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`Graph`::`chromaticPolynomial`

Calculates a chromatic polynomial

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Syntax

```Graph::chromaticPolynomial(`G`, `x`)
```

Description

`Graph::chromaticPolynomial(G, x)` returns the chromatic polynomial of the graph `G`. Evaluating the result at x = n, for any integer n, gives the number of possible ways to color the graph `G` using n colors such that no two adjacent vertices have the same color.

`G` must be an undirected graph: if an edge goes from a tob, another edge must go from b to a, for any two verticesa, b.

Examples

Example 1

We compute the chromatic polynomial of the complete graph with 5 vertices:

`f:= Graph::chromaticPolynomial(Graph::createCompleteGraph(5), x)`

There are 240 ways to color a complete graph with 5 vertices, since this is the number of bijective mappings between the set of colors and the set of vertices:

`f(5)`

`delete f:`

Example 2

Now let us delete one edge from a complete graph:

```G:= Graph::createCompleteGraph(5): G:= Graph::removeEdge(G, [[2, 3]]): G:= Graph::removeEdge(G, [[3, 2]])```

Now there are some additional possible colourings: vertices 2 and 3 may now have the same color, in five different ways; in each case, there must be one of the four remaining colors that does not occur at all. In each of the 20 cases, we are left with 3 vertices that form a complete graph and 3 colors, such that there are 6 colourings. Altogether this gives us 120 additional colourings:

`Graph::chromaticPolynomial(G, x)(5)`

Parameters

 `G` An undirected graph `x` An identifier

polynomial

References

See Birkhoff and Lewis, Chromatic Polynomials, Trans. AMS, Vol. 60, p.355–451, 1946.

Algorithms

Computing the chromatic polynomial of a graph with n vertices reduces to computing two chromatic polynomials of graphs with n - 1 vertices. The running time is hence roughly 2n.