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 A207881 #7 Jul 22 2025 20:55:24 %S A207881 9,81,221,575,1673,4881,14825,45411,141381,443159,1399925,4442717, %T A207881 14159449,45263327,145061281,465794335,1498036809,4823779033, %U A207881 15548389529,50156459339,161898944109,522854621863,1689250105197,5459434154405 %N A207881 Number of nX4 0..1 arrays avoiding 0 0 0 and 1 0 1 horizontally and 0 1 1 and 1 1 0 vertically. %C A207881 Column 4 of A207885 %H A207881 R. H. Hardin, <a href="/A207881/b207881.txt">Table of n, a(n) for n = 1..210</a> %F A207881 Empirical: a(n) = 8*a(n-1) -10*a(n-2) -67*a(n-3) +164*a(n-4) +194*a(n-5) -751*a(n-6) -172*a(n-7) +1630*a(n-8) -197*a(n-9) -1808*a(n-10) +490*a(n-11) +952*a(n-12) -320*a(n-13) -176*a(n-14) +64*a(n-15) for n>16 %e A207881 Some solutions for n=4 %e A207881 ..1..1..1..0....0..0..1..0....0..0..1..1....0..0..1..1....0..0..1..1 %e A207881 ..0..1..1..0....1..0..0..1....0..1..1..0....1..0..0..1....1..0..0..1 %e A207881 ..0..1..1..1....0..0..1..0....0..0..1..0....0..0..1..1....0..1..1..1 %e A207881 ..1..1..1..0....1..0..0..1....0..0..1..0....1..0..0..1....1..0..0..1 %K A207881 nonn %O A207881 1,1 %A A207881 _R. H. Hardin_ Feb 21 2012