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

Terms

a(0) =4a(1) =8a(2) =22a(3) =74a(4) =274a(5) =1058a(6) =4162a(7) =16514a(8) =65794a(9) =262658a(10) =1049602a(11) =4196354a(12) =16781314a(13) =67117058a(14) =268451842a(15) =1073774594a(16) =4295032834a(17) =17180000258a(18) =68719738882a(19) =274878431234