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A340864 Numbers k such that both sigma_{-1}(k) > 2 and sigma_0(k)/sigma_{-1}(k) are integers. 1
672, 30240, 32760, 2178540, 23569920, 45532800, 142990848, 459818240, 1379454720, 14182439040, 43861478400, 51001180160, 66433720320, 153003540480, 403031236608, 704575228896, 13661860101120, 181742883469056, 6088728021160320, 14942123276641920, 20158185857531904 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..21.

Wikipedia, Multiply perfect number

Wikipedia, Harmonic divisor number.

EXAMPLE

a(1) = 672 is the smallest number k that is both an Ore number and multiperfect such that sigma(k)/k > 2.

MATHEMATICA

Module[{a166069 = {120, 672, 30240, 32760, 523776, 2178540, 23569920, 45532800, 142990848, 459818240, 1379454720}, i, n, result = {}}, For[i = 1, i <= Length[a166069], i++, n = a166069[[i]]; If[Mod[DivisorSigma[0, n], DivisorSigma[-1, n]] == 0, AppendTo[result, n]]]; result]

CROSSREFS

Intersection of A001599 and A166069.

Cf. A007691, A325025.

Sequence in context: A321116 A234481 A234476 * A331666 A245782 A047728

Adjacent sequences:  A340861 A340862 A340863 * A340865 A340866 A340867

KEYWORD

nonn

AUTHOR

David Terr, Jan 24 2021

EXTENSIONS

Name changed by and more terms from Jinyuan Wang, Feb 11 2021

STATUS

approved

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Last modified June 16 21:55 EDT 2021. Contains 345080 sequences. (Running on oeis4.)