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A081756 Numbers n such that there is a proper divisor d of n satisfying sigma(d)=n. 4
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

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)

FORMULA

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

MAPLE

with(numtheory): P:= proc(q) local a, k, n; for n from 2 to q do a:=sort([op(divisors(n))]);

for k from 2 to nops(a)-1 do if n=sigma(a[k]) then print(n); break;

fi; od; od; end: P(10^15); # Paolo P. Lava, May 16 2017

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-Fran├žois Alcover, Oct 31 2019, after David Wasserman *)

CROSSREFS

Cf. A007691, A054030, A065125, A139256.

Sequence in context: A068418 A068414 A199316 * A307741 A027147 A095724

Adjacent sequences:  A081753 A081754 A081755 * A081757 A081758 A081759

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Apr 08 2003

EXTENSIONS

More terms from David Wasserman, Jun 28 2004

Description clarified by Ray Chandler, May 18 2017

STATUS

approved

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Last modified September 24 17:09 EDT 2020. Contains 337321 sequences. (Running on oeis4.)