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 A338539 #15 Oct 12 2024 16:37:25
%S A338539 36,72,100,108,144,180,196,200,216,225,252,288,300,324,360,392,396,
%T A338539 400,432,441,450,468,484,500,504,540,576,588,600,612,648,675,676,684,
%U A338539 700,720,756,784,792,800,828,864,882,936,968,972,980,1000,1008,1044,1080,1089
%N A338539 Numbers having exactly two non-unitary prime factors.
%C A338539 Numbers k such that A056170(k) = A001221(A057521(k)) = 2.
%C A338539 Numbers divisible by the squares of exactly two distinct primes.
%C A338539 Subsequence of A036785 and first differs from it at n = 44.
%C A338539 The asymptotic density of this sequence is (3/Pi^2)*(eta_1^2 - eta_2) = 0.0532928864..., where eta_j = Sum_{p prime} 1/(p^2-1)^j (Pomerance and Schinzel, 2011).
%H A338539 Amiram Eldar, <a href="/A338539/b338539.txt">Table of n, a(n) for n = 1..10000</a>
%H A338539 Carl Pomerance and Andrzej Schinzel, <a href="http://mjcnt.phystech.edu/en/article.php?id=4">Multiplicative Properties of Sets of Residues</a>, Moscow Journal of Combinatorics and Number Theory, Vol. 1, No. 1 (2011), pp. 52-66. See pp. 61-62.
%e A338539 36 = 2^2 * 3^2 is a term since it has exactly 2 prime factors, 2 and 3, that are non-unitary.
%t A338539 Select[Range[1000], Count[FactorInteger[#][[;;,2]], _?(#1 > 1 &)] == 2 &]
%Y A338539 Subsequence of A013929 and A036785.
%Y A338539 Cf. A001221, A056170, A057521, A190641, A338540, A338541, A338542, A347892 (subsequence).
%Y A338539 Cf. A154945 (eta_1), A324833 (eta_2).
%K A338539 nonn
%O A338539 1,1
%A A338539 _Amiram Eldar_, Nov 01 2020