2^p - 5 where p is prime.

Terms

a(0) =-1a(1) =3a(2) =27a(3) =123a(4) =2043a(5) =8187a(6) =131067a(7) =524283a(8) =8388603a(9) =536870907a(10) =2147483643a(11) =137438953467