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.
%I A186103 #24 Mar 17 2024 02:15:38 %S A186103 13,77,489,557,1101,1431,2409,8897,538209,2263024,8910721,13685781, %T A186103 17428321,962402769,128508838576 %N A186103 Numbers k such that k | sigma(k-1) + sigma(k+1). %C A186103 The calculations were produced by V. Astanoff and Richard Tobin. Tobin has found that there are no other solutions up to 2*10^9. %C A186103 a(16) > 10^13. - _Giovanni Resta_, Apr 04 2014 %H A186103 J. M. Bergot, et al., <a href="http://groups.google.com/group/sci.math/browse_thread/thread/ef1cbd9928d22851/93e6783834f39e56">How rare is 13?</a>, sci.math discussion %e A186103 Take consecutive numbers 8896, 8897, 8898 to find that sigma(8896)=17780 and sigma(8898)=17808, with sum 35588=4*8897. %o A186103 (PARI) sa=sigma(1);sb=sigma(2);for(n=2,1e10,sc=sigma(n+1);if((sa+sc)%n==0,print1(n", "));sa=sb;sb=sc) \\ _Charles R Greathouse IV_, Feb 13 2011 %K A186103 nonn,more %O A186103 1,1 %A A186103 _J. M. Bergot_, Feb 12 2011 %E A186103 a(15) from _Giovanni Resta_, Apr 04 2014