Coefficient of x^7 in expansion of (1+x+x^2)^n.

Terms

a(0) =4a(1) =30a(2) =126a(3) =393a(4) =1016a(5) =2304a(6) =4740a(7) =9042a(8) =16236a(9) =27742a(10) =45474a(11) =71955a(12) =110448a(13) =165104a(14) =241128a(15) =344964a(16) =484500a(17) =669294a(18) =910822a(19) =1222749a(20) =1621224a(21) =2125200a(22) =2756780a(23) =3541590a(24) =4509180a(25) =5693454a(26) =7133130a(27) =8872231a(28) =10960608a(29) =13454496