a(n) = 4^n + 2^n - 1.

Terms

a(0) =1a(1) =5a(2) =19a(3) =71a(4) =271a(5) =1055a(6) =4159a(7) =16511a(8) =65791a(9) =262655a(10) =1049599a(11) =4196351a(12) =16781311a(13) =67117055a(14) =268451839a(15) =1073774591a(16) =4295032831a(17) =17180000255a(18) =68719738879a(19) =274878431231