a(n) is the least k such that the denominator(sigma(sigma(k))/k) equals n.

Terms

a(0) =1a(1) =30a(2) =3a(3) =32a(4) =5a(5) =864a(6) =7a(7) =128a(8) =9a(9) =10a(10) =11a(11) =96a(12) =13a(13) =3570a(14) =45a(15) =256a(16) =17a(17) =216a(18) =19a(19) =40a(20) =147a(21) =22a(22) =23a(23) =6912a(24) =25a(25) =468a(26) =567a(27) =224a(28) =29a(29) =2520