Invitation link: https://tu-berlin.zoom.us/j/69716124232?pwd=dzFlcTFHMmFXRTE5QmZLaEV5N0FRUT09 Time: Monday, Jan 10, 2022 - 14:15 h Lecture: Vera Traub (ETH Zürich): Better-Than-2 Approximations for Weighted Tree AugmentationThe Weighted Tree Augmentation Problem (WTAP) is one of the most basic connectivity augmentation problems. It asks how to increase the edge-connectivity of a given graph from 1 to 2 in the cheapest possible way by adding some additional edges from a given set. There are many standard techniques that lead to a 2-approximation for WTAP, but despite much progress on special cases, the factor 2 remained unbeaten for several decades.
In this talk we present two algorithms for WTAP that improve on the longstanding approximation ratio of 2. The first algorithm is a relative greedy algorithm, which starts with a simple, though weak, solution and iteratively replaces parts of this starting solution by stronger components. This algorithm achieves an approximation ratio of (1 + ln 2 + ε) < 1.7. Second, we present a local search algorithm that achieves an approximation ratio of 1.5 + ε (for any constant ε > 0).
This is joint work with Rico Zenklusen. ===== There will be no colloquium on this Monday.