2*cos(2Pi/6) = 1
2*cos(2Pi/10) = 1.6180339887... and so 10, for instance, is not in this sequence.
CROSSREFS
KEYWORD
sign,fini,full
AUTHOR
Raphie Frank, Sep 30 2012
STATUS
approved