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%I #5 Dec 21 2017 14:30:20
%S 2,4,10,38,194,1378,13570,188546,3664898
%N Number of n X n ortho-projection matrices over GF(2). Also, the number of labeled ortho-projection graphs on n vertices.
%C A matrix over GF(2) is an ortho-projection if and only if the matrix is symmetric and idempotent. A labeled ortho-projection graph is a labeled, undirected pseudograph without multiple edges and without multiple loops whose adjacency matrix is an ortho-projection matrix over GF(2). These matrices and graphs arise naturally in low-dimensional topology.
%D B. Shtylla and L. Zulli, Ortho-projection graphs, in preparation.
%H L. Zulli, <a href="http://www.cs.lafayette.edu/~zullil/">Home Page</a> [broken link]
%e a(2)=4 because there are four 2 X 2 ortho-projection matrices over GF(2), namely [0 0 / 0 0], [0 0 / 0 1], [1 0 / 0 0], [1 0 / 0 1].
%Y Cf. A081081, A081082.
%K hard,more,nonn
%O 1,1
%A B. Shtylla and L. Zulli (shtyllab(AT)lafayette.edu, zullil(AT)lafayette.edu), Mar 05 2003
%E a(9) from Louis Zulli (zullil(AT)lafayette.edu), Aug 23 2004
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