This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A348412 #19 Oct 31 2021 15:59:57 %S A348412 2,6,12,30,56,84,168,270,280,540,616,840,992,1092,1344,2856,2976,3276, %T A348412 3780,4590,5320,5940,7560,12400,12420,14880,16256,16380,18848,24360, %U A348412 26784,36036,37200,37240,41664,48768,49140,55692,60480,65520,86304,86800,111720,128520 %N A348412 Numbers whose even divisors have an integer harmonic mean. %C A348412 The corresponding harmonic means are 2, 3, 4, 5, 6, 7, 9, 9, 10, 12, 11, 15, 10, 13, 16, 17, 15, ... %C A348412 Equivalently, even numbers k such that the harmonic mean of the divisors of k/2 is either an integer (A001599) or a half-integer (A348411). %H A348412 Amiram Eldar, <a href="/A348412/b348412.txt">Table of n, a(n) for n = 1..569</a> %e A348412 6 is a term since its even divisors are 2 and 6, and their harmonic mean, 1/((1/2 + 1/6)/2) = 3, is an integer. %t A348412 Select[Range[2, 10^5, 2], IntegerQ[HarmonicMean[Select[Divisors[#], EvenQ]]] &] %o A348412 (Python) %o A348412 from sympy import gcd, divisor_sigma %o A348412 A348412_list = [2*n for n in range(1,10**3) if (lambda x, y: 2*gcd(x,y*n)>=x)(divisor_sigma(n),divisor_sigma(n,0))] # _Chai Wah Wu_, Oct 20 2021 %o A348412 (PARI) isok(m) = if (! (m%2), my(d=select(x->!(x%2), divisors(m))); denominator(#d/sum(k=1, #d, 1/d[k])) == 1); \\ _Michel Marcus_, Oct 31 2021 %Y A348412 Cf. A001599, A074400, A348411. %Y A348412 A139256 is a subsequence. %K A348412 nonn %O A348412 1,1 %A A348412 _Amiram Eldar_, Oct 17 2021