Coefficient of x^4 in expansion of (1+x+x^2)^n.

Terms

a(0) =1a(1) =6a(2) =19a(3) =45a(4) =90a(5) =161a(6) =266a(7) =414a(8) =615a(9) =880a(10) =1221a(11) =1651a(12) =2184a(13) =2835a(14) =3620a(15) =4556a(16) =5661a(17) =6954a(18) =8455a(19) =10185a(20) =12166a(21) =14421a(22) =16974a(23) =19850a(24) =23075a(25) =26676a(26) =30681a(27) =35119a(28) =40020a(29) =45415