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 A186103 n such that n | sigma(n-1)+sigma(n+1). 3
 13, 77, 489, 557, 1101, 1431, 2409, 8897, 538209, 2263024, 8910721, 13685781, 17428321, 962402769, 128508838576 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The calculations were produced by V. Astanoff and Richard Tobin.  Tobin has found that there are no other solutions up to 2*10^9. a(16) > 10^13. - Giovanni Resta, Apr 04 2014 LINKS J. M. Bergot, et al., How rare is 13?, sci.math discussion EXAMPLE Take consecutive numbers 8896,8897,8898 to find that sigma(8896)=17780 and sigma(8898)=17808, with sum 35588=4*8897. MAPLE with(numtheory): P:=proc(q) local n; for n from 2 to q do if frac((sigma(n-1)+sigma(n+1))/n)=0 then print(n); fi; od; end: P(10^9); # Paolo P. Lava, Nov 23 2017 PROG (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 CROSSREFS Sequence in context: A044581 A126423 A072474 * A194713 A047638 A010929 Adjacent sequences:  A186100 A186101 A186102 * A186104 A186105 A186106 KEYWORD nonn,more AUTHOR J. M. Bergot, Feb 12 2011 EXTENSIONS a(15) from Giovanni Resta, Apr 04 2014 STATUS approved

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Last modified October 22 14:44 EDT 2019. Contains 328318 sequences. (Running on oeis4.)