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

Terms

a(0) =1a(1) =3a(2) =20a(3) =204a(4) =2784a(5) =47520a(6) =973440a(7) =23264640a(8) =635443200a(9) =19525847040a(10) =666654105600