a(n) = 2^n - 3.

Terms

a(0) =-2a(1) =-1a(2) =1a(3) =5a(4) =13a(5) =29a(6) =61a(7) =125a(8) =253a(9) =509a(10) =1021a(11) =2045a(12) =4093a(13) =8189a(14) =16381a(15) =32765a(16) =65533a(17) =131069a(18) =262141a(19) =524285a(20) =1048573a(21) =2097149a(22) =4194301a(23) =8388605a(24) =16777213a(25) =33554429a(26) =67108861a(27) =134217725a(28) =268435453a(29) =536870909