A340864 Numbers k such that both sigma_{-1}(k) > 2 and sigma_0(k)/sigma_{-1}(k) are integers.

672, 30240, 32760, 2178540, 23569920, 45532800, 142990848, 459818240, 1379454720, 14182439040, 43861478400, 51001180160, 66433720320, 153003540480, 403031236608, 704575228896, 13661860101120, 181742883469056, 6088728021160320, 14942123276641920, 20158185857531904

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