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

%I A081756 #25 Feb 23 2024 07:26:11
%S A081756 1,12,56,360,992,2016,16256,120960,131040,1571328,8714160,67100672,
%T A081756 94279680,182131200,571963392,1379454720,4428914688,5517818880,
%U A081756 17179738112,70912195200,153003540480,159991977600,175445913600,265734881280,274877382656,612014161920
%N A081756 Numbers n such that there is a proper divisor d of n satisfying sigma(d)=n.
%C A081756 A139256 is a subsequence. - _Michel Marcus_, Dec 02 2013
%H A081756 Ray Chandler, <a href="/A081756/b081756.txt">Table of n, a(n) for n = 1..1000</a> (using comment in formula section from _David Wasserman_ and b-files for A007691 and A054030)
%F A081756 Multiply A007691 by A054030 and sort the resulting sequence. - _David Wasserman_, Jun 28 2004
%t A081756 kmax = 10^12;
%t A081756 A007691 = Cases[Import["https://oeis.org/A007691/b007691.txt", "Table"], {_, _}][[All, 2]];
%t A081756 A054030 = Cases[Import["https://oeis.org/A054030/b054030.txt", "Table"], {_, _}][[All, 2]];
%t A081756 okQ[n_] := AnyTrue[Most[Divisors[n]], DivisorSigma[1, #] == n&];
%t A081756 {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_ *)
%Y A081756 Cf. A007691, A054030, A065125, A139256.
%K A081756 nonn
%O A081756 1,2
%A A081756 _Benoit Cloitre_, Apr 08 2003
%E A081756 More terms from _David Wasserman_, Jun 28 2004
%E A081756 Description clarified by _Ray Chandler_, May 18 2017