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 A211842 #7 May 04 2021 15:45:22 %S A211842 1,5,19,63,196,588,1731,5049,14689,42809,125364,369678,1099339, %T A211842 3300091,10006487,30658799,94932890,297079702,939477265,3001846219, %U A211842 9689289277,31586215805,103968208826,345457027212,1158435912189,3919502591649 %N A211842 Number of nonnegative integer arrays of length n+3 with new values 0 upwards introduced in order, no three adjacent elements all unequal, and containing the value 2. %C A211842 Column 1 of A211849. %H A211842 R. H. Hardin, <a href="/A211842/b211842.txt">Table of n, a(n) for n = 1..210</a> %e A211842 Some solutions for n=3 %e A211842 ..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0 %e A211842 ..1....1....1....1....0....1....0....1....0....1....1....1....0....1....0....1 %e A211842 ..1....1....1....1....1....0....1....1....1....0....1....1....0....0....1....1 %e A211842 ..1....1....1....2....1....0....0....0....1....1....2....2....1....0....1....2 %e A211842 ..2....1....2....2....2....0....0....0....2....1....1....2....1....2....1....2 %e A211842 ..2....2....1....3....2....2....2....2....1....2....2....1....2....2....2....2 %Y A211842 Cf. A211849. %K A211842 nonn %O A211842 1,2 %A A211842 _R. H. Hardin_ Apr 22 2012