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 A272078 #15 Mar 11 2020 11:56:37
%S A272078 13,17,21,23,27,31,33,37,53,55,63,67,72,75,77,81,87,89,91,97,98,103,
%T A272078 105,109,111,112,113,115,119,122,125,127,128,129,135,137,138,142,147,
%U A272078 148,149,151,153,155,161,162,163,167,172,174,179,185,189,192,197,200,208
%N A272078 Numbers k such that k^2 + 1 is product of 3 distinct primes.
%H A272078 Daniel Starodubtsev, <a href="/A272078/b272078.txt">Table of n, a(n) for n = 1..1000</a>
%e A272078 13 appears in the list because 13^2 + 1 = 170 = 2 * 5 * 17.
%e A272078 21 appears in the list because 21^2 + 1 = 442 = 2 * 13 * 17.
%t A272078 A272078 = {}; Do[ k = Last /@ FactorInteger[n^2 + 1]; If[k == {1, 1, 1}, AppendTo[A272078, n]], {n, 1000}]; A272078
%t A272078 Select[Range[1000], Last /@ FactorInteger[#^2 + 1] == {1, 1, 1} &]
%o A272078 (PARI) isok(k) = my(x=k^2+1); (omega(x)==3) && (bigomega(x)==3); \\ _Michel Marcus_, Mar 11 2020
%Y A272078 Cf. A007304, A046386, A176686, A176687.
%K A272078 nonn
%O A272078 1,1
%A A272078 _K. D. Bajpai_, Apr 19 2016