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

Terms

a(0) =2a(1) =11a(2) =65a(3) =407a(4) =2657a(5) =17831a(6) =121745a(7) =839927a(8) =5830337a(9) =40615751a(10) =283523825a(11) =1981521047a(12) =13858064417a(13) =96956119271a(14) =678491508305