Generalized Halfspaces in Restricted-orientation Convexity 1

Restricted-orientation convexity (O-convexity) is the study of geometric objects whose intersections with lines from some xed set O of orientations are empty or connected. The notion of O-convexity generalizes standard convexity, as well as several other types of nontraditional convexity. We introduce and study O-halfspaces, which are analogs of standard halfspaces in the theory of O-convexity, and directed O-halfspaces, which are restricted O-halfspaces. We explore some of the basic properties of them and outline their relationships to O-convex sets and O-connected sets, which are restricted O-convex sets, in two and more dimensions.