Coloring Copoints of a Planar Point Set Walter Morris Department of Mathematical Sciences George Mason University 4400 University Drive, Fairfax, VA 22030, USA Abstract To a set of n points in the plane, one can associate a graph that has less than n2 vertices and has the property that k-cliques in the graph correspond vertex sets of convex k-gons in the point set. We prove an upper bound of 2k¡1 on the size of a planar point set for which the graph has chromatic number k, matching the bound con- jectured by Szekeres for the clique number. Constructions of Erd}os and Szekeres are shown to yield graphs that have very low chromatic number. The constructions are carried out in the context of pseudoline arrangements. 1 Introduction Let X be a ¯nite set of points in IR2. We will assume that X is in general position, that is, no three points of X are on a line. A subset C of X is called closed if C = K \ X for some convex subset K of IR2. The set C of closed subsets of X, partially ordered by inclusion, is a lattice. If x 2 X and A is a closed subset of X that does not contain x and is inclusion-maximal among all closed subsets of X that do not contain x, then A is called a copoint of X attached to x. In the more general context of antimatroids, or convex geometries, co- points have been studied in [1], [2] and [3]. It is shown in these references that every copoint is attached to a unique point of X, and that the copoints are the meet-irreducible elements of the lattice of closed sets. Following [2], 1 we use the notation M(X) to denote the set of copoints of X, partially or- dered by inclusion. Edelman and Saks [2] initiated the study of the convex dimension of X, which is the smallest number of chains needed to cover M(X). Their investigations applied to convex geometries in general. The paper [4] studied the speci¯c case of convex geometries de¯ned by planar point sets. It is very easy to ¯nd the copoints of a planar point set, and to see that there are less than n2 of them. Denote by ®(A) the unique point to which a copoint A is attached. We will then let ®¡1(x) denote the set of copoints that are attached to an element x. If a subset A of X is a copoint attached to a point x 2 X, then the convex hull of A is disjoint from x, because A 2 C. There is therefore a line ` through x so that A is in one of the open halfspaces determined by `. Furthermore, there can be no other points of X in this open halfspace, by the maximality of A. We can list all of the copoints attached to a point x by rotating a directed line around x. Start with a vertical line through x directed from bottom to top. We may assume, without loss of generality, that no two points of X are on the same vertical line. Call the part of the line above x the head and the part of the line below x the tail. Rotate the line clockwise around x, noting the order in which the points of Xnx are met by the line. If a point y is met by the head of the line, write y, and if y is met by the tail of the line, write ¡y. The sequence of 2jXj ¡ 2 symbols written as the line makes a complete revolution around x, viewed as a circular sequence, is called the circular local sequence of x. At one or more places in the circular local sequence of x there will be an element y followed by an element ¡z. At such a place we can ¯nd a copoint. Let m be a line through x of which y and z are on the same side. Let H be the open halfspace de¯ned by m that contains y and z. Then H \ X is a copoint attached to x. Central to our investigations are three graphs, each having vertex set M(X). 1. The graph G(X) has an edge between copoints A1 and A2 if and only if A1 and A2 are incomparable sets. The chromatic number Â(G(X)) is the smallest number of chains into which M(X) can be partitioned. By Dilworth's Theorem, Â(G(X)) is also equal to !(G(X)), the size of the largest clique in G(X). 2. The graph ¡(X) has an edge between copoints A1 and A2 if and only if ®(A1) 2 A2 [ ®(A2) and ®(A2) 2 A1 [ ®(A1). 2 w {x,y,u,v,z} {x,y,w,z,v} {u,w,z,v,y} z v {x,y,u,v} {x,y,w,z} {u,w,z,v} y u x {x,y,u} {x,v,u} {x,y,w} {x,w,z} {u,w,z} {u,w,v} Figure 1: A six point set and its poset of copoints 3. The graph G(X) has an edge between copoints A1 and A2 if and only if ®(A1) 2 A2 and ®(A2) 2 A1. Proposition 1.1 Every edge of G(X) is an edge of ¡(X), and every edge of ¡(X) is an edge of G(X). Proof. The ¯rst part is easy. For each v 2 X, ¡(X) contains the edge set of a clique formed by the copoints A for which ®(A) = v, and these are the edges of ¡(X) that are not in G(X). To show the second part, note that if ®(A1) 2 A2 and ®(A2) 2 A1 then A1 and A2 are incomparable, because ®(A1) 2= A1 and ®(A2) 2= A2. If ®(A1) = ®(A2) and A1 ⊆ A2, then A1 = A2, because A1 is a maximal convex subset of Xn®(A1). If X is the six point set in Figure 1, then the copoints are fx; y; ug; fx; u; vg, fx; y; wg; fx; w; zg; fu; v; wg; fu; w; zg; fx; y; u; vg; fx; y; w; zg; fu; v; w; zg, fx; y; u; v; zg; fx; y; w; z; vg; fu; v; w; z; yg. The copoint fx; y; ug, for exam- ple, is attached to the point v because every closed superset of fx; y; ug con- tains v. The circular local sequence of x is (w; z; y; v; u; ¡w; ¡z; ¡y; ¡v; ¡u), and the circular local sequence of y is (u; ¡w; ¡z; x; ¡v; ¡u; w; z; ¡x; v). The six three-element copoints induce a clique in G(X), but in ¡(X) they only induce a cycle of length six and in G(X) they induce a graph with three disjoint edges. The set ffx; y; ug; fx; u; vg; fx; y; w; z; vg; fy; u; v; w; zgg in- duces a four-clique in G(X). The chromatic number of G(X) is six, while the chromatic number of ¡(X) is four. Proposition 1.2 If Y is a k-clique in G(X), then f®(A): A 2 Y g is the vertex set of a convex k-gon in X. If P is the vertex set of a convex k-gon in X, then there is a k-clique Y in G(X) so that P = f®(A): A 2 Y g. 3 Proof. Suppose Y is a k-clique in G(X). If A 2 Y , then f®(B): B 2 Y nfAgg ⊆ A, because Y is a clique. Thus ®(A) can be separated from conv(f®(B): B 2 Y nfAgg) by a hyperplane, which means that ®(A) is a vertex of conv(f®(B): B 2 Y g). Next, suppose P is the vertex set of a convex k-gon in X. For each x 2 P , let A(x) be a copoint attached to x containing P nfxg. Then fA(x): x 2 P g is a k-clique in G(X). The correspondence between k-cliques in G(X) and convex k-gons in X is not bijective. For a point x in a set P that is the vertex set of a k-gon, there may be several copoints attached to x that contain P nfxg.A k-gon in X therefore corresponds to a complete k-partite graph in G(X) that becomes a clique in ¡(X). The Proposition shows that the largest number of vertices of a convex polygon in X equals !(G(X), the size of the largest clique in G(X). The graphs G(X); ¡(X) and G(X) can be relatively dense, so drawing them directly does not yield much intuition into their structure. In the next section, we show that copoints of a ¯nite point set are much easier to identify and study if one considers the dual setting of line arrangements. In that setting, a copoint corresponds to a face of the arrangement that has exactly one of its bounding lines above it or exactly one of its bounding lines below it. We summarize known and new results about the parameters ! and  for the graphs under study. ² Erd}osand Szekeres ([5], see also [6]) gave examples of point sets X with jXj = 2k¡1 and !(G(X)) = k. They also conjectured [7] that there are no larger point sets with !(G(X))³ = k´. The currently best 2k¡3 upper bound on jXj when !(G(X)) = k is k¡2 , due to [8].