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 A206990 #7 Jul 22 2025 19:53:58 %S A206990 13,169,840,6510,46878,329952,2371090,16939450,120961040,864671390, %T A206990 6178621414,44150290200,315500062138,2254527357770,16110599565656, %U A206990 115124947986150,822671737926942,5878731842796456,42008857944366290 %N A206990 Number of nX4 0..1 arrays avoiding 0 0 0 horizontally and 1 1 1 vertically. %C A206990 Column 4 of A206994 %H A206990 R. H. Hardin, <a href="/A206990/b206990.txt">Table of n, a(n) for n = 1..210</a> %F A206990 Empirical: a(n) = 2*a(n-1) +17*a(n-2) +135*a(n-3) +74*a(n-4) -7*a(n-5) -1496*a(n-6) +492*a(n-7) -890*a(n-8) +4192*a(n-9) -1374*a(n-10) +1082*a(n-11) -3316*a(n-12) +622*a(n-13) +112*a(n-14) +684*a(n-15) -125*a(n-16) -42*a(n-17) -43*a(n-18) +7*a(n-19) +2*a(n-20) +a(n-21) for n>23 %e A206990 Some solutions for n=4 %e A206990 ..1..1..0..1....0..0..1..1....1..0..1..0....1..0..0..1....0..0..1..0 %e A206990 ..0..0..1..0....1..0..1..1....1..0..0..1....0..1..1..0....0..0..1..1 %e A206990 ..0..1..1..1....0..1..0..0....0..1..0..0....1..0..1..1....0..1..0..1 %e A206990 ..1..0..0..1....1..0..0..1....0..1..0..0....1..1..0..1....0..0..1..0 %K A206990 nonn %O A206990 1,1 %A A206990 _R. H. Hardin_ Feb 14 2012