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Counting perfect matchings as fast as Ryser

Published: 17 January 2012 Publication History

Abstract

We show that there is a polynomial space algorithm that counts the number of perfect matchings in an n-vertex graph in O*(2n/2) ⊂ O(1.415n) time. (O*(f(n)) suppresses functions polylogarithmic in f(n)). The previously fastest algorithms for the problem was the exponential space O*(((1 + √5)/2)n) ⊂ O(1.619n) time algorithm by Koivisto, and for polynomial space, the O(1.942n) time algorithm by Nederlof. Our new algorithm's runtime matches up to polynomial factors that of Ryser's 1963 algorithm for bipartite graphs. We present our algorithm in the more general setting of computing the hafnian over an arbitrary ring, analogously to Ryser's algorithm for permanent computation.
We also give a simple argument why the general exact set cover counting problem over a slightly superpolynomial sized family of subsets of an n element ground set cannot be solved in O*(2(1−ε1)n) time for any ε1 > 0 unless there are O*(2(1−ε2)n) time algorithms for computing an n x n 0/1 matrix permanent, for some ε2 > 0 depending only on ε1.

References

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O. Amini, F. V. Fomin, and S. Saurabh. Counting Subgraphs via Homomorphisms. In proceedings of the 36th ICALP, pp. 71--82, 2009.
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E. Bax and J. Franklin. A Permanent Algorithm with exp{(n 1/3/2ln(n))} Expected Speedup for 0--1 Matrices. Algorithmica, Volume 32, Number 1, 157--162, 2002.
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A. Björklund and T. Husfeldt. Exact algorithms for Exact Satisfiability and Number of Perfect Matchings. Algorithmica 52, pp. 226--249, 2008.
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A. Björklund, T. Husfeldt, and M. Koivisto. Set partitioning via inclusion--exclusion. SIAM Journal on Computing, Volume 39, Issue 2, pp. 546--563, 2009.
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M. Koivisto. Partitioning into sets of bounded cardinality. 4th International Workshop on Parameterized and Exact Computation (IWPEC), LNCS 5917, pp. 258--263, Springer, 2009.
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J. Nederlof. Inclusion exclusion for hard problems. Masters thesis, Utrecht University, August 2008.
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J. Nederlof. Fast polynomial-space algorithms using Mobius inversion: Improving on Steiner Tree and related problems. Submitted, available at http://folk.uib.no/jne061/Steinerfull.pdf, 2010.
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H. J. Ryser. Combinatorial Mathematics, The Carus mathematical monographs, The Mathematical Association of America, 1963.
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P. Sankowski. Alternative algorithms for counting all matchings in a graph. In proceedings of the 20th STACS, pp. 427--438, 2003.
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R. A. Servedio and A. Wan. Computing sparse permanents faster. Information Processing Letters, Volume 96 Issue 3, 15 November 2005.
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SODA '12: Proceedings of the twenty-third annual ACM-SIAM symposium on Discrete algorithms
January 2012
1764 pages

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  • Kyoto University: Kyoto University

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Society for Industrial and Applied Mathematics

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Published: 17 January 2012

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