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 A019347 #16 Mar 22 2019 15:56:34
%S A019347 3,13,17,23,37,43,47,53,67,73,83,103,107,113,137,157,163,167,173,223,
%T A019347 227,233,257,263,277,283,293,313,317,337,347,353,367,383,397,433,443,
%U A019347 463,467,487,503,547,557,563,587,593,607,613,647,653,673,677,683,727,743,773,797
%N A019347 Primes with primitive root 20.
%H A019347 Vincenzo Librandi, <a href="/A019347/b019347.txt">Table of n, a(n) for n = 1..1000</a>
%H A019347 <a href="/index/Pri#primes_root">Index entries for primes by primitive root</a>
%t A019347 pr=20; Select[Prime[Range[200]], MultiplicativeOrder[pr, # ] == #-1 &]
%t A019347 Join[{3,13,17},Select[Prime[Range[200]],PrimitiveRoot[#,20]==20&]] (* _Harvey P. Dale_, Mar 22 2019 *)
%K A019347 nonn
%O A019347 1,1
%A A019347 _David W. Wilson_