%I
%S 2,3,5,7,11,13,17,19,23,31,37,41,47,53,59,61,67,73,79,83,89,97,101,
%T 103,107,109,131,137,139,149,151,157,163,167,173,179,181,191,193,199,
%U 223,227,229,233,241,251,257,263,269,271,277,283,293,307,311,313,317
%N Primes p such that x^7 = 3 has a solution mod p.
%C Complement of A042969 relative to A000040.
%C Differs from A042966 first at index 98.  _R. J. Mathar_, Mar 13 2013
%H Vincenzo Librandi, <a href="/A216881/b216881.txt">Table of n, a(n) for n = 1..1000</a>
%t ok[p_] := Reduce[Mod[x^7  3, p] == 0, x, Integers] =!= False; Select[Prime[Range[150]], ok]
%o (MAGMA) [p: p in PrimesUpTo(500)  exists(t){x: x in ResidueClassRing(p)  x^7 eq 3}];
%K nonn,easy
%O 1,1
%A _Vincenzo Librandi_, Sep 19 2012
