a(n) = A255281(2^n-1).

Terms

a(0) =1a(1) =7a(2) =31a(3) =127a(4) =511a(5) =2031a(6) =8043a(7) =31735a(8) =125063a(9) =492367a(10) =1937763a(11) =7624303a(12) =29995559a(13) =118000431a(14) =464192219a(15) =1826013415a(16) =7183010967a(17) =28255752751a(18) =111149170563a(19) =437224979743