E.g.f. 1/(1-2x-x^4).

Terms

a(0) =1a(1) =2a(2) =8a(3) =48a(4) =408a(5) =4320a(6) =54720a(7) =806400a(8) =13587840a(9) =257644800a(10) =5428684800a(11) =125817753600