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 A316351 #23 Sep 17 2024 04:03:43 %S A316351 47,73,83,123,133,157,173,177,183,187,191,203,213,217,233,237,242,253, %T A316351 255,265,273,278,293,302,307,313,317,319,327,333,337,343,353,377,387, %U A316351 395,401,403,411,413,421,423,437,438,467,473,477,483,487,489,497,499,507 %N A316351 Numbers k such that k^2 + 1 has exactly four distinct prime factors. %H A316351 Amiram Eldar, <a href="/A316351/b316351.txt">Table of n, a(n) for n = 1..10000</a> %e A316351 For k = 133, k^2 + 1 = 17690 = 2*5*29*61 which has 4 distinct prime factors, so 133 is a term. %e A316351 For k = 157, k^2 + 1 = 24650 = 2*5*5*17*29 which has 4 distinct prime factors, so 157 is a term. %t A316351 Select[Range@510, PrimeNu[#^2 + 1] == 4 &] (* _Robert G. Wilson v_, Jul 15 2018 *) %o A316351 (PARI) isok(n) = omega(n^2+1) == 4; \\ _Michel Marcus_, Jun 30 2018 %Y A316351 Cf. A001221, A002522, A033993. %K A316351 nonn %O A316351 1,1 %A A316351 _Gordon Elliot Michaels_, Jun 29 2018