a(n) = 8^n - 4^n + 1.

Terms

a(0) =1a(1) =5a(2) =49a(3) =449a(4) =3841a(5) =31745a(6) =258049a(7) =2080769a(8) =16711681a(9) =133955585a(10) =1072693249a(11) =8585740289a(12) =68702699521a(13) =549688705025