## Abstract

The chromatic polynomial PΓ(x) of a graph Γ is a polynomial whose value at the positive integer k is the number of proper k-colourings of Γ. If G is a group of automorphisms of Γ, then there is a polynomial OP_{Γ,G}(x), whose value at the positive integer k is the number of orbits of G on proper k-colourings of Γ. It is known that real chromatic roots cannot be negative, but they are dense in [57,00), Here we discuss the location of real orbital chromatic roots. We show, for example, that they are dense in ℝ, but under certain hypotheses, there are zero-free regions. We also look at orbital flow roots. Here things are more complicated because the orbit count is given by a multivariate polynomial; but it has a natural univariate specialization, and we show that the roots of these polynomials are dense in the negative real axis.

Original language | English |
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Pages (from-to) | 401-407 |

Number of pages | 7 |

Journal | Combinatorics Probability and Computing |

Volume | 16 |

Issue number | 3 |

DOIs | |

Publication status | Published - 1 May 2007 |