a(n) is the smallest 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) =21a(6) =0a(7) =93a(8) =217a(9) =381a(10) =651a(11) =0a(12) =2667a(13) =8191a(14) =11811a(15) =24573a(16) =57337a(17) =82677a(18) =172011a(19) =393213a(20) =761763a(21) =1572861a(22) =2752491a(23) =5332341a(24) =11010027a(25) =21845397a(26) =48758691a(27) =85327221a(28) =199753347a(29) =341310837