An Algebraic Proof of the Dichotomy for Graph Orientation Problems with Forbidden Tournaments

Abstract. For a set [Formula: see text] of finite tournaments, the [Formula: see text]-free orientation problem is the problem of deciding if a given finite undirected graph can be oriented in such a way that the resulting oriented graph does not contain any member of [Formula: see text]. Using the theory of smooth approximations, we give a new shorter proof of the complexity dichotomy for such problems obtained recently by Bodirsky and Guzmán-Pro. In fact, our approach yields a complexity dichotomy for a considerably larger class of computational problems, where one is given an undirected graph along with additional local constraints on the allowed orientations. Moreover, the border between tractable and hard problems is also described by a decidable algebraic condition.