Adding Isolated Vertices Makes Some Online Algorithms Optimal
An unexpected difference between online and offline algorithms is observed. The natural greedy algorithms are shown to be worst case online optimal for Online Independent Set and Online Vertex Cover on graphs with “enough” isolated vertices, Freckle Graphs. For Online Dominating Set, the greedy algorithm is shown to be worst case online optimal on graphs with at least one isolated vertex. These algorithms are not online optimal in general. The online optimality results for these greedy algorithms imply optimality according to various worst case performance measures, such as the competitive ratio. It is also shown that, despite this worst case optimality, there are Freckle graphs where the greedy independent set algorithm is objectively less good than another algorithm.
It is shown that it is NP-hard to determine any of the following for a given graph: the online independence number, the online vertex cover number, and the online domination number.
The authors would like to thank Lene Monrad Favrholdt for interesting and helpful discussions.
- 2.Angelopoulos, S., Dorrigiv, R., López-Ortiz, A.: On the separation and equivalence of paging strategies. In: 18th ACM-SIAM Symposium on Discrete Algorithms (SODA), pp. 229–237 (2007)Google Scholar
- 4.Boyar, J., Favrholdt, L.M., Medvedev, P.: The relative worst order ratio of online bipartite graph coloring, Unpublished manuscriptGoogle Scholar
- 5.Garey, M.R., Johnson, D.S.: Computers and Intractability: A Guide to the Theory of NP-Completeness. W. H. Freeman & Co., New York (1979). Under Dominating Set on p. 190Google Scholar
- 6.Gyárfás, A., Király, Z., Lehel, J.: On-line competitive coloring algorithms. Tech. rep. TR-9703-1, Institute of Mathematics at Eötvös Loránd University (1997). http://www.cs.elte.hu/tr97/
- 9.Halldórsson, M.M.: Online coloring known graphs. In: 10th ACM-SIAM Symposium on Discrete Algorithms (SODA), pp. 917–918 (1999)Google Scholar