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 A104021 #21 Feb 13 2021 01:11:49
%S A104021 4,9,16,18,25,36,48,49,50,64,75,81,98,100,121,144,147,162,169,180,192,
%T A104021 196,225,242,245,256,289,294,300,320,324,338,361,363,400,405,441,448,
%U A104021 450,484,507,529,567,576,578,588,605,625,648,676,700,720,722,726,729
%N A104021 Numbers that can be represented as k^2*j with 0 < j <= k and gcd(k, j) = 1.
%C A104021 Essentially the same as A102646. - _Georg Fischer_, Oct 07 2018
%t A104021 Take[ Union[ Flatten[ Table[ If[ GCD[k, j] == 1, k^2*j, {}], {k, 27}, {j, k - 1}]]], 55]
%Y A104021 Cf. A102448, A104020, A104023, A104026.
%K A104021 nonn
%O A104021 1,1
%A A104021 _Leroy Quet_ and _Robert G. Wilson v_, Feb 24 2005