Algorithms for Secretary Problems on Graphs and Hypergraphs

  • Authors:
  • Nitish Korula;Martin Pál

  • Affiliations:
  • Dept. of Computer Science, University of Illinois, Urbana 61801;Google Inc., New York, NY 10011

  • Venue:
  • ICALP '09 Proceedings of the 36th Internatilonal Collogquium on Automata, Languages and Programming: Part II
  • Year:
  • 2009

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Abstract

We examine online matching problems with applications to Internet advertising reservation systems. Consider an edge-weighted bipartite graph G (L *** R , E ). We develop an 8-competitive algorithm for the following secretary problem: Initially given R , and the size of L , the algorithm receives the vertices of L sequentially, in a random order. When a vertex l *** L is seen, all edges incident to l are revealed, together with their weights. The algorithm must immediately either match l to an available vertex of R , or decide that l will remain unmatched. In [5], the authors show a 16-competitive algorithm for the transversal matroid secretary problem, which is the special case with weights on vertices, not edges. (Equivalently, one may assume that for each l *** L , the weights on all edges incident to l are identical.) We use a very similar algorithm, but simplify and improve the analysis to obtain a better competitive ratio for the more general problem. Our analysis is easily extended to obtain competitive algorithms for a class of similar problems, such as to find disjoint sets of edges in hypergraphs where edges arrive online. We also introduce secretary problems with adversarially chosen groups . Finally, we give a 2e -competitive algorithm for the secretary problem on graphic matroids, where, with edges appearing online, the goal is to find a maximum-weight acyclic subgraph of a given graph.