a(n) = 4^(n+1) - 3.

Terms

a(0) =1a(1) =13a(2) =61a(3) =253a(4) =1021a(5) =4093a(6) =16381a(7) =65533a(8) =262141a(9) =1048573a(10) =4194301a(11) =16777213a(12) =67108861a(13) =268435453a(14) =1073741821a(15) =4294967293a(16) =17179869181a(17) =68719476733a(18) =274877906941