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

Terms

a(0) =1a(1) =2a(2) =7a(3) =29a(4) =121a(5) =497a(6) =2017a(7) =8129a(8) =32641a(9) =130817a(10) =523777a(11) =2096129a(12) =8386561a(13) =33550337a(14) =134209537a(15) =536854529a(16) =2147450881a(17) =8589869057a(18) =34359607297a(19) =137438691329a(20) =549755289601