E.g.f. 1/((1-x)(1-x-x^2)).

Terms

a(0) =1a(1) =2a(2) =8a(3) =42a(4) =288a(5) =2400a(6) =23760a(7) =272160a(8) =3548160a(9) =51891840a(10) =841881600a(11) =15008716800a(12) =291711974400