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 A221453 #7 Jul 23 2025 01:57:13 %S A221453 0,1,2,7,25,102,456,2219,11640,65364,390646,2472719,16508791, %T A221453 115839661,851634863,6542134884,52384882759,436290809772, %U A221453 3772143122161,33797290492507,313304547351868,3000631062271288,29651208549845055 %N A221453 Number of 0..n arrays of length n with each element unequal to at least one neighbor, with new values introduced in 0..n order. %C A221453 Diagonal of A221459 %H A221453 R. H. Hardin, <a href="/A221453/b221453.txt">Table of n, a(n) for n = 1..182</a> %e A221453 Some solutions for n=6 %e A221453 ..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0 %e A221453 ..1....1....1....1....1....1....1....1....1....1....1....1....1....1....1....1 %e A221453 ..0....2....1....0....2....2....1....0....2....2....0....1....0....1....1....1 %e A221453 ..2....1....2....1....3....0....2....2....1....3....2....2....1....2....2....0 %e A221453 ..2....3....1....1....0....0....2....3....2....2....1....2....2....1....3....0 %e A221453 ..3....4....2....2....3....3....1....0....3....0....2....0....0....3....0....1 %K A221453 nonn %O A221453 1,3 %A A221453 _R. H. Hardin_ Jan 17 2013