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Enumerating Contingency Tables via Random Permanents

by: Alexander Barvinok
Combinatorics, Probability and Computing, Vol. 17, No. 01. (2007), pp. 1-19.


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Given <em>m</em> positive integers <em>R</em> = (<em>r<sub>i</sub></em>), <em>n</em> positive integers <em>C</em> = (<em>c<sub>j</sub></em>) such that Σ<em>r<sub>i</sub></em> = Σ<em>c<sub>j</sub></em> = <em>N</em>, and <em>mn</em> non-negative weights <em>W</em>=(<em>w<sub>ij</sub></em>), we consider the total weight <em>T</em>=<em>T</em>(<em>R, C</em>; <em>W</em>) of non-negative integer matrices <em>D</em>=(<em>d<sub>ij</sub></em>) with the row sums <em>r<sub>i</sub></em>, column sums <em>c<sub>j</sub></em>, and the weight of <em>D</em> equal to <span style="vertical-align:middle"><img src="/fulltext_content/CPC/CPC17_01/S0963548307008668_inline1.gif" alt="$∏ w_ij^d_ij$"></img></span>. For different choices of <em>R</em>, <em>C</em>, and <em>W</em>, the quantity <em>T</em>(<em>R,C</em>; <em>W</em>) specializes to the permanent of a matrix, the number of contingency tables with prescribed margins, and the number of integer feasible flows in a network. We present a randomized algorithm whose complexity is polynomial in <em>N</em> and which computes a number <em>T</em>′=<em>T</em>′(<em>R,C</em>;<em>W</em>) such that <em>T</em>′ ≤ <em>T</em> ≤ α(<em>R,C</em>)<em>T</em>′ where <span style="vertical-align:middle"><img src="/fulltext_content/CPC/CPC17_01/S0963548307008668_inline2.gif" alt="$α(R,C) = \min \bigl{∏ r_i! r_i^-r_i, \ ∏ c_j! c_j^-c_j \bigr} N^N/N!$"></img></span>. In many cases, ln <em>T</em>′ provides an asymptotically accurate estimate of ln <em>T</em>. The idea of the algorithm is to express <em>T</em> as the expectation of the permanent of an <em>N</em> × <em>N</em> random matrix with exponentially distributed entries and approximate the expectation by the integral <em>T</em>′ of an efficiently computable log-concave function on <span style="vertical-align:middle"><img src="/fulltext_content/CPC/CPC17_01/xs211D.gif" alt="xs211D"></img></span><sup>mn</sup>.


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