Jacob Holm receives Best Paper Award at SODA’18 – University of Copenhagen

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21 November 2017

Jacob Holm receives Best Paper Award at SODA’18

Best Paper Award

PhD student at DIKU Jacob Holm has in cooperation with Aaron Bernstein (Columbia University, USA) and Eva Rotenberg (DTU) received the Best Paper Award at ACM-SIAM Symposium on Discrete Algorithm (SODA’18) in New Orleans, USA for the article Online Bipartite Matching with Amortized O(log² n) Replacements.

The SODA symposium, which is the leading algorithm event, focuses on research topics related to efficient algorithms and data structures for discrete problems.

Jacob Holms paper is 1 out of 10 accepted papers at SODA'18 authored or co-authored by members of the new research centre at DIKU, Basic Algorithms Research Copenhagen (BARC). 

The work on the article has been made while Eva Rotenberg, co-author of the article, also was PhD student at DIKU.

Abstract

In the online bipartite matching problem with replacements, all the vertices on one side of the bipartition are given, and the vertices on the other side arrive one by one with all their incident edges. The goal is to maintain a maximum matching while minimizing the number of changes (replacements) to the matching. We show that the greedy algorithm that always takes the shortest augmenting path from the newly inserted vertex (denoted the SAP protocol) uses at most amortized O(log² n)  replacements per insertion, where n  is the total number of vertices inserted. This is the first analysis to achieve a polylogarithmic number of replacements for any replacement strategy, almost matching the Ω(log n)  lower bound. The previous best known strategy achieved amortized O(√n)  replacements [Bosek, Leniowski, Sankowski, Zych, FOCS 2014]. For the SAP protocol in particular, nothing better than then trivial O(n)  bound was known except in special cases.
Our analysis immediately implies the same upper bound of O(log² n)  reassignments for the capacitated assignment problem, where each vertex on the static side of the bipartition is initialized with the capacity to serve a number of vertices.
We also analyze the problem of minimizing the maximum server load. We show that if the final graph has maximum server load L , then the SAP protocol makes amortized O(min{L log² n, √n log n})  reassignments. We also show that this is close to tight because Ω(min{L, √n})  reassignments can be necessary.