A111308 Sum_{k=1..n} k!! * (n+1-k)!!, where k!! = k*(k-2)*(k-4)..*(1 or 2).

0, 1, 4, 10, 28, 71, 204, 556, 1716, 5049, 16884, 53418, 193500, 654639, 2560140, 9209016, 38697300, 147193425, 661035060, 2645445870, 12627252540, 52926443415, 267088884300, 1167777120420, 6200158778100, 28178714373705

Offset: 0

Author

Leroy Quet, Nov 03 2005

Keywords

• easy
• nonn

Examples

			a(5) = 1 *5*3*1 + 2 *4*2 + 3*1 *3*1 + 4*2 *2 + 5*3*1 *1 = 15 + 16 + 9 + 16 + 15 = 71.
		

Crossrefs

Cf. A006882.

Programs

Mathematica

Table[ Sum[k!!(n + 1 - k)!!, {k, n}], {n, 25}] (* Robert G. Wilson v *)

Extensions

More terms from Robert G. Wilson v, Nov 04 2005