A208124 a(1)=2, a(n) = (4n/3)*(2n-1)!! (see A001147) for n>1.

2, 8, 60, 560, 6300, 83160, 1261260, 21621600, 413513100, 8729721000, 201656555100, 5059746291600, 137034795397500, 3984550204635000, 123805667072587500, 4093840724533560000, 143540290403957947500, 5319434291440794525000, 207753461493493252837500, 8528826313943407221750000

Offset: 1

Author

N. J. A. Sloane, Mar 27 2012

Keywords

• nonn

Links

• Bela Bollobas, The number of 1-factors in 2k-connected graphs, J. Combin. Theory Ser. B 25 (1978), no. 3, 363--366. MR0516268 (80m:05060). See Eq. (2).

Programs

Mathematica

a[n_] := If[n == 1, 2, (4n/3)*(2n - 1)!!]; Array[a, 20] (* Amiram Eldar, Dec 01 2018 *)

Formula

(n-1)*a(n) =n*(2n-1)*a(n-1), n>2. - R. J. Mathar, Mar 27 2012