# chromaticNumber -- computes the chromatic number of a hypergraph

## Synopsis

• Usage:
c = chromaticNumber H
• Inputs:
• H, ,
• Outputs:
• c, an integer, the chromatic number of H

## Description

Returns the chromatic number, the smallest number of colors needed to color the vertices of a graph. This method is based upon a result of Francisco-Ha-Van Tuyl that relates the chromatic number to an ideal membership problem.

 i1 : S = QQ[a..f]; i2 : c4 = cycle(S,4) -- 4-cycle; chromatic number = 2 o2 = Graph{edges => {{a, b}, {b, c}, {c, d}, {a, d}}} ring => S vertices => {a, b, c, d, e, f} o2 : Graph i3 : c5 = cycle(S,5) -- 5-cycle; chromatic number = 3 o3 = Graph{edges => {{a, b}, {b, c}, {c, d}, {d, e}, {a, e}}} ring => S vertices => {a, b, c, d, e, f} o3 : Graph i4 : k6 = completeGraph S -- complete graph on 6 vertices; chormatic number = 6 o4 = Graph{edges => {{a, b}, {a, c}, {a, d}, {a, e}, {a, f}, {b, c}, {b, d}, {b, e}, {b, f}, {c, d}, {c, e}, {c, f}, {d, e}, {d, f}, {e, f}}} ring => S vertices => {a, b, c, d, e, f} o4 : Graph i5 : chromaticNumber c4 o5 = 2 i6 : chromaticNumber c5 o6 = 3 i7 : chromaticNumber k6 o7 = 6

## Caveat

This method should not be used with a hypergraph that has an edge of cardinality one since no coloring is possible.

## Ways to use chromaticNumber :

• "chromaticNumber(HyperGraph)"

## For the programmer

The object chromaticNumber is .