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%I #6 Feb 07 2022 21:39:37
%S 1,1,1,1,6,5,4,829,707,2938,55779,60084
%N Number of unlabeled directed graphs on n nodes without a subgraph isomorphic to directed K_{2,2} that have the maximal number of edges (=A191873(n)).
%C When adjacency matrices viewed as those of bipartite graphs, these bipartite graphs are pairwise isomorphic for each n <= 12, except for n = 5 and n = 8 with 2 and 3 distinct bipartite graphs, respectively.
%e For n = 7 there are 4 such graphs with A191873(7) = 21 edges, described by the following adjacency matrices:
%e [0111000] [0110100] [0110010] [0010101]
%e [1010100] [1011000] [1000011] [1001001]
%e [1100010] [1000101] [0101001] [0100011]
%e [0100101] [1100010] [1100100] [1010010]
%e [0010011] [0010011] [1011000] [0111000]
%e [1001001] [0101001] [0010101] [1100100]
%e [0001110] [0001110] [0001110] [0001110]
%e Absence of directed K_{2,2} subgraph means that these matrices do not contain a rectangle with four 1s at the corners.
%Y Unlabeled version of A191874.
%Y Directed version of A335820.
%Y Cf. A191873.
%K nonn,more
%O 1,5
%A _Max Alekseyev_, Feb 07 2022
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