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 A086976 #17 Feb 16 2025 08:32:50
%S A086976 2,19,712,87685,31102080,28757840751
%N A086976 Number of monotonic matrices of order n.
%C A086976 A monotonic matrix of order n is an n × N matrix in which every element is either 0 or contains a number from the set {1, 2, ..., n} subject to 3 conditions:
%C A086976 the filled-in entries in each row are strictly increasing;
%C A086976 the filled-in entries in each column are strictly decreasing;
%C A086976 for two filled-in cells with same entry, the one further right is higher (the positive slope condition).
%C A086976 See A070214 for more information.
%H A086976 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/MonotonicMatrix.html">Monotonic Matrix</a>
%Y A086976 Cf. A070214.
%K A086976 nonn,more
%O A086976 1,1
%A A086976 _Eric W. Weisstein_, Jul 25 2003
%E A086976 a(4) from _Rob Pratt_, Aug 23 2003
%E A086976 a(5)-a(6) from _Giovanni Resta_, Jul 28 2015