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%I #18 Sep 24 2018 02:05:15
%S 47,73,83,123,133,157,173,177,183,187,191,203,213,217,233,237,242,253,
%T 255,265,273,278,293,302,307,313,317,319,327,333,337,343,353,377,387,
%U 395,401,403,411,413,421,423,437,438,467,473,477,483,487,489,497,499,507
%N Numbers k such that k^2 + 1 has exactly four distinct prime factors.
%e For k = 133, k^2 + 1 = 17690 = 2*5*29*61 which has 4 distinct prime factors, so 133 is a term.
%e For k = 157, k^2 + 1 = 24650 = 2*5*5*17*29 which has 4 distinct prime factors, so 157 is a term.
%t Select[Range@510, PrimeNu[#^2 + 1] == 4 &] (* _Robert G. Wilson v_, Jul 15 2018 *)
%o (PARI) isok(n) = omega(n^2+1) == 4; \\ _Michel Marcus_, Jun 30 2018
%Y Cf. A001221, A002522, A033993.
%K nonn
%O 1,1
%A _Gordon Elliot Michaels_, Jun 29 2018
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