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

Terms

a(0) =3a(1) =12a(2) =66a(3) =408a(4) =2658a(5) =17832a(6) =121746a(7) =839928a(8) =5830338a(9) =40615752a(10) =283523826a(11) =1981521048a(12) =13858064418a(13) =96956119272a(14) =678491508306