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 A217639 #8 Jul 22 2025 23:41:59 %S A217639 3,9,27,91,293,959,3121,10171,33141,107985,351855,1146471,3735623, %T A217639 12172023,39660897,129229689,421077529,1372024391,4470556607, %U A217639 14566706329,47463649813,154653907527,503918919165,1641951898617,5350079020329 %N A217639 Number of nX2 arrays of the minimum value of corresponding elements and their horizontal, vertical or antidiagonal neighbors in a random 0..2 nX2 array. %C A217639 Column 2 of A217645 %H A217639 R. H. Hardin, <a href="/A217639/b217639.txt">Table of n, a(n) for n = 1..210</a> %F A217639 Empirical: a(n) = 2*a(n-1) +3*a(n-2) +3*a(n-3) +2*a(n-4) -a(n-6). %F A217639 Empirical: G.f.: x*(3+3*x+x^3-3*x^4+x^5) / ( 1-2*x-3*x^2-3*x^3-2*x^4+x^6 ). - _R. J. Mathar_, Jul 10 2015 %e A217639 Some solutions for n=3 %e A217639 ..2..1....1..1....0..0....2..0....2..1....1..1....0..0....0..0....1..0....2..2 %e A217639 ..1..0....1..1....0..1....0..0....0..0....1..0....0..1....0..0....0..0....2..1 %e A217639 ..0..0....2..2....2..2....0..0....0..0....0..0....1..1....0..2....0..0....1..1 %K A217639 nonn %O A217639 1,1 %A A217639 _R. H. Hardin_ Oct 09 2012