A038894 - OEIS

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

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

MATHEMATICA

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

PROG

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

CROSSREFS