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

Terms

a(0) =7a(1) =23a(2) =79a(3) =287a(4) =1087a(5) =4223a(6) =16639a(7) =66047a(8) =263167a(9) =1050623a(10) =4198399a(11) =16785407a(12) =67125247a(13) =268468223a(14) =1073807359a(15) =4295098367a(16) =17180131327a(17) =68720001023a(18) =274878955519