2^p - 7 where p is prime.

Terms

a(0) =-3a(1) =1a(2) =25a(3) =121a(4) =2041a(5) =8185a(6) =131065a(7) =524281a(8) =8388601a(9) =536870905a(10) =2147483641a(11) =137438953465