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

Terms

a(0) =1a(1) =7a(2) =31a(3) =127a(4) =511a(5) =2047a(6) =8191a(7) =32767a(8) =131071a(9) =524287a(10) =2097151a(11) =8388607a(12) =33554431a(13) =134217727a(14) =536870911a(15) =2147483647a(16) =8589934591a(17) =34359738367a(18) =137438953471a(19) =549755813887