a(n) = (16^n-1)/3.

Terms

a(0) =0a(1) =5a(2) =85a(3) =1365a(4) =21845a(5) =349525a(6) =5592405a(7) =89478485a(8) =1431655765a(9) =22906492245a(10) =366503875925