a(n) = n! + n*(n-1)!!.

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =36a(5) =160a(6) =810a(7) =5376a(8) =41160a(9) =366336a(10) =3638250a(11) =39959040a(12) =479126340a(13) =6227619840a(14) =87180183090