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

Terms

a(0) =1a(1) =1a(2) =7a(3) =1a(4) =31a(5) =1a(6) =127a(7) =1a(8) =511a(9) =1a(10) =2047a(11) =1a(12) =8191a(13) =1a(14) =32767a(15) =1a(16) =131071a(17) =1a(18) =524287a(19) =1a(20) =2097151a(21) =1a(22) =8388607a(23) =1a(24) =33554431a(25) =1a(26) =134217727a(27) =1a(28) =536870911a(29) =1