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 A004042 #29 Feb 16 2025 08:32:28
%S A004042 0,142857,5882352941176470,526315789473684210,4347826086956521739130,
%T A004042 3448275862068965517241379310,
%U A004042 2127659574468085106382978723404255319148936170,1694915254237288135593220338983050847457627118644067796610
%N A004042 Periods of reciprocals of A006883, starting with first nonzero digit.
%C A004042 A variant of A180340.
%H A004042 Ray Chandler, <a href="/A004042/b004042.txt">Table of n, a(n) for n = 1..61</a>
%H A004042 C. A. Van Cott, <a href="https://doi.org/10.1080/10724117.2020.1809284">The Integer Hokey Pokey</a>, Math Horizons, Vol. 28, pp. 24-27, November 2020.
%H A004042 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/FullReptendPrime.html">Full Reptend Prime</a>
%H A004042 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/CyclicNumber.html">Cyclic Number</a>
%t A004042 f[n_]:=Block[{q},q=Last[First[RealDigits[1/n]]];If[IntegerQ[q],q={}];FromDigits[q]]; q=0;lst={};Do[If[StringLength[ToString[f[n]]]>q,AppendTo[lst,f[n]];q=StringLength[ToString[f[n]]]],{n,5!}];lst (* _Vladimir Joseph Stephan Orlovsky_, May 21 2009 *)
%Y A004042 Cf. A180340.
%K A004042 nonn,base
%O A004042 1,2
%A A004042 _N. J. A. Sloane_
%E A004042 Definition corrected by _Max Alekseyev_, Feb 12 2012