Theoretical computer science · Oct 6, 2026

Hardness of coloring three-colorable graphs

It is NP-hard to color a three-colorable graph using any fixed number c ≥ 3 of colors. More strongly, for every fixed 0<δ<1/30\lt \delta\lt 1/3, a deterministic polynomial-time reduction from 3SAT produces simple unweighted graphs that are three-colorable in the satisfiable case and have no independent set of size δn\delta n otherwise, where n is the number of vertices.

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