a(n) = 1 + 2*n*n!.

Terms

a(0) =1a(1) =3a(2) =9a(3) =37a(4) =193a(5) =1201a(6) =8641a(7) =70561a(8) =645121a(9) =6531841a(10) =72576001a(11) =878169601a(12) =11496038401a(13) =161902540801