2^(n-th semiprime) - 1.

Terms

a(0) =15a(1) =63a(2) =511a(3) =1023a(4) =16383a(5) =32767a(6) =2097151a(7) =4194303a(8) =33554431a(9) =67108863a(10) =8589934591a(11) =17179869183a(12) =34359738367a(13) =274877906943a(14) =549755813887