a(n) = 4^n + 8^n + 9^n.

Terms

a(0) =3a(1) =21a(2) =161a(3) =1305a(4) =10913a(5) =92841a(6) =797681a(7) =6896505a(8) =59889473a(9) =521900361a(10) =4561574801a(11) =39975188505a(12) =351165790433