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

Terms

a(0) =0a(1) =-1a(2) =3a(3) =5a(4) =15a(5) =29a(6) =63a(7) =125a(8) =255a(9) =509a(10) =1023a(11) =2045a(12) =4095a(13) =8189a(14) =16383a(15) =32765a(16) =65535a(17) =131069a(18) =262143a(19) =524285a(20) =1048575a(21) =2097149a(22) =4194303a(23) =8388605a(24) =16777215a(25) =33554429a(26) =67108863a(27) =134217725a(28) =268435455a(29) =536870909