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

Terms

a(0) =1a(1) =0a(2) =4a(3) =12a(4) =144a(5) =1200a(6) =15840a(7) =211680a(8) =3467520a(9) =61689600a(10) =1241049600a(11) =27223257600a(12) =654316185600