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 A223921 #6 Jul 23 2025 04:39:13 %S A223921 15,225,2111,14123,74040,323394,1226491,4157102,12856218,36834899, %T A223921 98904313,251078009,606789828,1403779390,3122765753,6704628063, %U A223921 13936852281,28123662137,55220811463,105715542647,197678421680 %N A223921 Number of 4Xn 0..2 arrays with rows and antidiagonals unimodal and columns nondecreasing. %C A223921 Row 4 of A223918 %H A223921 R. H. Hardin, <a href="/A223921/b223921.txt">Table of n, a(n) for n = 1..210</a> %F A223921 Empirical: a(n) = (1/106748928000)*n^16 + (1/2668723200)*n^15 + (29/2335132800)*n^14 + (10013/37362124800)*n^13 + (133979/28740096000)*n^12 + (37333/574801920)*n^11 + (49631/65318400)*n^10 + (1761593/261273600)*n^9 + (219704797/5225472000)*n^8 + (49791911/261273600)*n^7 + (925916249/1437004800)*n^6 + (287406331/179625600)*n^5 + (44691980741/15567552000)*n^4 + (315864449/86486400)*n^3 + (14884357/4324320)*n^2 + (558703/360360)*n + 1 %e A223921 Some solutions for n=3 %e A223921 ..0..1..0....0..1..1....0..1..0....0..0..1....0..0..0....0..2..0....0..0..1 %e A223921 ..0..1..0....0..1..1....0..2..0....0..1..2....1..0..0....1..2..0....2..2..1 %e A223921 ..2..1..0....1..1..1....0..2..0....1..1..2....1..0..0....1..2..1....2..2..1 %e A223921 ..2..1..1....1..1..1....0..2..1....1..1..2....1..0..0....2..2..1....2..2..1 %K A223921 nonn %O A223921 1,1 %A A223921 _R. H. Hardin_ Mar 29 2013