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

Terms

a(0) =0a(1) =1a(2) =-3a(3) =7a(4) =-15a(5) =31a(6) =-63a(7) =127a(8) =-255a(9) =511a(10) =-1023a(11) =2047a(12) =-4095a(13) =8191a(14) =-16383a(15) =32767a(16) =-65535a(17) =131071a(18) =-262143a(19) =524287a(20) =-1048575a(21) =2097151a(22) =-4194303a(23) =8388607a(24) =-16777215a(25) =33554431a(26) =-67108863a(27) =134217727a(28) =-268435455a(29) =536870911