List coloring

In graph theory, a branch of mathematics, list coloring is a type of graph coloring where each vertex can be restricted to a list of allowed colors, first studied by Vizing and by Erdős, Rubin, and Taylor.

Definition

Given a graph G and given a set L(v) of colors for each vertex v (called a list), a list coloring is a choice function that maps every vertex v to a color in the list L(v). As with graph coloring, a list coloring is generally assumed to be proper, meaning no two adjacent vertices receive the same color. A graph is k-choosable (or k-list-colorable) if it has a proper list coloring no matter how one assigns a list of k colors to each vertex. The choosability (or list colorability or list chromatic number) ch(G) of a graph G is the least number k such that G is k-choosable.

More generally, for a function f assigning a positive integer f(v) to each vertex v, a graph G is f-choosable (or f-list-colorable) if it has a list coloring no matter how one assigns a list of f(v) colors to each vertex v. In particular, if $f(v) = k$ for all vertices v, f-choosability corresponds to k-choosability.

Example

File:List-coloring-K-3-27.svg

A list coloring instance on the complete bipartite graph K_3,27 with three colors per vertex. No matter which colors are chosen for the three central vertices, one of the outer 27 vertices will be uncolorable, showing that the list chromatic number of K_3,27 is at least four.

Let q be a positive integer, and let G be the complete bipartite graph K_q,q^q. Let the available colors be represented by the q² different two-digit numbers in radix q. On one side of the bipartition, let the q vertices be given sets of colors {i0, i1, i2, ...} in which the first digits are equal to each other, for each of the q possible choices of the first digit i. On the other side of the bipartition, let the q^q vertices be given sets of colors {0a, 1b, 2c, ...} in which the first digits are all distinct, for each of the q^q possible choices of the q-tuple (a, b, c, ...). For instance, for q = 2, this construction produces the graph K_2,4. In this graph, the two vertices on one side of the bipartition have color sets {00,01} and {10,11} and the four vertices on the other side of the bipartition have color sets {00,10}, {00,11}, {01,10}, and {01,11}. The illustration shows a larger example of the same construction, with q = 3.

Then, G does not have a list coloring for L: no matter what set of colors is chosen for the vertices on the small side of the bipartition, this choice will conflict with all of the colors for one of the vertices on the other side of the bipartition. For instance if the vertex with color set {00,01} is colored 01, and the vertex with color set {10,11} is colored 10, then the vertex with color set {01,10} cannot be colored. Therefore, the list chromatic number of G is at least q + 1.

Similarly, if $n=\binom{2k-1}{k}$, then the complete bipartite graph K_n,n is not k-choosable. For, suppose that 2k − 1 colors are available in total, and that, on a single side of the bipartition, each vertex has available to it a different k-tuple of these colors than each other vertex. Then, each side of the bipartition must use at least k colors, for otherwise some vertex would remain uncolored, but this implies that some two adjacent vertices have the same color. In particular, the utility graph K_3,3 has chromatic index at least three, and the graph K_10,10 has chromatic index at least four.

Properties

Choosability ch(G) satisfies the following properties for a graph G with n vertices, chromatic number χ(G), and maximum degree Δ(G):

1. ch(G) ≥ χ(G). A k-list-colorable graph must in particular have a list coloring when every vertex is assigned the same list of k colors, which corresponds to a usual k-coloring.
2. ch(G) cannot be bounded in terms of chromatic number in general, that is, ch(G) ≤ f(χ(G)) does not hold in general for any function f. In particular, as the complete bipartite graph examples show, there exist graphs with χ(G) = 2 but with ch(G) arbitrarily large.
3. ch(G) ≤ χ(G) ln(n).
4. ch(G) ≤ Δ(G) + 1.
5. ch(G) ≤ 5 if G is a planar graph.
6. ch(G) ≤ 3 if G is a bipartite planar graph.

Computing choosability and (a,b)-choosability

Two algorithmic problems have been considered in the literature:

1. k-choosability: decide whether a given graph is k-choosable for a given k, and
2. (j,k)-choosability: decide whether a given graph is f-choosable for a given function $f : V \to \{j,\dots,k\}$.

It is known that k-choosability in bipartite graphs is $\Pi^p_2$-complete for any k ≥ 3, and the same applies for 4-choosability in planar graphs, 3-choosability in planar triangle-free graphs, and (2,3)-choosability in bipartite planar graphs. For P₅-free graphs, that is, graphs excluding a 5-vertex path graph, k-choosability is fixed-parameter tractable.

It is possible to test whether a graph is 2-choosable in linear time by repeatedly deleting vertices of degree zero or one until reaching the 2-core of the graph, after which no more such deletions are possible. The initial graph is 2-choosable if and only if its 2-core is either an even cycle or a theta graph formed by three paths with shared endpoints, with two paths of length two and the third path having any even length.

Applications

List coloring arises in practical problems concerning channel/frequency assignment.

See also

• List edge-coloring

References

Further reading

• , Chapter 34 Five-coloring plane graphs.
• Diestel, Reinhard. Graph Theory. 3rd edition, Springer, 2005. Chapter 5.4 List Colouring. electronic edition available for download