Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 5000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".
Related to class numbers of cyclotomic fields, although the precise relationship is not clear to me. Compare A000927.
Mermin, N. David; Rokhsar, Daniel S.; and Wright, David C.; Beware of 46-fold symmetry: the classification of two-dimensional quasicrystallographic lattices. Phys. Rev. Lett. 58 (1987), 2099-2101.
Table of n, a(n) for n=2..49.
See A193175 for values of 2n such that a(n) = 1.
Sequence in context: A083985 A109848 A104146 * A133705 A046111 A131779
Adjacent sequences: A193176 A193177 A193178 * A193180 A193181 A193182
N. J. A. Sloane, Jul 17 2011