## Abstract

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.

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 language | English |
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Title of host publication | Combinatorial Algorithms : 26th International Workshop, IWOCA 2015, Verona, Italy, October 5-7, 2015, Revised Selected Papers |

Editors | Zsuzsanna Lipták, William F. Smyth |

Publisher | Springer |

Publication date | 2016 |

Pages | 65-76 |

ISBN (Print) | 978-3-319-29515-2 |

ISBN (Electronic) | 978-3-319-29516-9 |

DOIs | |

Publication status | Published - 2016 |

Event | 26th International Workshop on Combinatorial Algorithms - Verona, Italy Duration: 5. Oct 2015 → 7. Oct 2015 |

### Conference

Conference | 26th International Workshop on Combinatorial Algorithms |
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Country | Italy |

City | Verona |

Period | 05/10/2015 → 07/10/2015 |

Series | Lecture Notes in Computer Science |
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Volume | 9538 |

ISSN | 0302-9743 |