Abstract
We study the maximization version of the fundamental graph coloring problem. Here the goal is to color the vertices of a k-colorable graph with k colors so that a maximum fraction of edges are properly colored (i.e. their endpoints receive different colors). A random k-coloring properly colors an expected fraction \(1-\frac{1}{k}\) of edges. We prove that given a graph promised to be k-colorable, it is NP-hard to find a k-coloring that properly colors more than a fraction \(\approx 1-\frac{1}{33 k}\) of edges. Previously, only a hardness factor of \(1- O\bigl(\frac{1}{k^2}\bigr)\) was known. Our result pins down the correct asymptotic dependence of the approximation factor on k. Along the way, we prove that approximating the Maximum 3-colorable subgraph problem within a factor greater than \(\frac{32}{33}\) is NP-hard.
Using semidefinite programming, it is known that one can do better than a random coloring and properly color a fraction \(1-\frac{1}{k} +\frac{2 \ln k}{k^2}\) of edges in polynomial time. We show that, assuming the 2-to-1 conjecture, it is hard to properly color (using k colors) more than a fraction \(1-\frac{1}{k} + O\left(\frac{\ln k}{k^2}\right)\) of edges of a k-colorable graph.
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Guruswami, V., Sinop, A.K. (2009). Improved Inapproximability Results for Maximum k-Colorable Subgraph. In: Dinur, I., Jansen, K., Naor, J., Rolim, J. (eds) Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques. APPROX RANDOM 2009 2009. Lecture Notes in Computer Science, vol 5687. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-03685-9_13
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DOI: https://doi.org/10.1007/978-3-642-03685-9_13
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