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 A207657 #7 Jul 22 2025 20:38:24 %S A207657 10,100,292,870,2774,9060,30440,103838,359206,1254128,4412792, %T A207657 15616086,55522546,198135096,709186104,2544574170,9148317066, %U A207657 32944585328,118801548180,428898598218,1549885877794,5605176866756,20284633906700 %N A207657 Number of nX4 0..1 arrays avoiding 0 0 0 and 1 1 1 horizontally and 0 0 1 and 1 0 0 vertically. %C A207657 Column 4 of A207661 %H A207657 R. H. Hardin, <a href="/A207657/b207657.txt">Table of n, a(n) for n = 1..210</a> %F A207657 Empirical: a(n) = 8*a(n-1) -9*a(n-2) -67*a(n-3) +135*a(n-4) +215*a(n-5) -508*a(n-6) -375*a(n-7) +781*a(n-8) +426*a(n-9) -467*a(n-10) -263*a(n-11) +81*a(n-12) +51*a(n-13) -4*a(n-14) -3*a(n-15) for n>16 %e A207657 Some solutions for n=4 %e A207657 ..1..0..1..1....1..1..0..1....0..1..0..0....0..1..0..1....0..1..0..1 %e A207657 ..0..0..1..1....1..1..0..1....1..0..1..1....0..0..1..0....1..1..0..0 %e A207657 ..1..0..1..0....0..1..0..0....1..1..0..1....0..1..0..1....1..0..0..1 %e A207657 ..1..0..0..1....1..0..0..1....1..0..1..1....0..1..1..0....1..1..0..0 %K A207657 nonn %O A207657 1,1 %A A207657 _R. H. Hardin_ Feb 19 2012