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In the mathematical field of [[graph theory]], the '''bipartite dimension''' of an [[graph]] G=(V,E) is the minimum number of [[biclique]]s, that is, complete bipartite subgraphs, needed to [[covering (graph theory)|cover]] all edges in E. A collection of bicliques covering all edges in G is called a '''biclique edge cover''', or sometimes '''biclique cover'''. The bipartite dimension of G is often denoted by the symbol d(G).
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==Bipartite dimension of some special graphs==
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Fishburn and Hammer determine the bipartite dimension for some special graphs. For example, the path <math>P_n</math>
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has <math>d(P_n) = \lfloor n/2 \rfloor</math>, the cycle <math>C_n</math> has <math>d(C_n)=\lceil n/2\rceil</math>,
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and the complete graph <math>K_n</math> has <math>d(K_n) = \lceil\log n \rceil</math>.
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==Computational complexity==
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Determining d(G) is an [[optimization problem]]. The [[decision problem]] for bipartite dimension can be phrased as:
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:INSTANCE: A graph <math>G=(V,E)</math> and a positive integer <math>k</math>.
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:QUESTION: Does G admit a biclique edge cover containing at most <math>k</math> bicliques?
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This problem is [[NP-complete]], even for [[bipartite graph]]s, as proved by Orlin. It appears as problem '''GT18''' in
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Garey and Johnson's classical book on NP-completeness. This decision problem is a rather straightforward reformulation of
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another decision problem on families of finite sets, the ''set basis problem'', proved
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to be NP-complete earlier by Stockmeyer, and appearing as problem '''SP7''' in Garey and Johnson's book.
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:INSTANCE: A collection of finite sets <math>S_1,\ldots,S_n</math> and a positive integer <math>k</math>.
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:QUESTION: Can we choose at most <math>k</math> subsets such that their union equals <math>\bigcup_{i=1}^n S_i</math>, the union of all <math>S_i</math>?
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To transform a bipartite graph <math>G=(U,V,E)</math> into an instance of the latter problem, simply take
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<math>U = \bigcup_{i=1}^n S_i</math>, <math>V=\{S_1,\ldots, S_n\}</math>,
and <math> \{\{u,S_i\} \mid u \in S_i\}</math> as edge set. The converse direction is similar.
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==References==
* P. C. Fishburn and P. L. Hammer. Bipartite dimensions and bipartite degrees of graphs. ''Discrete Mathematics'', 160:127-148, 1996
* M. R. Garey and D. S. Johnson. ''[[Computers and Intractability: A Guide to the Theory of NP-Completeness]]''. Freeman, 1979.
* S. Monson, N. Pullman, and R. Rees: A survey of clique and biclique coverings and factorizations of (0,1)-matrices. ''Bulletin of the ICA'', vol 14, pp. 17--86, 1995.
* J. Orlin. Contentment in Graph Theory: Covering Graphs with Cliques. ''Indagationes Mathematicae'', 80:406–424, 1977.
* Larry J. Stockmeyer. The set basis problem is NP-complete. Technical Report RC-5431, IBM, 1975.

==See also==
[[List of NP-complete problems]]

==External Links==
[http://11011110.livejournal.com/134829.html blog entry about bipartite dimension by David Eppstein]

Revision as of 21:53, 10 October 2008