# chromatic_polynomial#

chromatic_polynomial(G)[source]#

Returns the chromatic polynomial of G

This function computes the chromatic polynomial via an iterative version of the deletion-contraction algorithm.

The chromatic polynomial X_G(x) is a fundamental graph polynomial invariant in one variable. Evaluating X_G(k) for an natural number k enumerates the proper k-colorings of G.

There are several equivalent definitions; here are three:

Def 1 (explicit formula): For G an undirected graph, c(G) the number of connected components of G, E the edge set of G, and G(S) the spanning subgraph of G with edge set S [1]:

$X_G(x) = \sum_{S \subseteq E} (-1)^{|S|} x^{c(G(S))}$

Def 2 (interpolating polynomial): For G an undirected graph, n(G) the number of vertices of G, k_0 = 0, and k_i the number of distinct ways to color the vertices of G with i unique colors (for i a natural number at most n(G)), X_G(x) is the unique Lagrange interpolating polynomial of degree n(G) through the points (0, k_0), (1, k_1), dots, (n(G), k_{n(G)}) [2].

Def 3 (chromatic recurrence): For G an undirected graph, G-e the graph obtained from G by deleting edge e, G/e the graph obtained from G by contracting edge e, n(G) the number of vertices of G, and e(G) the number of edges of G [3]:

$\begin{split}X_G(x) = \begin{cases} x^{n(G)}, & \text{if e(G)=0} \\ X_{G-e}(x) - X_{G/e}(x), & \text{otherwise, for an arbitrary edge e} \end{cases}\end{split}$

This formulation is also known as the Fundamental Reduction Theorem [4].

Parameters:
GNetworkX graph
Returns:
instance of sympy.core.add.Add

A Sympy expression representing the chromatic polynomial for G.

Notes

Interpretation of the coefficients is discussed in [5]. Several special cases are listed in [2].

The chromatic polynomial is a specialization of the Tutte polynomial; in particular, X_G(x) = T_G(x, 0) [6].

The chromatic polynomial may take negative arguments, though evaluations may not have chromatic interpretations. For instance, X_G(-1) enumerates the acyclic orientations of G [7].

References

[1]

D. B. West, “Introduction to Graph Theory,” p. 222

[2] (1,2)

E. W. Weisstein “Chromatic Polynomial” MathWorld–A Wolfram Web Resource https://mathworld.wolfram.com/ChromaticPolynomial.html

[3]

D. B. West, “Introduction to Graph Theory,” p. 221

[4]

J. Zhang, J. Goodall, “An Introduction to Chromatic Polynomials” https://math.mit.edu/~apost/courses/18.204_2018/Julie_Zhang_paper.pdf

[5]

R. C. Read, “An Introduction to Chromatic Polynomials” Journal of Combinatorial Theory, 1968 https://math.berkeley.edu/~mrklug/ReadChromatic.pdf

[6]

W. T. Tutte, “Graph-polynomials” Advances in Applied Mathematics, 2004 https://www.sciencedirect.com/science/article/pii/S0196885803000411

[7]

R. P. Stanley, “Acyclic orientations of graphs” Discrete Mathematics, 2006 https://math.mit.edu/~rstan/pubs/pubfiles/18.pdf

Examples

>>> C = nx.cycle_graph(5)
>>> nx.chromatic_polynomial(C)
x**5 - 5*x**4 + 10*x**3 - 10*x**2 + 4*x

>>> G = nx.complete_graph(4)
>>> nx.chromatic_polynomial(G)
x**4 - 6*x**3 + 11*x**2 - 6*x