%I #9 Sep 08 2022 08:44:53
%S 11,13,19,23,29,31,37,41,53,61,71,73,89,97,101,103,107,113,137,139,
%T 149,157,173,179,181,191,197,199,223,229,233,239,241,257,263,271,277,
%U 281,283,293,307,313,317,347,349
%N Primes p such that x^4 = 21 has no solution mod p.
%C Complement of A040149 relative to A000040. - _Vincenzo Librandi_, Sep 18 2012
%H Vincenzo Librandi, <a href="/A040150/b040150.txt">Table of n, a(n) for n = 1..1000</a>
%t ok[p_]:= Reduce[Mod[x^4 - 21, p] == 0, x, Integers] == False; Select[Prime[Range[100]], ok] (* _Vincenzo Librandi_, Sep 18 2012 *)
%o (Magma) [p: p in PrimesUpTo(500) | not exists{x : x in ResidueClassRing(p) | x^4 eq 21} ]; // _Vincenzo Librandi_, Sep 18 2012
%K nonn,easy
%O 1,1
%A _N. J. A. Sloane_.
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