### Abstract

On-line coloring of a graph is the following process. The graph is given vertex by vertex (with adjacencies to the previously given vertices) and for the actual vertex a color different from the colors of the neighbors must be irrevocably assigned. The on-line chromatic number of a graph G, χ*(G) is the minimum number of colors needed to color on-line the vertices of G (when it is given in the worst order). A graph G is on-line k-critical if χ*(G^{1}) = k, but χ*(G^{1}) < k for all proper induced subgraphs G^{1} ⊂ G. We show that there are finitely many (51) connected on-line 4-critical graphs but infinitely many disconnected ones. This implies that the problem whether χ*(G) ≤3 is polynomially solvable for connected graphs but leaves open whether this remains true without assuming connectivity. Using the structure descriptions of connected on-line 3-chromatic graphs we obtain one algorithm which colors all on-line 3-chromatic graphs with 4 colors. It is a tight result. This is a companion paper of [1] in which we analyze the structure of triangle-free on-line 3-chromatic graphs.

Original language | English |
---|---|

Pages (from-to) | 99-122 |

Number of pages | 24 |

Journal | Discrete Mathematics |

Volume | 177 |

Issue number | 1-3 |

DOIs | |

Publication status | Published - Dec 1 1997 |

### Fingerprint

### ASJC Scopus subject areas

- Theoretical Computer Science
- Discrete Mathematics and Combinatorics

### Cite this

*Discrete Mathematics*,

*177*(1-3), 99-122. https://doi.org/10.1016/S0012-365X(96)00359-7