a(n) = 1^n + 2^n + 8^n.

Terms

a(0) =3a(1) =11a(2) =69a(3) =521a(4) =4113a(5) =32801a(6) =262209a(7) =2097281a(8) =16777473a(9) =134218241a(10) =1073742849a(11) =8589936641a(12) =68719480833a(13) =549755822081