Erdos-Faber-Lovász conjecture
In graph theory, the Erdős-Faber-Lovász conjecture (1972) is a very deep problem about the coloring of graphs. It says: - The union of k copies of k-cliquess intersecting in at most one vertex pairwise is k-chromatic.
Erdős originally offered US$50 for proving the conjecture in the affirmative, and later raised the reward to US$500. It is easy to show that the desired chromatic number is less than 1 + k √(k − 1).
See also Erdős conjecture
References - Erdős, Paul (1981). On the combinatorial problems I would most like to see solved. Combinatorica 1, 25–42.
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