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 A251802 #6 Jun 02 2025 10:32:26 %S A251802 1740,28932,348679,3352272,27291608,195753488,1271711747,7634258216, %T A251802 42987423526,229660644080,1174599406053,5792185593840,27697451048182, %U A251802 129039115827548,587993778478925,2629056738236432,11566062999511464 %N A251802 Number of (2+1)X(n+1) 0..3 arrays with no 2X2 subblock having x11-x00 less than x10-x01. %C A251802 Row 2 of A251801 %H A251802 R. H. Hardin, <a href="/A251802/b251802.txt">Table of n, a(n) for n = 1..210</a> %F A251802 Empirical: a(n) = 40*a(n-1) -755*a(n-2) +8946*a(n-3) -74677*a(n-4) +467156*a(n-5) -2274363*a(n-6) +8833486*a(n-7) -27833039*a(n-8) +71958408*a(n-9) -153781873*a(n-10) +272810702*a(n-11) -402324879*a(n-12) +492639700*a(n-13) -498877265*a(n-14) +414825042*a(n-15) -280100140*a(n-16) +151065320*a(n-17) -63500432*a(n-18) +20037984*a(n-19) -4463424*a(n-20) +625536*a(n-21) -41472*a(n-22) %e A251802 Some solutions for n=2 %e A251802 ..3..3..2....1..1..0....2..3..2....0..3..0....2..1..2....0..3..3....2..2..1 %e A251802 ..0..1..3....0..1..3....0..0..3....0..0..3....2..3..2....0..0..3....1..1..3 %e A251802 ..0..0..0....0..2..0....0..1..0....1..1..1....1..0..2....0..1..3....0..1..1 %K A251802 nonn %O A251802 1,1 %A A251802 _R. H. Hardin_, Dec 09 2014