2^p - 3 where p is prime.

Terms

a(0) =1a(1) =5a(2) =29a(3) =125a(4) =2045a(5) =8189a(6) =131069a(7) =524285a(8) =8388605a(9) =536870909a(10) =2147483645a(11) =137438953469