E.g.f.: Sum_{n>=0} x^n * (1 + x^n)^n / n!.

Terms

a(0) =1a(1) =1a(2) =3a(3) =1a(4) =25a(5) =1a(6) =721a(7) =1a(8) =6721a(9) =181441a(10) =151201a(11) =1a(12) =203575681a(13) =1a(14) =121080961a(15) =108972864001a(17) =1a(19) =1a(23) =1