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

Terms

a(0) =2a(1) =4a(2) =11a(3) =37a(4) =137a(5) =529a(6) =2081a(7) =8257a(8) =32897a(9) =131329a(10) =524801a(11) =2098177a(12) =8390657a(13) =33558529a(14) =134225921a(15) =536887297a(16) =2147516417a(17) =8590000129a(18) =34359869441a(19) =137439215617a(20) =549756338177