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

Terms

a(0) =2a(1) =1a(2) =11a(3) =7a(4) =47a(5) =31a(6) =191a(7) =127a(8) =767a(9) =511a(10) =3071a(11) =2047a(12) =12287a(13) =8191a(14) =49151a(15) =32767a(16) =196607a(17) =131071a(18) =786431a(19) =524287a(20) =3145727a(21) =2097151a(22) =12582911a(23) =8388607a(24) =50331647a(25) =33554431a(26) =201326591a(27) =134217727a(28) =805306367a(29) =536870911