2^(n-1) - (prime(n) mod n).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =15a(5) =31a(6) =61a(7) =125a(8) =251a(9) =503a(10) =1015a(11) =2047a(12) =4094a(13) =8191a(14) =16382a(15) =32763a(16) =65528a(17) =131065a(18) =262134a(19) =524277a(20) =1048566a(21) =2097139a(22) =4194290a(23) =8388591a(24) =16777194a(25) =33554409a(26) =67108842a(27) =134217705a(28) =268435434a(29) =536870889