# chromatic number
## English
### Noun

chromatic number (plural chromatic numbers)
* (graph theory) The smallest number of colours needed to colour a given graph (i.e., to assign a colour to each vertex such that no two vertices connected by an edge have the same colour).
    The chromatic number of a complete graph $K_{n}$ is $n$; the chromatic number of a bipartite graph $K_{n,m}$ is 2.
** 1995, J. A. Bondy, “1: Basic Graph Theory: Paths and Circuits”, in Ronald L. Graham, Martin Grötschel, László Lovász, editors, Handbook of Combinatorics, Volume 1, Elsevier (North-Holland), page 48:The chromatic number of a graph $G$ is the minimum value of $k$ for which $G$ is $k$-colourable, and is denoted by $\chi (G)$. […] A more essential use of the chromatic number was made by Gallai (1968a) and Roy (1967), who discovered a simple relationship between the chromatic number of a digraph and the length of a longest directed path in the digraph, where the chromatic number $\chi (D)$ of a digraph $D$ is defined to be the chromatic number of its underlying graph $G(D)$.
** 2004, Monia Discepoli, Ivan Gerace, Riccardo Mariani, Andrea Remigi, A Spectral Technique to Solve the Chromatic Number Problem in Circulant Graphs, Antonio Laganà, et al. (editors), Computational Science and Its Applications, ICCSA 2004: International Conference, Proceedings, Part 3, Springer, LNCS 3045, page 745,
    The CHROMATIC NUMBER is the minimum number of colors by means of which it is possible to color a graph in such a way that each vertex has a different color with respect to the adjacent vertices. Such a problem is an NP-hard problem [14] and [it] is even hard to obtain a good approximation of the solution in a polynomial time [17]. Although in a lot of computational problems the cost decreases when these problems are restricted to circulant graphs [6, 9], the CHROMATIC NUMBER problem is NP-hard even restrecting to circulant graphs [9]. Moreover the problem of finding a good approximation of the CHROMATIC NUMBER problem on circulant graphs is also NP-hard.
** 2009, Gary Chartrand, Ping Zhang, Chromatic Graph Theory, Taylor & Francis Group (CRC Press / Chapman & Hall), page 149,
    There is no general formula for the chromatic number of a graph. Consequently, we will often be concerned and must be content with (1) determining the chromatic number of some classes of interest and (2) determining upper and/or lower bounds for the chromatic number of a graph.

#### Usage notes

* Not to be confused with chromatic index (aka edge-chromatic number), which is the equivalent minimum number for an edge colouring.
* The chromatic number of a graph $G$ is often denoted $\chi (G)$.

#### Synonyms

* (smallest number of colours needed to colour the vertices of a graph): vertex chromatic number (used to differentiate from edge chromatic number, etc.)

#### Derived terms

* chromatic number problem
* edge chromatic number
* harmonious chromatic number
* total chromatic number

#### Related terms

* achromatic number
* chromatic index
* chromatic polynomial

#### Translations
### See also

* k-colouring
* k-chromatic

### Further reading

* Graph coloring on Wikipedia.Wikipedia
* Edge coloring on Wikipedia.Wikipedia
* Total coloring on Wikipedia.Wikipedia
* Hadwiger conjecture (graph theory) on Wikipedia.Wikipedia