cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A291881 Numbers n such that sigma(sigma(n)) = sigma(sigma(n)-n) + sigma(n); that is, f(g(n)) = g(f(n)) where f = A000203 and g = A001065.

Original entry on oeis.org

2, 38040, 51888, 236644, 260880, 3097024, 5283852, 5667312, 11777472, 46120848, 52981252, 69128640, 121352208, 330364848, 485906400, 662736600, 769422720, 1111869360, 1267978320, 1272335760, 1426817904, 1807128528, 2107406448, 2381691312, 2452404544, 2691587568
Offset: 1

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Author

Altug Alkan, Sep 05 2017

Keywords

Comments

Initial motivation for this sequence was that question: Can be an odd number k such that f(g(k)) = g(f(k)) where f = A000203 and g = A001065?
Non-abundant terms are 2, 236644, 52981252,...
If an odd term exists, it is larger than 2*10^11. - Giovanni Resta, Sep 15 2017

Examples

			38040 is a term because sigma(38040) = 114480 and sigma(114480) = sigma(76440) + 114480.
		

Crossrefs

Programs

  • Mathematica
    inQ[n_] :=  DivisorSigma[1, DivisorSigma[1, n]] == DivisorSigma[1, DivisorSigma[1, n] - n] + DivisorSigma[1, n]; (* Robert G. Wilson v, Sep 10 2017 *)
  • PARI
    a001065(n) = sigma(n)-n;
    isok(n) = sigma(a001065(n))==a001065(sigma(n));