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 A042209 #18 Sep 06 2021 19:02:52
%S A042209 1,10,501,5020,251501,2520030,126253001,1265050040,63378755001,
%T A042209 635052600050,31816008757501,318795140175060,15971573017510501,
%U A042209 160034525315280070,8017697838781514001,80337012913130420080,4024868343495302518001,40329020447866155600090
%N A042209 Denominators of continued fraction convergents to sqrt(630).
%H A042209 Vincenzo Librandi, <a href="/A042209/b042209.txt">Table of n, a(n) for n = 0..200</a>
%H A042209 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (0,502,0,-1).
%F A042209 G.f.: -(x^2 - 10*x - 1) / (x^4 - 502*x^2 + 1). - _Colin Barker_, Dec 04 2013
%t A042209 Denominator[Convergents[Sqrt[630], 30]] (* _Vincenzo Librandi_, Jan 16 2014 *)
%t A042209 LinearRecurrence[{0,502,0,-1},{1,10,501,5020},20] (* _Harvey P. Dale_, Sep 06 2021 *)
%Y A042209 Cf. A042208, A040604.
%K A042209 nonn,frac,easy
%O A042209 0,2
%A A042209 _N. J. A. Sloane_
%E A042209 More terms from _Colin Barker_, Dec 04 2013