A348867 -id:A348867 - cp's OEIS frontend

A348868 Numbers whose numerator and denominator of the harmonic mean of their divisors are both 5-smooth numbers.

1, 2, 3, 5, 6, 8, 10, 15, 24, 27, 28, 30, 40, 54, 84, 120, 135, 140, 216, 224, 270, 420, 496, 672, 756, 775, 819, 1080, 1120, 1488, 1550, 1638, 2176, 2325, 2480, 3360, 3780, 4095, 4650, 6048, 6200, 6528, 6552, 7440, 8190, 10880, 11375, 13392, 18600, 20925, 21700

Offset: 1

Amiram Eldar, Nov 02 2021

nonn

The terms that are also harmonic numbers (A001599) are those whose harmonic mean of divisors (A001600) is a 5-smooth number. Of the 937 harmonic numbers below 10^14, 83 are terms in this sequence.

If k1 and k2 are coprime terms, then k1*k2 is also a term. In particular, if k is an odd term, then 2*k is also a term.

			8 is a term since the harmonic mean of its divisors is 32/15 and both 32 = 2^5 and 15 = 3*5 are 5-smooth numbers.

Amiram Eldar, Table of n, a(n) for n = 1..234

Cf. A001599, A001600, A051037, A099377, A099378.
A348867 is a subsequence.
Similar sequences: A074266, A348658, A348659.

smQ[n_] := n == 2^IntegerExponent[n, 2] * 3^IntegerExponent[n, 3] * 5^IntegerExponent[n, 5]; h[n_] := DivisorSigma[0, n]/DivisorSigma[-1, n]; q[n_] := smQ[Numerator[(hn = h[n])]] && smQ[Denominator[hn]]; Select[Range[22000], q]

cp's OEIS Frontend

A348868 Numbers whose numerator and denominator of the harmonic mean of their divisors are both 5-smooth numbers.

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