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 A151731 #15 May 30 2025 11:00:46 %S A151731 0,2,6,14,20,32,40,54,70,94,108,128,152,172,188,224,256,300,344,380, %T A151731 416,464,504,552,598,658,728,772,816,880,940,1000,1076,1148,1212,1276, %U A151731 1360,1454,1556,1624,1708,1796,1912,2004,2124,2250,2376,2480 %N A151731 Number of ON states after n generations of cellular automaton based on square grid with each cell adjacent to its eight neighbors. %C A151731 A cell is turned ON if exactly two of its eight neighbors is ON. An ON cell remains ON forever. %C A151731 We start with two edge-adjacent ON cells. %H A151731 Rémy Sigrist, <a href="/A151731/b151731.txt">Table of n, a(n) for n = 0..8192</a> %H A151731 Rémy Sigrist, <a href="/A151731/a151731.gp.txt">PARI program for A151731</a> %H A151731 Rémy Sigrist, <a href="/A151731/a151731.png">Illustration of the structure at stage 512</a> %o A151731 (PARI) \\ See Links section. %Y A151731 See A151725, A151726 for the same CA except that exactly one neighbor must be ON for a cell to turn ON. %Y A151731 Cf. A151732, A151733, A151734. %K A151731 nonn %O A151731 0,2 %A A151731 _N. J. A. Sloane_, Jun 15 2009