a(n) = 2^(2*n+1) - 3*2^n - 1.

Terms

a(0) =1a(1) =19a(2) =103a(3) =463a(4) =1951a(5) =7999a(6) =32383a(7) =130303a(8) =522751a(9) =2094079a(10) =8382463a(11) =33542143a(12) =134193151a(13) =536821759a(14) =2147385343a(15) =8589737983a(16) =34359345151a(17) =137438167039a(18) =549754241023