Coefficient of x^6 in expansion of (1+x+x^2)^n.

Terms

a(0) =1a(1) =10a(2) =45a(3) =141a(4) =357a(5) =784a(6) =1554a(7) =2850a(8) =4917a(9) =8074a(10) =12727a(11) =19383a(12) =28665a(13) =41328a(14) =58276a(15) =80580a(16) =109497a(17) =146490a(18) =193249a(19) =251713a(20) =324093a(21) =412896a(22) =520950a(23) =651430a(24) =807885a(25) =994266a(26) =1214955a(27) =1474795a(28) =1779121a(29) =2133792