(5^n+(-1)^n)/2.

Terms

a(0) =1a(1) =2a(2) =13a(3) =62a(4) =313a(5) =1562a(6) =7813a(7) =39062a(8) =195313a(9) =976562a(10) =4882813a(11) =24414062a(12) =122070313a(13) =610351562a(14) =3051757813a(15) =15258789062a(16) =76293945313a(17) =381469726562