Abstract

Homomorphisms between graphs are studied as a generalization of colorings and of chromatic number. We investigate here homomorphisms from orientations of undirected planar graphs to graphs (not necessarily planar) containing as few digons as possible. We relate the existence of such homomorphisms to girth and it appears that these questions remain interesting even under large girth assumption in the range where the chromatic number is an easy invariant. In particular we prove that every orientation of any large girth planar graph is 5-colorable and classify those digraphs on 3, 4 and 5 vertices which color all large girth oriented planar graphs. 1 Introduction and statement of results Given graphs G = (V; E) and G 0 = (V 0 ; E 0 ) a homomorphism from G to G 0 is any mapping f : V ! V 0 satisfying [x; y] 2 E =) [f(x); f(y)] 2 E 0 : This work has been done while the author was visiting the University of Bordeaux I and was partly supported by GA CR 2167. y With the suppo...

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