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 A038894 Primes p such that 21 is not a square mod p. 1
 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 Sequence in context: A068807 A241732 A154812 * A207039 A095743 A106984 Adjacent sequences:  A038891 A038892 A038893 * A038895 A038896 A038897 KEYWORD nonn AUTHOR STATUS approved

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Last modified May 20 13:18 EDT 2019. Contains 323422 sequences. (Running on oeis4.)