a(n) = 4^n + 3.

Terms

a(0) =4a(1) =7a(2) =19a(3) =67a(4) =259a(5) =1027a(6) =4099a(7) =16387a(8) =65539a(9) =262147a(10) =1048579a(11) =4194307a(12) =16777219a(13) =67108867a(14) =268435459a(15) =1073741827a(16) =4294967299a(17) =17179869187a(18) =68719476739a(19) =274877906947