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