A000779 a(n) = 2*(2n-1)!!-(n-1)!*2^(n-1), where (2n-1)!! is A001147(n).

1, 4, 22, 162, 1506, 16950, 224190, 3408930, 58596930, 1123663590, 23782729950, 550718680050, 13849716607650, 375904338960150, 10952237584237950, 340947694234397250, 11294123783425733250, 396665528378000631750

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn
• easy

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.

Crossrefs

Cf. A001147.

Programs

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

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 *)

Extensions

More terms from Sean A. Irvine, Jun 13 2011