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 A161504 #14 Jul 26 2020 13:41:41
%S A161504 2,11,19,23,31,41,43,53,61,73,83,103,107,127,137,149,157,167,179,191,
%T A161504 199,211,229,233,241,251,263,271,283,293,313,317,337,347,359,367,379,
%U A161504 389,397,401,409,419,421,431,439,443,461,463,503,523,547,557,569,577,587
%N A161504 Primes congruent to {1, 2, 10, 11, 19, 20} mod 21.
%C A161504 The cyclotomic polynomial Phi(21p,x) is flat only for p in this sequence.
%H A161504 Harvey P. Dale, <a href="/A161504/b161504.txt">Table of n, a(n) for n = 1..1001</a> [a(1)=2 inserted by _Georg Fischer_, Jul 26 2020]
%F A161504 Union of A124826, A141888, A141892, A141893, A141897, A141898.
%t A161504 Flatten[Table[Select[21*n+{-10,-2,-1,1,2,10}, PrimeQ], {n,50}]]
%t A161504 Select[Prime[Range[2,250]],MemberQ[{1,2,10,11,19,20},Mod[#,21]]&] (* _Harvey P. Dale_, May 03 2019 *)
%Y A161504 Cf. A117223, A124826, A141888, A141892, A141893, A141897, A141898, A160495.
%K A161504 nonn
%O A161504 1,1
%A A161504 _T. D. Noe_, Jun 17 2009
%E A161504 a(1)=2 inserted by _Georg Fischer_, Jul 26 2020