a(n) = 2^n + 2*(-1)^n - 1.

Terms

a(0) =2a(1) =-1a(2) =5a(3) =5a(4) =17a(5) =29a(6) =65a(7) =125a(8) =257a(9) =509a(10) =1025a(11) =2045a(12) =4097a(13) =8189a(14) =16385a(15) =32765a(16) =65537a(17) =131069a(18) =262145a(19) =524285a(20) =1048577a(21) =2097149a(22) =4194305a(23) =8388605a(24) =16777217a(25) =33554429a(26) =67108865a(27) =134217725a(28) =268435457a(29) =536870909