a(n) = (4^n + 11)/3.

Terms

a(0) =4a(1) =5a(2) =9a(3) =25a(4) =89a(5) =345a(6) =1369a(7) =5465a(8) =21849a(9) =87385a(10) =349529a(11) =1398105a(12) =5592409a(13) =22369625a(14) =89478489a(15) =357913945a(16) =1431655769a(17) =5726623065a(18) =22906492249a(19) =91625968985a(20) =366503875929