Expansion of e.g.f.: (1+x+x^2+x^3)^x.

Terms

a(0) =1a(1) =0a(2) =2a(3) =3a(4) =20a(5) =-30a(6) =594a(7) =-840a(8) =14384a(9) =-167832a(10) =2300040a(11) =-17190360a(12) =153272712a(13) =-2775904560a(14) =51294972624a(15) =-651268374120