a(n) = 4^n - 2*n - 1.

Terms

a(0) =1a(1) =11a(2) =57a(3) =247a(4) =1013a(5) =4083a(6) =16369a(7) =65519a(8) =262125a(9) =1048555a(10) =4194281a(11) =16777191a(12) =67108837a(13) =268435427a(14) =1073741793a(15) =4294967263a(16) =17179869149a(17) =68719476699a(18) =274877906905