%I #15 Sep 08 2022 08:44:58
%S 2,3,5,11,17,23,31,41,47,53,59,83,89,101,107,109,131,137,149,157,167,
%T 173,179,191,223,227,229,233,251,257,263,269,277,283,293,307,311,317,
%U 347,353,359,383,389,397,401,419,431,433,439,443,457,461,467,479,499
%N Primes p such that x^21 = 2 has a solution mod p.
%C Complement of A059310 relative to A000040. - _Vincenzo Librandi_, Sep 14 2012
%H R. J. Mathar, <a href="/A049553/b049553.txt">Table of n, a(n) for n = 1..1000</a>
%H <a href="/index/Pri#smp">Index entries for related sequences</a>
%t ok[p_]:= Reduce[Mod[x^21 - 2, p] == 0, x, Integers] =!= False; Select[Prime[Range[150]], ok] (* _Vincenzo Librandi_, Sep 14 2012 *)
%o (Magma) [p: p in PrimesUpTo(500) | exists(t){x : x in ResidueClassRing(p) | x^21 eq 2}]; // _Vincenzo Librandi_, Sep 14 2012
%Y Cf. A000040, A059310.
%K nonn,easy
%O 1,1
%A _N. J. A. Sloane_
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