a(n) = n! + n!!.

Terms

a(0) =2a(1) =2a(2) =4a(3) =9a(4) =32a(5) =135a(6) =768a(7) =5145a(8) =40704a(9) =363825a(10) =3632640a(11) =39927195a(12) =479047680a(13) =6227155935a(14) =87178936320