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 A266258 #14 Feb 16 2025 08:33:28
%S A266258 0,2,3,2,7,2,11,2,15,2,19,2,23,2,27,2,31,2,35,2,39,2,43,2,47,2,51,2,
%T A266258 55,2,59,2,63,2,67,2,71,2,75,2,79,2,83,2,87,2,91,2,95,2,99,2,103,2,
%U A266258 107,2,111,2,115,2,119,2,123,2,127,2,131,2,135,2,139,2
%N A266258 Number of OFF (white) cells in the n-th iteration of the "Rule 11" elementary cellular automaton starting with a single ON (black) cell.
%D A266258 S. Wolfram, A New Kind of Science, Wolfram Media, 2002; p. 55.
%H A266258 Robert Price, <a href="/A266258/b266258.txt">Table of n, a(n) for n = 0..999</a>
%H A266258 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ElementaryCellularAutomaton.html">Elementary Cellular Automaton</a>
%H A266258 <a href="/index/Ce#cell">Index entries for sequences related to cellular automata</a>
%H A266258 <a href="https://oeis.org/wiki/Index_to_Elementary_Cellular_Automata">Index to Elementary Cellular Automata</a>
%F A266258 Conjectures from _Colin Barker_, Dec 27 2015 and Apr 14 2019: (Start)
%F A266258 a(n) = (2*(-1)^n*n+2*n-3*(-1)^n+1)/2 for n>0
%F A266258 a(n) = 2*a(n-2)-a(n-4) for n>4.
%F A266258 G.f.: x*(2+3*x-2*x^2+x^3) / ((1-x)^2*(1+x)^2).
%F A266258 (End)
%t A266258 rule=11; rows=20; ca=CellularAutomaton[rule,{{1},0},rows-1,{All,All}]; (* Start with single black cell *) catri=Table[Take[ca[[k]],{rows-k+1,rows+k-1}],{k,1,rows}]; (* Truncated list of each row *) nbc=Table[Total[catri[[k]]],{k,1,rows}]; (* Number of Black cells in stage n *) Table[Length[catri[[k]]]-nbc[[k]],{k,1,rows}] (* Number of White cells in stage n *)
%Y A266258 Cf. A266253.
%K A266258 nonn,easy
%O A266258 0,2
%A A266258 _Robert Price_, Dec 25 2015
%E A266258 Conjectures from _Colin Barker_, Apr 14 2019