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 A070443 #27 Jan 09 2025 02:49:56
%S A070443 0,1,4,9,16,4,15,7,1,18,16,16,18,1,7,15,4,16,9,4,1,0,1,4,9,16,4,15,7,
%T A070443 1,18,16,16,18,1,7,15,4,16,9,4,1,0,1,4,9,16,4,15,7,1,18,16,16,18,1,7,
%U A070443 15,4,16,9,4,1,0,1,4,9,16,4,15,7,1,18,16,16,18,1,7,15,4,16,9,4,1,0,1,4,9
%N A070443 a(n) = n^2 mod 21.
%H A070443 G. C. Greubel, <a href="/A070443/b070443.txt">Table of n, a(n) for n = 0..1000</a>
%H A070443 Sathwik Karnik, <a href="https://arxiv.org/abs/1702.08066">On the classification and algorithmic analysis of Carmichael numbers</a>, arXiv:1702.08066 [math.NT], 2016. See Table 1.
%H A070443 <a href="/index/Rec#order_21">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1).
%F A070443 From _R. J. Mathar_, Jul 27 2015: (Start)
%F A070443 a(n) = a(n-21).
%F A070443 G.f.: -x *(1+x) *(x^18 +3*x^17 +6*x^16 +10*x^15 -6*x^14 +21*x^13 -14*x^12 +15*x^11 +3*x^10 +13*x^9 +3*x^8 +15*x^7 -14*x^6 +21*x^5 -6*x^4 +10*x^3 +6*x^2 +3*x+1) / ( (x-1) *(1+x^6+x^5+x^4+x^3+x^2+x) *(1+x+x^2) *(1-x+x^3-x^4+x^6-x^8+x^9-x^11+x^12) ). (End)
%t A070443 Table[Mod[n^2,21],{n,0,200}] (* _Vladimir Joseph Stephan Orlovsky_, Apr 23 2011 *)
%t A070443 PowerMod[Range[0,90],2,21] (* _Harvey P. Dale_, Jan 19 2013 *)
%o A070443 (PARI) a(n)=n^2%21 \\ _Charles R Greathouse IV_, Apr 06 2016
%K A070443 nonn,easy
%O A070443 0,3
%A A070443 _N. J. A. Sloane_, May 12 2002