OFFSET
1,2
LINKS
Nathaniel Johnston, Table of n, a(n) for n = 1..250
J. R. Stembridge, Some combinatorial aspects of reduced words in finite Coxeter groups, Trans. Amer. Math. Soc. 349 (1997), no. 4, 1285-1332.
MAPLE
seq(2*doublefactorial(2*n-1)-(n-1)!*2^(n-1), n=1..18); # Nathaniel Johnston, Jun 23 2011
MATHEMATICA
Table[2*(2n-1)!! - (n-1)!*2^(n-1), {n, 1, 20}] (* Jean-François Alcover, Feb 11 2016 *)
PROG
(Magma) A001147:=func< n | n eq 0 select 1 else &*[ k: k in [1..2*n-1 by 2] ] >; [ 2*A001147(n)-Factorial(n-1)*2^(n-1): n in [1..20] ]; // Klaus Brockhaus, Jun 22 2011
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
More terms from Sean A. Irvine, Jun 13 2011
STATUS
approved