a(n) = (5^n + 3)/2.

Terms

a(0) =2a(1) =4a(2) =14a(3) =64a(4) =314a(5) =1564a(6) =7814a(7) =39064a(8) =195314a(9) =976564a(10) =4882814a(11) =24414064a(12) =122070314a(13) =610351564a(14) =3051757814a(15) =15258789064a(16) =76293945314a(17) =381469726564