Polychromatic Coloring for Half-Planes

We prove that for every integer k, every finite set of points in the plane can be k-colored so that every half-plane that contains at least 2k−1 points, also contains at least one point from every color class. We also show that the bound 2k−1 is best possible. This improves the best previously known lower and upper bounds of $\frac{4}{3}k$ and 4k−1 respectively. As a corollary, we also show that every finite set of half-planes can be k colored so that if a point p belongs to a subset Hp of at least 4k−3 of the half-planes then Hp contains a half-plane from every color class. This improves the best previously known upper bound of 8k−3. Another corollary of our first result is a new proof of the existence of small size e-nets for points in the plane with respect to half-planes.