A038894 Primes p such that 21 is not a square mod p.

2, 11, 13, 19, 23, 29, 31, 53, 61, 71, 73, 97, 103, 107, 113, 137, 139, 149, 157, 179, 181, 191, 197, 199, 223, 229, 233, 239, 241, 263, 271, 281, 283, 307, 313, 317, 347, 349, 359, 367, 389, 397, 401, 409, 431, 433, 439, 443, 449, 491, 523, 557, 569, 577, 599, 601

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn

Comments

Primes p such that 1+5*x+x^2 is irreducible over GF(p). - Joerg Arndt, Jul 19 2012

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Programs

Mathematica

Select[Prime@Range[120], JacobiSymbol[21, #] == -1 &] (* Vincenzo Librandi, Sep 08 2012 *)

PARI

is(n)=!issquare(21,n) && isprime(n) \\ Charles R Greathouse IV, Nov 21 2014