Coefficient of x^n in the expansion of (1+x+x^3)^n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =4a(4) =13a(5) =31a(6) =76a(7) =211a(8) =589a(9) =1597a(10) =4351a(11) =12046a(12) =33496a(13) =93094a(14) =259351a(15) =725089a(16) =2031709a(17) =5701189a(18) =16023181a(19) =45104044a(20) =127134283a(21) =358764613a(22) =1013494318a(23) =2865933907a(24) =8111573416a(25) =22977551656a(26) =65138143006a(27) =184789086106a(28) =524571000799