# Chapter 6: Colouring Graphs

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1 17 Coloring Vertices Chapter 6: Colouring Graphs 1. Sections 17-19: Qualitative (Can we color a graph with given colors?) Section 20: Quantitative (How many ways can the coloring be done?) 2. Definitions: If G is a graph without loops, then G is k-colorable if we can assign one of k colors to each vertex in such a way that no two adjacent vertices have the same color. If G is k-colorable but not (k 1)-colorable, then the chromatic number (χ(g)) of G is k (G is k-chromatic). 3. Examples: No upper bound on chromatic number of a general graph: χ(k n ) = n. χ(g) = 1 if and only if G is a null graph. χ(g) = 2 if and only if G is a non-null bipartite graph. Examples where χ(g) = 3: C n and P n, where n is odd, Petersen graph Examples where χ(g) = 4: W n, where n is even 4. Theorem If G is a simple graph with largest vertex degree, then G is ( + 1)-colorable. Induction on V (G) = n. Let G be a simple graph with n vertices. Delete any vertex v: G := G v has n 1 vertices and largest vertex degree at most. By IH, G is ( + 1) colorable. The vertices adjacent to v have at most colors. Color v with any other color. 5. Theorem: (Brooks Theorem, 1941) If G is a simple connected graph which is not a complete graph, and if the largest vertex-degree of G is ( 3), then G is -colorable. (Proof in Section 18) 6. Both previous theorems are useful if vertex degrees are approximately the same. They tell little if the graph has a few vertices of large degree. (i.e.k 1,n ) 7. Theorem Every simple planar graph is 6-colorable. Induction on V (G) = n. (Trivial for n 6) Let G be a simple, planar, with n vertices, and assume that all simple planar graphs with at most n 1 vertices are 6-colorable. By Theorem 13.6 (p. 68), G contains a vertex, v, of degree at most 5. G := G v is thus 6-colorable. Color G with coloring of G and coloring v with a color different from the (at most 5) adjacent vertices.

2 8. Theorem: Every simple planar graph is 5-colorable. Induction on V (G) = n. (Trivial for n < 6) Let G be a simple, planar, with n vertices, and assume that all simple planar graphs with at most n 1 vertices are 5-colorable. By Theorem 13.6 (p.68), G contains a vertex, v, of degree at most 5. G := G v is thus 5-colorable. If deg(v) < 5, then v can be colored by any color not adjacent to v. Assume deg(v) = 5, adjacent to vertices v 1,..., v 5. If v 1,..., v 5 are mutually adjacent, then G contains K 5 as a subgraph, which is impossible since G is planar. Hence at least 2 vertices (WLOG v 1 and v 2 ) are not adjacent. Contract edges vv 1 and vv 2. Result is planar with fewer than n vertices 5-colorable. Now color v 1 and v 2 with the color originally assigned to v (w/ edges contracted). A 5-coloring of G is obtained by color v differently than the (at most 4) colors assigned to v 1,..., v Theorem: (Appel and Haken, 1976) Every simple planar graph is 4-colorable. 10. Example: (p. 86, pr. 17.7) Let G be a simple graph with n vertices, which is regular of degree d. By considering the number of vertices that can be assigned the came color, prove that χ(g) n/(n d). 11. Example: (p. 86, pr. 17.8) Let G be a simple planar graph containing no triangles. (a) Using Euler s formula, show that G contains a vertex of degree at most 3. (b) Use induction to show that G is 4-colorable. (In fact, it can be proved that G is 3-colorable.)

3 19 Colouring Maps 1. The 4-color problem Whether a map can be colored with 4 colors so that no 2 adjacent countries are shown in the same color. Raised by Francis Gurthrie in Presented to the general public (London Mathematical Society) by Cayley in Kempe published an incorrect proof in 1879, modified by Heawood in 1890 into a proof of the five color theorem. First generally accepted proof by Appel and Haken in 1977 (builds on Kempe s ideas). First shows that every plane triangulation must contain at least one of 1,482 unavoidable configurations. Second, a computer is used to show that each configuration is reducible, meaning that any plane triangulation containing such a configuration can be 4-colored by piecing together 4-colorings of smaller plane triangulations. Together, these produce an inductive proof that all plane triangulations, and hence all planar graphs can be 4-colored. Proof criticized, responded with 741 page long algorithmic version of their proof. Shorter proof more readily verifiable given by N. Robertson, D. Sanders, P.D. Seymour, and R. Thomas in Definitions: A map is a 3-connected plane graph; it contains no cutsets with 1 or 2 edges, no vertices of degree 1 or 2. A map is k-colorable(f) if its faces can be colored with k colors with no adjacent faces having the same color. A graph is k-colorable(v) if its k-colorable, as in Section Theorem: Let G be a plane graph without loops, and let G be geometric dual of G. Then G is k-colorable(v) G is k-colorable(f). Assume G is simple and connected G is a map. Assuming we have a k-coloring(v) of G, color each face of G with the color of the corresponding vertex in G. No two adjacent faces of G have the same color because the vertices they correspond to in G are adjacent and have different colors. Thus, G is k-colorable(f). Suppose we have a k-coloring(f) of G. k-color the vertices of G so that each vertex has the color of the face in G containing it. Again, no two adjacent vertices of G have the same color, and G is k-colorable.

4 4. Example: (p. 92, pr. 19.3) Give an example of a plane graph that is both 2-colorable(f) and 2-colorable(v). 5. Theorem: A map G is 2-colorable(f) G is an Eulerian graph. For every v V (G), even # of faces at v since covered with 2 colors. G is Eulerian since every vertex hence has even degree. Alternate Proof in whole: By Exercise 15.9, G is Eulerian G is bipartite. A connected graph without loops hence is 2-colorable it is bipartite. 6. Corollary: The four-color theorem for maps is equivalent to the four-color theorem for planar graphs. Proof in book. 7. Theorem: Let G be a cubic map. Then G is 3-colorable(f) each face is bounded by an even number of edges. Given any face F, the faces surrounding F must alternate in color. There must be an even number of them, so each face is bounded by even # of edges. We prove the dual result. Assume G is a simple connected plane graph where each face is a triangle and every vertex has even degree ( G is Eulerian). We must prove that G is 3-colorable(v) with colors r, y, g. By Theorem, G Eulerian G is 2-colorable(f), with colors black and whit. Color any white face so that r, y, g appear in clockwise order, counter-clockwise black faces. Vertex coloring is extended to the whole graph. 8. Theorem: In order to prove the four-color theorem, it is sufficient to prove that each cubic map is 4-colorable(f). Corollary above implies enough to show that 4-colorability(f) of every cubic map 4-colorability(f) of any map. Let G be a map, and assume that every cubic map is 4-colorable(f). Remove vertices of degree 2 without affecting coloring. Only remains to eliminate vertices of degree 4. If v has degree n 4, then cover v with an n-gon patch. Repeating this process for all such vertices, we obtain a cubic map that s 4-colorable(f) by hypothesis. 4-coloring of faces of G is obtained by shrinking each patch to a single vertex and reinstating each patch of degree 2.

5 20 Colouring Edges 1. Definitions: G is k-colorable(e) (or k-edge colorable) if its edges can be colored with k colors so that no two adjacent edges have the same color. The chromatic index, χ (G) is the number k such that G is k-colorable(e) but not (k 1)- colorable(e). 2. Theorem (Vizing, 1964) If G is a simple graph with largest vertex-degree, then 3. Chromatic index for particular graphs: χ (C 2n ) = 2, and χ (C 2n+1 ) = 3. χ (W n ) = n 1, if n 4. χ (G) + 1. Example: (p. 95, pr. 20.5) Chromatic index of Platonic graphs? 4. Theorem: χ (K n ) = n if n is odd (n 1), and χ (K n ) = n 1 if n is even. Assume n 3. (Otherwise trivial) If n is odd, place the vertices as a regular n-gon. Color n-cycle with a different color for each edge. Color remaining edges with color of boundary edge parallel to it. K n is not (n 1)-colorable(e), as the largest number of edges of the same color is (n 1)/2. It follows that K n has at most (n 1)/2 χ (K n ) = (n 1) 2 /2 edges. If n is even, K n can be built by attaching a new vertex to all vertices in K n 1. Color K n 1 as before. One color is missing at each vertex, and the colors are all different. Color new edges of K n with the missing colors.

6 5. Theorem: The four-color theorem is equivalent to the statement that χ (G) = 3 for each cubic map G. Since G is cubic, each vertex is surrounded by a tetrahedron. Assume G is 4-colored(f) by α = (1, 0), β = (0, 1), γ = (1, 1), and δ = (0, 0). Construct a 3-coloring(e) by coloring each edge e by the sum of the colors of the two adjacent faces, mod 2. (Example) cannot occur in edge coloring as 2 faces adjacent to each edge must have different colors. Furthermore, no two adjacent edges can share the same color. Suppose we have a 3-coloring(e) for G: α, β, γ edge of each color at each vertex. Subgraph determine by edges of any pair of colors (i.e. α & β) is 2-regular, hence Eulerian. Theorem 19.1 (2-colorable(f) Eulerian) implies that we can color its faces with two colors, 0 and 1. Doing this for each pair of colors, each edge is assigned two colors (x, y), where each is 0 or 1. Since coordinates of two adjacent faces must differ in at least one place, (1,0), (0,1), (1,1), (0,0) give the required 4-coloring(f). 6. Theorem: (König 1916) If G is a bipartite graph with largest vertex-degree, then χ (G) =. Proof in Book. 7. Corollary: χ (K r,s = max(r, s). 8. Example: (p. 95, pr. 20.7) Prove that if G is a cubic Hamiltonian graph, then χ (G) = 3.

7 21 Chromatic Polynomials 1. P G (k) = # of ways to color the vertices of G with k-colors (P G (k) is the chromatic function of G, soon to be chromatic polynomial.) 2. Chromatic Functions: Determine P G (k) for paths, trees, K n, C n. In each case, determine how many ways they can be colored with 5 or 6 colors. Observe that if k < χ(g), then P G (k) = 0, and k χ(g) P G (k) > 0. Four-color Theorem is equivalent to: If G is a simple planar graph, then P G (4) > Theorem: Let G be a simple graph, and let G e and G\e be the graphs obtained from G by deleting and contracting e. Then Give an example. (Book includes some G/e.) P G (k) = P G e (k) P G\e (k). Equivalent to showing that P G e (k) = P G (k) + P G\e (k). Let e = vw. # of k-colorings of G e in which v and w have different colors is unchanged if e is added equals P G (k). # of k-colorings of G e in which v and w have the same color is unchanged if v and w are identified with each other equals P G\e (k). Thus, P G e (k) = P G (k) + P G\e (k). 4. Corollary: The chromatic function of a simple graph is a polynomial. Induction on the number of edges. Basis: 0 edges P G (k) = k n, where n is the number of vertices (components). Assume true for graphs with m or fewer edges, and let G have m + 1 edges. G e and G\e have m edges, and hence P G e (k) and P G\e (k) are polynomials. Result follows since P G (k) = P G e (k) P G\e (k). Note: Quickly follows that G has n vertices P G (k) is of degree n, and the coefficient of k n is Example: (p. 99, pr. 21.4) (a) Prove that the chromatic polynomial of K 2,s is k(k 1) s + k(k 1)(k 2) s. (b) Prove that the chromatic polynomial of C n is (k 1) n + ( 1) n (k 1).

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