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EJC
2008

# A simple proof for open cups and caps

8 years 2 months ago
Let X be a set of points in general position in the plane. General position means that no three points lie on a line and no two points have the same x-coordinate. Y X is a cup, resp. cap, if the points of Y lie on the graph of a convex, resp. concave function. Denote the points of Y by p1, p2, . . . , pm according to the increasing x-coordinate. The set Y is open in X if there is no point of X above the polygonal line p1, p2, . . . , pm. Valtr [12] showed that for every positive integers k and l there exists a positive integer g(k, l) such that any g(k, l)point set in the plane in general position contains an open k-cup or an open l-cap. This is a generalization of the Erdos-Szekeres theorem on cups and caps. We show a simple proof for this theorem and we also show better recurreces for g(k, l). This theorem implies results on empty polygons in k -convex sets proved by K
Jakub Cerný
 Added 10 Dec 2010 Updated 10 Dec 2010 Type Journal Year 2008 Where EJC Authors Jakub Cerný