login
Primes p such that 21 is not a square mod p.
2

%I #11 Nov 21 2014 16:17:56

%S 2,11,13,19,23,29,31,53,61,71,73,97,103,107,113,137,139,149,157,179,

%T 181,191,197,199,223,229,233,239,241,263,271,281,283,307,313,317,347,

%U 349,359,367,389,397,401,409,431,433,439,443,449,491,523,557,569,577,599,601

%N Primes p such that 21 is not a square mod p.

%C Primes p such that 1+5*x+x^2 is irreducible over GF(p). - _Joerg Arndt_, Jul 19 2012

%H Vincenzo Librandi, <a href="/A038894/b038894.txt">Table of n, a(n) for n = 1..1000</a>

%t Select[Prime@Range[120], JacobiSymbol[21, #] == -1 &] (* _Vincenzo Librandi_, Sep 08 2012 *)

%o (PARI) is(n)=!issquare(21,n) && isprime(n) \\ _Charles R Greathouse IV_, Nov 21 2014

%K nonn

%O 1,1

%A _N. J. A. Sloane_.