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 A206873 #7 Jul 22 2025 19:45:30 %S A206873 12,144,889,7675,63866,502864,4108471,33311703,269021206,2180747052, %T A206873 17657728229,142945674729,1157610400394,9373284938378,75896278735065, %U A206873 614558095591053,4976204942982610,40293442480994080,326265843009762463 %N A206873 Number of 4Xn 0..1 arrays avoiding 0 0 0 horizontally and 1 0 1 vertically. %C A206873 Row 4 of A206871 %H A206873 R. H. Hardin, <a href="/A206873/b206873.txt">Table of n, a(n) for n = 1..210</a> %F A206873 Empirical: a(n) = 2*a(n-1) +22*a(n-2) +195*a(n-3) +237*a(n-4) +211*a(n-5) -2946*a(n-6) -1034*a(n-7) -4204*a(n-8) +13726*a(n-9) +2298*a(n-10) +9852*a(n-11) -18860*a(n-12) -5516*a(n-13) -8002*a(n-14) +9398*a(n-15) +3229*a(n-16) +2616*a(n-17) -2128*a(n-18) -839*a(n-19) -433*a(n-20) +161*a(n-21) +68*a(n-22) +28*a(n-23) %e A206873 Some solutions for n=4 %e A206873 ..0..1..1..0....0..1..0..0....0..0..1..1....0..0..1..1....1..0..0..1 %e A206873 ..0..1..1..1....1..0..1..1....0..1..0..1....1..0..1..1....1..1..0..0 %e A206873 ..0..1..0..1....1..0..0..1....0..1..0..0....0..0..1..0....1..0..1..0 %e A206873 ..1..0..0..1....1..0..0..1....0..1..0..0....0..1..0..0....1..0..0..1 %K A206873 nonn %O A206873 1,1 %A A206873 _R. H. Hardin_ Feb 13 2012