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

Terms

a(0) =1a(1) =4a(2) =18a(3) =114a(4) =936a(5) =9480a(6) =114480a(7) =1607760a(8) =25764480a(9) =464123520a(10) =9286099200a(11) =204334099200