A081756 Numbers n such that there is a proper divisor d of n satisfying sigma(d)=n.

1, 12, 56, 360, 992, 2016, 16256, 120960, 131040, 1571328, 8714160, 67100672, 94279680, 182131200, 571963392, 1379454720, 4428914688, 5517818880, 17179738112, 70912195200, 153003540480, 159991977600, 175445913600, 265734881280, 274877382656, 612014161920

Offset: 1

Author

Benoit Cloitre, Apr 08 2003

Keywords

• nonn

Comments

A139256 is a subsequence. - Michel Marcus, Dec 02 2013

Links

• Ray Chandler, Table of n, a(n) for n = 1..1000 (using comment in formula section from _David Wasserman_ and b-files for A007691 and A054030)

Crossrefs

Cf. A007691, A054030, A065125, A139256.

Programs

Mathematica

kmax = 10^12;
A007691 = Cases[Import["https://oeis.org/A007691/b007691.txt", "Table"], {, }][[All, 2]];
A054030 = Cases[Import["https://oeis.org/A054030/b054030.txt", "Table"], {, }][[All, 2]];
okQ[n_] := AnyTrue[Most[Divisors[n]], DivisorSigma[1, #] == n&];
{1}~Join~Reap[Do[k = A007691[[i]]*A054030[[j]]; If[k <= kmax, Sow[k]], {i, Length[A007691]}, {j, Length[A054030]}]][[2, 1]] // Union // Select[#, okQ]& (* Jean-François Alcover, Oct 31 2019, after David Wasserman *)

Formula

Multiply A007691 by A054030 and sort the resulting sequence. - David Wasserman, Jun 28 2004

Extensions

More terms from David Wasserman, Jun 28 2004

Description clarified by Ray Chandler, May 18 2017