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 A251220 #7 Jun 26 2025 12:27:14 %S A251220 14,305,20782,4462968,3007738372,6373427948731,42432102644410392, %T A251220 887822645668106069016,58374742677350528694024651, %U A251220 12061621459257582136492647314788,7831831916823870434473136772446378463 %N A251220 Number of (n+1) X (n+1) 0..1 arrays with no 2 X 2 subblock having the minimum of its diagonal elements greater than the absolute difference of its antidiagonal elements. %C A251220 Diagonal of A251228. %H A251220 R. H. Hardin, <a href="/A251220/b251220.txt">Table of n, a(n) for n = 1..15</a> %e A251220 Some solutions for n=3 %e A251220 ..1..0..0..1....1..0..1..0....1..1..0..0....0..1..1..0....1..1..1..0 %e A251220 ..1..0..0..1....0..0..1..1....0..1..0..1....1..0..1..0....1..0..1..1 %e A251220 ..0..0..0..1....0..0..0..0....0..1..1..1....1..1..1..1....0..0..0..1 %e A251220 ..0..0..0..0....1..1..0..1....0..0..0..1....0..0..1..0....1..0..1..1 %Y A251220 Cf. A251228. %K A251220 nonn %O A251220 1,1 %A A251220 _R. H. Hardin_, Nov 30 2014