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On a problem of P. Erdős. (English) Zbl 0612.05032
For \(\alpha\) real, \(0<\alpha <2\), and n an integer, let G(n,\(\alpha)\) be the graph whose vertices are the points of the unit sphere \(S^{n- 1}=\{x=(x_ 1,...,x_ n)\in {\mathbb{R}}^ n:\sum (x_ i)^ 2=1\}\), and which has an edge between two points if and only if they are a distance of \(\alpha\) apart. \(\chi\) (G(n,\(\alpha)\)) is the chromatic number of the graph G(n,\(\alpha)\), finite because \(S^{n-1}\) is a compact metric space. It is shown that for each \(\alpha\), \(\chi\) (G(n,\(\alpha)\))\(\to \infty\) as \(n\to \infty\). The proof uses known results on the chromatic number of Kneser graphs in a critical manner.
05C15 Coloring of graphs and hypergraphs
05C99 Graph theory
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[1] B. BOLLOBÁS: Extremal Graph Theory. Academic Press, London-New York-San Francisco, 1978. · Zbl 0844.05054
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