a(n) = (Product_{d=1..n-1} (2^d-1)) mod (2^n-1).

Terms

a(0) =0a(1) =1a(2) =3a(3) =6a(4) =5a(5) =0a(6) =7a(7) =60a(8) =301a(9) =837a(10) =11a(11) =2835a(12) =13a(13) =11811a(14) =13454a(15) =2040a(16) =17a(17) =179361a(18) =19a(19) =639375a(20) =1082802a(21) =2818719a(22) =23a(23) =12878775a(24) =28142451a(25) =44845725a(26) =131853841a(27) =161290635a(28) =29a(29) =911545173