**Date: **
17:00,
Tuesday, August 4, 2015

**Speaker: **
Stephan Holzer

**Notes: **

**Time: 16:00** **Day: Tuesday Room: Menger room, Favoritenstrasse 11, 3rd floor**

We study several classical graph-problems such as computing all pairs shortest paths, as well as the related problems of computing the diameter, center and girth of a network in a distributed setting.The model of distributed computation we consider is: in each synchronous round,each node can transmit a different (but short) message to each of its neighbors. For the above mentioned problems, the talk will cover algorithms running in time O(n) in the unweighted case, as well as lower bounds showing that this is essentially optimal. After extending these results to approximation algorithms and according lower bounds, the talk will provide insights into distributed verification problems. That is, we study problems such as verifying that a subgraph H of a graph G is a minimum spanning tree. It will turn out that in our setting, e.g., computing a spanning tree can take much more time than actually computing a spanning tree of G. As an application of these results we derive strong unconditional time lower bounds on the hardness of distributed approximation for many classical optimization problems including minimum spanning tree, shortest paths, and minimum cut. Many of these results are the first non-trivial lower bounds for both exact and approximate distributed computation and they resolve previous open questions. Our result implies that there can be no distributed approximation algorithm for minimum spanning tree that is significantly faster than the current exact algorithm, for any approximation factor.