2^(prime(n) - 2) - 1.

Terms

a(0) =0a(1) =1a(2) =7a(3) =31a(4) =511a(5) =2047a(6) =32767a(7) =131071a(8) =2097151a(9) =134217727a(10) =536870911a(11) =34359738367a(12) =549755813887