a(n) = 7^n-4^n+1^n.

Terms

a(0) =1a(1) =4a(2) =34a(3) =280a(4) =2146a(5) =15784a(6) =113554a(7) =807160a(8) =5699266a(9) =40091464a(10) =281426674a(11) =1973132440a(12) =13824509986a(13) =96821901544a(14) =677954637394