a(n) is the largest number k such that sigma(k) = 2^n or 0 if no such k exists.

Terms

a(0) =1a(1) =0a(2) =3a(3) =7a(4) =0a(5) =31a(6) =0a(7) =127a(8) =217a(9) =381a(10) =889a(11) =0a(12) =3937a(13) =8191a(14) =11811a(15) =27559a(16) =57337a(17) =131071a(18) =253921a(19) =524287a(20) =1040257a(21) =1777447a(22) =4063201a(23) =7281799a(24) =16646017a(25) =32247967a(26) =66584449a(27) =116522119a(28) =225735769a(29) =516026527