On 17.9.2015 at 12:20 in S6, there is the following noon lecture:
Drawing graphs using a small number of obstacles
An obstacle representation of a graph G is a set of points in the plane representing the vertices of G, together with a set of polygonal obstacles such that two vertices of G are connected by an edge in G if and only if the line segment between the corresponding points avoids all the obstacles. The obstacle number obs(G) of G is the minimum number of obstacles in an obstacle representation of G.
We provide the first non-trivial general upper bound on the obstacle number of graphs by showing that every n-vertex graph satisfies obs(G) <= 2n log(n). This refutes a conjecture of Mukkamala, Pach, and Pálvölgyi. For bipartite n-vertex graphs, we improve this bound to n-1. Both bounds apply even when the obstacles are required to be convex.
We also prove a lower bound Omega(hn) on the number of n-vertex graphs with obstacle number at most h for h < n and an asymptotically matching lower bound Omega(n^(4/3)M^(2/3))
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