a(n) = [x^n] 1/(1 + x + x^2 + x^3)^n.

Terms

a(0) =1a(1) =-1a(2) =1a(3) =-1a(4) =5a(5) =-26a(6) =91a(7) =-246a(8) =597a(9) =-1540a(10) =4576a(11) =-14521a(12) =44915a(13) =-132328a(14) =380290a(15) =-1102076a(16) =3268437a(17) =-9838428a(18) =29616364a(19) =-88538500a(20) =263489380a(21) =-785026110a(22) =2348923875a(23) =-7053379710a(24) =21204016275a(25) =-63716916276a(26) =191394838116a(27) =-575200476046