This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A216881 #10 Sep 08 2022 08:46:03
%S A216881 2,3,5,7,11,13,17,19,23,31,37,41,47,53,59,61,67,73,79,83,89,97,101,
%T A216881 103,107,109,131,137,139,149,151,157,163,167,173,179,181,191,193,199,
%U A216881 223,227,229,233,241,251,257,263,269,271,277,283,293,307,311,313,317
%N A216881 Primes p such that x^7 = 3 has a solution mod p.
%C A216881 Complement of A042969 relative to A000040.
%C A216881 Differs from A042966 first at index 98. - _R. J. Mathar_, Mar 13 2013
%H A216881 Vincenzo Librandi, <a href="/A216881/b216881.txt">Table of n, a(n) for n = 1..1000</a>
%t A216881 ok[p_] := Reduce[Mod[x^7 - 3, p] == 0, x, Integers] =!= False; Select[Prime[Range[150]], ok]
%o A216881 (Magma) [p: p in PrimesUpTo(500) | exists(t){x: x in ResidueClassRing(p) | x^7 eq 3}];
%K A216881 nonn,easy
%O A216881 1,1
%A A216881 _Vincenzo Librandi_, Sep 19 2012