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 A347222 Numbers k for which sigma(k)/k = 12/5. 0
 30, 140, 2480, 6200, 40640, 167751680, 42949345280, 687193456640, 11529215040699760640, 13292279957849158723273463079769210880, 957809713041180536473966890421518190654986607740846080, 65820182292848241686198767302293614551117361591934715588918640640 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This sequence will contain terms of the form 5*P, where P is a perfect number (A000396) not divisible by 5. Proof: sigma(5*P)/(5*P) = sigma(5)*sigma(P)/(5*P) = 6*(2*P)/(5*P) = 12/5. QED Terms ending in "30", "40", or "80" have this form. Example: a(n) = 5*A000396(n) for n = 1, 2, 3 and a(n) = 5*A000396(n-1) for n = 5..12. LINKS G. P. Michon, Multiperfect Numbers and Hemiperfect Numbers Walter Nissen, Abundancy: Some Resources (preliminary version 4) EXAMPLE 6200 is a term, since sigma(6200)/6200 = 14880/6200 = 12/5. MATHEMATICA Select[Range[5*10^8], DivisorSigma[1, #]/# == 12/5 &] Do[If[DivisorSigma[1, k]/k == 12/5, Print[k]], {k, 5*10^8}] CROSSREFS Cf. A000203, A000396. Subsequence of A005101 and A218407. Sequence in context: A064495 A267904 A218407 * A229450 A124958 A126417 Adjacent sequences: A347219 A347220 A347221 * A347223 A347224 A347225 KEYWORD nonn AUTHOR Timothy L. Tiffin, Aug 23 2021 EXTENSIONS a(9)-a(10) from Michel Marcus, Aug 24 2021 a(11)-a(12) from David A. Corneth, Aug 24 2021 STATUS approved

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Last modified March 30 19:10 EDT 2023. Contains 361622 sequences. (Running on oeis4.)