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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =5a(6) =9a(7) =16a(8) =29a(9) =53a(10) =97a(11) =178a(12) =327a(13) =601a(14) =1105a(15) =2032a(16) =3737a(17) =6873a(18) =12641a(19) =23250a(20) =42763a(21) =78653a(22) =144665a(23) =266080a(24) =489397a(25) =900141a(26) =1655617a(27) =3045154a(28) =5600911a(29) =10301681