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 A331644 #8 Jan 25 2020 17:55:44 %S A331644 1,1,13,313,14797,1095601,119621653,17943752233,3550217905597, %T A331644 894951415465441,280109128908963493,106563994919500072153, %U A331644 48431841090878234155597,25916389944261925021739281,16128299505539619187866596533,11549663940282035980807700520073 %N A331644 Number of nonnegative integer matrices with n columns and any number of distinct nonzero rows with column sums 2. %H A331644 Andrew Howroyd, <a href="/A331644/b331644.txt">Table of n, a(n) for n = 0..200</a> %e A331644 The a(2) = 13 matrices are: %e A331644 [1 1] [1 1] [1 0] [1 0] [0 1] [0 1] %e A331644 [1 0] [0 1] [1 1] [0 1] [1 1] [1 0] %e A331644 [0 1] [1 0] [0 1] [1 1] [1 0] [1 1] %e A331644 . %e A331644 [2 1] [2 0] [1 2] [1 0] [0 2] [0 1] [2 2] %e A331644 [0 1] [0 2] [1 0] [1 2] [2 0] [2 1] %Y A331644 Row n=2 of A331568. %Y A331644 Cf. A331645. %K A331644 nonn %O A331644 0,3 %A A331644 _Andrew Howroyd_, Jan 23 2020