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%I #21 Feb 24 2024 01:04:54
%S 1,1919,2759,11219,27089,50039,58463,100127,113831,115289,120203,
%T 131879,148511,233729,244319,308039,461099,554063,596447,1406303,
%U 1486619,2285519,2880989,5138783,5369111,5521619,5736743,6621383,7496279,7683191,8571527,8848619
%N Numbers k such that sigma(k+1) = 3 * sigma(k).
%H Donovan Johnson, <a href="/A077087/b077087.txt">Table of n, a(n) for n = 1..500</a>
%e k = 1: sigma(2)/sigma(1) = 3/1 = 3;
%e k = 9563231: sigma(k+1)/sigma(k) = 31026240/10342080 = 3.
%t Do[s=Mod[a=DivisorSigma[1, n+1], b=DivisorSigma[1, n]]; If[Equal[s, 0]&&Equal[a/b, 3], Print[n]], {n, 1, 10000000}]
%t Position[Partition[DivisorSigma[1,Range[9*10^6]],2,1],_?(3#[[1]] == #[[2]]&),1,Heads->False]//Flatten (* _Harvey P. Dale_, Oct 03 2017 *)
%Y Cf. A000203, A002961, A067081, A077086, A272027 (3*sigma(n)).
%K nonn
%O 1,2
%A _Labos Elemer_, Oct 31 2002
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