Adding Isolated Vertices Makes Some Greedy Online Algorithms Optimal

Joan Boyar, Christian Kudahl

Research output: Contribution to journalJournal articleResearchpeer-review


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.

Original languageEnglish
JournalDiscrete Applied Mathematics
Pages (from-to)12-21
Publication statusPublished - 10. Sep 2018


  • Greedy algorithm
  • Isolated vertices
  • Online algorithms
  • Online independence number

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