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 A242121 #19 Dec 12 2024 09:29:44
%S A242121 2,3,5,7,11,13,17,19,2,8,10,16,20,1,5,11,17,19,4,8,10,16,20,5,13,17,
%T A242121 19,2,4,8,1,5,11,13,2,4,10,16,20,5,11,13,2,4,8,10,1,13,17,19,2,8,10,
%U A242121 20,5,11,17,19,4,8,10,20,13,17,19,2,16,1,11,13,17,2,10
%N A242121 a(n) = n-th prime modulo 21.
%H A242121 Vincenzo Librandi, <a href="/A242121/b242121.txt">Table of n, a(n) for n = 1..1000</a>
%F A242121 Sum_k={1..n} a(k) ~ (21/2)*n. - _Amiram Eldar_, Dec 12 2024
%t A242121 Mod[Prime[Range[100]], 21]
%o A242121 (Magma) [p mod(21): p in PrimesUpTo(500)];
%o A242121 (PARI) a(n) = prime(n) % 21; \\ _Michel Marcus_, May 05 2014
%o A242121 (Sage) [mod(p, 21) for p in primes(500)] # _Bruno Berselli_, May 05 2014
%Y A242121 Cf. similar sequences listed in A242119.
%K A242121 nonn,easy
%O A242121 1,1
%A A242121 _Vincenzo Librandi_, May 05 2014