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 A074266 #7 Jun 30 2020 05:34:59
%S A074266 1,216,468,810,1550,1638,3744,10880,11340,13965,21700,23716,40176,
%T A074266 45847,50274,56896,80262,90720,97969,126360,128744,137940,139159,
%U A074266 161728,173600,189728,224450,319579,434511,482790,515450,526500,555660
%N A074266 Numbers k such that the harmonic mean of the divisors of k is the square of a rational number.
%H A074266 Amiram Eldar, <a href="/A074266/b074266.txt">Table of n, a(n) for n = 1..500</a>
%e A074266 The harmonic mean of the divisors of 468 is 324/49 = (18/7)^2, the square of a rational number, so 468 is a term of the sequence.
%t A074266 H[l_] := Module[{m, s}, m = Length[l]; s = 0; For[i = 1, i <= m, i++, s = s + (1/l[[i]])]; s = s/m; s = 1/s; s] r = {}; Do[d = Divisors[n]; h = H[d]; num = Numerator[h]; den = Denominator[h]; If[IntegerQ[num^(1/2)] && IntegerQ[den^(1/2)], r = Append[r, n]], {n, 1, 10^6}]; r
%Y A074266 Cf. A001599, A099377, A099378.
%K A074266 nonn
%O A074266 1,2
%A A074266 _Joseph L. Pe_, Sep 20 2002