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Search: id:A081080
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| A081080 |
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Number of n X n ortho-projection matrices over GF(2). Also, the number of labeled ortho-projection graphs on n vertices. |
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4
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OFFSET
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1,1
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COMMENT
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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.
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REFERENCES
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B. Shtylla and L. Zulli, Ortho-projection graphs, in preparation.
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LINKS
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L. Zulli, Home Page
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EXAMPLE
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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].
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CROSSREFS
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Cf. A081081, A081082.
Sequence in context: A002577 A076132 A047142 this_sequence A109460 A108801 A111022
Adjacent sequences: A081077 A081078 A081079 this_sequence A081081 A081082 A081083
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KEYWORD
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hard,more,nonn
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AUTHOR
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B. Shtylla and L. Zulli (shtyllab(AT)lafayette.edu, zullil(AT)lafayette.edu), Mar 05 2003
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EXTENSIONS
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a(9) from Louis Zulli (zullil(AT)lafayette.edu), Aug 23 2004
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