a(n) = n! + 2^n - 1.

Terms

a(0) =1a(1) =2a(2) =5a(3) =13a(4) =39a(5) =151a(6) =783a(7) =5167a(8) =40575a(9) =363391a(10) =3629823a(11) =39918847a(12) =479005695a(13) =6227028991a(14) =87178307583