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 A174969 #19 Apr 03 2023 10:36:12 %S A174969 21,57,91,111,133,183,273,343,381,507,553,651,703,813,871,931,993, %T A174969 1057,1191,1261,1333,1407,1561,1641,1807,1893,1981,2071,2163,2257, %U A174969 2353,2451,2653,2757,2863,3081,3193,3423,3661,3783,4033,4161,4291,4557,4693,4971 %N A174969 Composites of form n^2 + n + 1. %D A174969 L. Poletti, Le serie dei numeri primi appartenente alle due forme quadratiche (A) n^2+n+1 e (B) n^2+n-1 per l'intervallo compreso entro 121 milioni, e cioè per tutti i valori di n fino a 11000, Atti della Reale Accademia Nazionale dei Lincei, Memorie della Classe di Scienze Fisiche, Matematiche e Naturali, s. 6, v. 3 (1929), pages 193-218. %H A174969 Michel Marcus, <a href="/A174969/b174969.txt">Table of n, a(n) for n = 1..10000</a> %H A174969 C. K. Caldwell, <a href="https://t5k.org/glossary/page.php?sort=Composite">Composite Numbers</a> %e A174969 n=1 gives 1^2+1+1=3, which is prime and so not a term, and similarly for n=2,3; n=4 gives 21=3*7, which is a(1). %p A174969 with(numtheory):for n from 0 to 200 do:x:=n^2+n+1: if type(x,prime)=false then print (x):else fi:od: %t A174969 Select[Array[ #^2+#+1&,6!,2],!PrimeQ[ # ]&] (* _Vladimir Joseph Stephan Orlovsky_, Apr 07 2010 *) %o A174969 (PARI) isok(k) = (k>1) && !isprime(k) && issquare(4*k-3); \\ _Michel Marcus_, Apr 20 2022 %Y A174969 Cf. A174967, A002384, A002383, A110284. %K A174969 nonn %O A174969 1,1 %A A174969 _Michel Lagneau_, Apr 02 2010 %E A174969 Example edited and keyword uned removed by _D. S. McNeil_, Nov 17 2010