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 A280434 #6 Apr 21 2020 12:58:16 %S A280434 0,4,21,80,225,548,1211,2512,4989,9604,18073,33432,61035,110276, %T A280434 197581,351568,621945 %N A280434 Number of n X n 0..1 arrays with no element equal to more than one of its horizontal and vertical neighbors, with the exception of exactly one element, and with new values introduced in order 0 sequentially upwards. %C A280434 Diagonal of A280440. %e A280434 Some solutions for n=4 %e A280434 ..0..0..1..0. .0..1..0..1. .0..1..0..1. .0..0..1..1. .0..0..1..0 %e A280434 ..1..1..0..1. .1..0..1..0. .1..0..1..0. .0..1..0..0. .1..0..1..0 %e A280434 ..0..0..1..0. .0..1..0..0. .0..1..0..1. .1..0..1..1. .0..1..0..1 %e A280434 ..0..1..0..1. .1..0..1..0. .0..1..1..0. .0..1..0..0. .0..1..0..1 %Y A280434 Cf. A280440. %K A280434 nonn %O A280434 1,2 %A A280434 _R. H. Hardin_, Jan 03 2017