OFFSET
1,1
COMMENTS
x*cos(x) - sin(x) = Sum_{n>=1} (-1)^n/a(n) * x^(2*n+1). - James R. Buddenhagen, Nov 21 2013
Also the number of adjacency matrices for the n-helm graph. - Eric W. Weisstein, May 25 2017
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..100
Eric Weisstein's World of Mathematics, Adjacency Matrix.
Eric Weisstein's World of Mathematics, Helm Graph.
FORMULA
zeta(2*n+1) = Integral_{u=0..Pi/2} (sin(u)*log(sin(u))^(2*n+1)/(cos(u)^3))*(-2^(2*n+1)/(n*a(n)). Verified for n=1 to 4 on Wolfram Alpha. - Jean-Claude Babois, Oct 28 2014
Sum_{n>=1} (-1)^(n+1)/a(n) = sin(1)-cos(1) = (-1)*A143624. - Amiram Eldar, Apr 12 2021
MAPLE
A174549 := proc(n) (1+2*n)*(2*n-1)! ; end proc: # R. J. Mathar, Jan 13 2011
MATHEMATICA
Table[(2*n - 1)! + (2*n)!, {n, 15}] (* T. D. Noe, Nov 21 2013 *)
PROG
(Magma) [Factorial(2*n-1) + Factorial(2*n): n in [1..15]]; // Vincenzo Librandi, Aug 04 2011
(PARI) a(n)=(2*n-1)!*(2*n+1) \\ Charles R Greathouse IV, Nov 21 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Paul Curtz, Mar 22 2010
STATUS
approved