OFFSET
1,2
COMMENTS
a(n) equals (-1)^n times the coefficient of sqrt(1-x^2)*(arcsin x)^2 in int (arcsin x)^(2n-1) dx. - John M. Campbell, Jul 20 2011
For n >= 4, also the number of distinct adjacency matrices of the n-Moebius ladder. - Eric W. Weisstein, Mar 31 2017
LINKS
Eric Weisstein's World of Mathematics, Adjacency Matrix
Eric Weisstein's World of Mathematics, Moebius Ladder
FORMULA
a(n) = (n-1)*(2*n-1)*(2*n-3)!
a(n) = (2n-1)!/2 = A009445(n)/2, for n>1. - Wesley Ivan Hurt, Mar 31 2015
EXAMPLE
For example: if n=1 then no polygon exists with 2 different sides. If n=2 then the polygon has 4 different sides A, B, C, D. In this case 3 different types of such 4-angle exist: (A, B, C, D), (A, B, D, C), (A, C, B, D).
MAPLE
MATHEMATICA
nn = 32; a = Log[1/(1 - x^2)^(1/4)] - x^2/4; Prepend[Select[Range[0, nn]! CoefficientList[Series[a, {x, 0, nn}], x], # > 0 &], 0] (* Geoffrey Critzer, Dec 10 2011 *)
Join[{0}, (2 Range[2, 20] - 1)!/2] (* Wesley Ivan Hurt, Mar 31 2015 *)
PROG
(PARI) a(n)=(2*n-1)!\2 \\ Charles R Greathouse IV, Dec 10 2011
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Sergey L. Dolmatov, Almir Dzhumaev (aalma(AT)mail.ru), Aug 18 2003
STATUS
approved