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 A221439 #8 Jul 23 2025 01:55:54 %S A221439 1,9,221,18393,6415089,8569227073,43045366565233,827667742380908273, %T A221439 60943705772313538144913,17137605013408451421126281145, %U A221439 18409652574456111249919293718583541 %N A221439 Hilltop maps: number of n X n binary arrays indicating the locations of corresponding elements not exceeded by any horizontal or antidiagonal neighbor in a random 0..1 n X n array. %C A221439 Diagonal of A221446 %H A221439 R. H. Hardin, <a href="/A221439/b221439.txt">Table of n, a(n) for n = 1..14</a> %e A221439 Some solutions for n=3 %e A221439 ..1..1..1....1..0..1....0..1..0....1..1..0....0..1..0....1..1..1....1..0..0 %e A221439 ..0..0..1....1..0..0....1..1..0....0..1..0....1..0..1....1..1..0....1..1..1 %e A221439 ..1..1..0....0..1..1....1..1..0....0..1..0....1..1..0....1..1..0....0..1..1 %K A221439 nonn %O A221439 1,2 %A A221439 _R. H. Hardin_ Jan 17 2013