Geodesic
On acyclic colorings of direct products
Discussiones Mathematicae. Graph Theory, Tome 28 (2008) no. 2, pp. 323-333.

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A coloring of a graph G is an acyclic coloring if the union of any two color classes induces a forest. It is proved that the acyclic chromatic number of direct product of two trees T₁ and T₂ equals minΔ(T₁) + 1, Δ(T₂) + 1. We also prove that the acyclic chromatic number of direct product of two complete graphs Kₘ and Kₙ is mn-m-2, where m ≥ n ≥ 4. Several bounds for the acyclic chromatic number of direct products are given and in connection to this some questions are raised.
Keywords: coloring, acyclic coloring, distance-two coloring, direct product 
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Špacapan, Simon; Horvat, Aleksandra. On acyclic colorings of direct products. Discussiones Mathematicae. Graph Theory, Tome 28 (2008) no. 2, pp. 323-333. http://geodesic.mathdoc.fr/item/DMGT_2008_28_2_a7/

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