Polynomial-time Approximation of Independent Set Parameterized by Treewidth
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We prove the following result about approximating the maximum independent set in a graph. Informally, we show that any approximation algorithm with a “non-trivial” approximation ratio (as a function of the number of vertices of the input graph G) can be turned into an approximation algorithm achieving almost the same ratio, albeit as a function of the treewidth of G. More formally, we prove that for any function f, the existence of a polynomial time (n/f(n))-approximation algorithm yields the existence of a polynomial time O(tw ·logf(tw)/f(tw))-approximation algorithm, where n and tw denote the number of vertices and the width of a given tree decomposition of the input graph. By pipelining our result with the state-of-the-art O(n · (loglog n)^2/log^3 n)-approximation algorithm by Feige (2004), this implies an O(tw · (loglog tw)^3/log^3 tw)-approximation algorithm.
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