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%I #27 Aug 30 2019 21:51:13
%S 7,19,31,43,79,127,139,163,199,211,223,283,331,379,463,487,571,607,
%T 631,691,739,751,811,823,859,907,1039,1063,1087,1123,1231,1279,1291,
%U 1327,1423,1447,1459,1483,1567,1579,1627,1663,1699,1723,1747,1831,1951,1987,1999
%N Primes p such that 3 is a primitive root modulo p while 27 is not.
%C Primes p such that 3 is a primitive root modulo p (i.e., p is in A019334) and that p == 1 (mod 3).
%C According to Artin's conjecture, the number of terms <= N is roughly ((2/5)*C)*PrimePi(N), where C is the Artin's constant = A005596, PrimePi = A000720. Compare: the number of terms of A001122 that are no greater than N is roughly C*PrimePi(N).
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ArtinsConstant.html">Artin's constant</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Artin%27s_conjecture_on_primitive_roots">Artin's conjecture on primitive roots</a>
%o (PARI) forprime(p=5, 2000, if(znorder(Mod(3, p))==(p-1) && p%3==1, print1(p, ", ")))
%Y Complement of A019353 with respect to A019334.
%Y Cf. also A005596, A000720.
%Y Primes p such that 3 is a primitive root modulo p and that p == 1 (mod q): this sequence (q=3), A323617 (q=5), A323628 (q=7).
%K nonn
%O 1,1
%A _Jianing Song_, Aug 30 2019
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