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 A218421 #6 Jul 22 2025 23:56:34 %S A218421 5,49,357,2725,20793,158429,1207525,9203377,70144889,534620525, %T A218421 4074695269,31055936969,236697756661,1804029548133,13749697742817, %U A218421 104795505274517,798715588594961,6087537722095689,46397135660186377 %N A218421 Hilltop maps: number of nX3 binary arrays indicating the locations of corresponding elements not exceeded by any horizontal, vertical or antidiagonal neighbor in a random 0..1 nX3 array. %C A218421 Column 3 of A218426 %H A218421 R. H. Hardin, <a href="/A218421/b218421.txt">Table of n, a(n) for n = 1..210</a> %F A218421 Empirical: a(n) = 5*a(n-1) +16*a(n-2) +28*a(n-3) +17*a(n-4) +7*a(n-5) -3*a(n-6) +3*a(n-7) -8*a(n-8) -a(n-10) +a(n-11) %e A218421 Some solutions for n=3 %e A218421 ..0..1..0....1..0..0....0..1..1....1..1..0....1..0..0....0..1..0....0..1..1 %e A218421 ..1..1..1....1..1..0....0..0..0....0..1..1....0..0..1....1..0..1....0..0..0 %e A218421 ..1..1..1....1..0..1....0..1..1....0..1..1....1..0..0....0..0..0....1..1..0 %K A218421 nonn %O A218421 1,1 %A A218421 _R. H. Hardin_ Oct 28 2012