Constant term in the reduction of the polynomial 1+x+x^2+...+x^n by x^3->x^2+2x+1. See Comments.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =6a(6) =12a(7) =25a(8) =53a(9) =113a(10) =242a(11) =519a(12) =1114a(13) =2392a(14) =5137a(15) =11033a(16) =23697a(17) =50898a(18) =109323a(19) =234814a(20) =504356a(21) =1083305a(22) =2326829a(23) =4997793a(24) =10734754a(25) =23057167a(26) =49524466a(27) =106373552a(28) =228479649a(29) =490751217