a(n) = (5^n + 1)/2.

Terms

a(0) =1a(1) =3a(2) =13a(3) =63a(4) =313a(5) =1563a(6) =7813a(7) =39063a(8) =195313a(9) =976563a(10) =4882813a(11) =24414063a(12) =122070313a(13) =610351563a(14) =3051757813a(15) =15258789063a(16) =76293945313a(17) =381469726563