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.

A073500 Numbers k such that sigma(k) = sigma(k+1)+sigma(k-1).

Original entry on oeis.org

954, 1300, 11142, 38214, 89484, 152154, 167784, 254136, 303280, 473316, 574716, 813920, 918080, 1307022, 2008360, 3676242, 9878244, 10368512, 12548754, 19411480, 22493262, 42413484, 77766486, 81513816, 157874000, 240828972
Offset: 1

Views

Author

Benoit Cloitre, Aug 27 2002

Keywords

Crossrefs

Cf. A000203 (sigma), A076528.

Programs

  • Mathematica
    s1 = s2 = s3 = 0; Do[ s1 = s2; s2 = s3; s3 = DivisorSigma[1, n]; If[ s2 == s1 + s3, Print[n - 1]], {n, 1, 10^8}]
  • PARI
    list(lim) = {my(s1 = 0, s2 = 0, s3); for(k = 1, lim, s3 = sigma(k); if(s2 == s1 + s3, print1(k-1, ", ")); s1 = s2; s2 = s3);} \\ Amiram Eldar, May 09 2025

Formula

a(n) = A076528(n)-1 - Alex Ratushnyak, Jul 06 2013.

Extensions

Edited and extended by Robert G. Wilson v, Oct 24 2002
a(25)-a(26) from Donovan Johnson, Feb 01 2009