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 A038894 #13 Jul 08 2025 22:27:17 %S A038894 2,11,13,19,23,29,31,53,61,71,73,97,103,107,113,137,139,149,157,179, %T A038894 181,191,197,199,223,229,233,239,241,263,271,281,283,307,313,317,347, %U A038894 349,359,367,389,397,401,409,431,433,439,443,449,491,523,557,569,577,599,601 %N A038894 Primes p such that 21 is not a square mod p. %C A038894 Primes p such that 1+5*x+x^2 is irreducible over GF(p). - _Joerg Arndt_, Jul 19 2012 %H A038894 Vincenzo Librandi, <a href="/A038894/b038894.txt">Table of n, a(n) for n = 1..1000</a> %t A038894 Select[Prime@Range[120], JacobiSymbol[21, #] == -1 &] (* _Vincenzo Librandi_, Sep 08 2012 *) %o A038894 (PARI) is(n)=!issquare(21,n) && isprime(n) \\ _Charles R Greathouse IV_, Nov 21 2014 %K A038894 nonn %O A038894 1,1 %A A038894 _N. J. A. Sloane_