(7*5^n+1)/4.

Terms

a(0) =2a(1) =9a(2) =44a(3) =219a(4) =1094a(5) =5469a(6) =27344a(7) =136719a(8) =683594a(9) =3417969a(10) =17089844a(11) =85449219a(12) =427246094a(13) =2136230469a(14) =10681152344a(15) =53405761719a(16) =267028808594