# MT4514: Graph Theory. Colva M. Roney-Dougal

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1 MT4514: Graph Theory Colva M. Roney-Dougal February 5, 2009

2 Contents 1 Introduction 3 1 About the course Some introductory examples Basic definitions and isomorphism 8 1 Some definitions Isomorphism Some families of graphs Degree sequences Paths on graphs 15 1 Definitions Eulerian circuits Hamiltonian Circuits Planarity and Colouring 21 1 Euler s Formula and Kuratowski s Theorem Regular Polyhedra Colouring Planar Graphs Colouring non-planar graphs The chromatic polynomial The chromatic number of the line and plane Marriage, Matchings and Connectivity 33 1 Hall s Marriage Theorem Edge colourings of graphs Factorisation of graphs Connectivity of graphs Trees 42 1 Characterisation of Trees Counting Trees Ramsey Theory 48 1 Graphs with no triangles Ramsey Numbers Ramsey numbers for several colours Adjacency matrices 53 1 Definitions and basic results Eigenvalues The eigenvalue method

3 Chapter 1 Introduction 1. About the course The course will not be solely based on a single book. Therefore, the best study source will be the lecture notes. Some useful texts are: 1. Robin Wilson, Introduction to Graph Theory 2. Robin Wilson and John Watkins, Graphs an Introductory Approach. 3. Frank Harary, Graph Theory. 4. Norman Biggs, Discrete Mathematics All these books, as well as all tutorial sheets and solutions, will be available in Mathematics/Physics library on short loan. Also, any other book containing in its title the words such as graph theory, discrete mathematics, combinatorics is likely to contain material relevant to the course. It will be useful to bring coloured pens or pencils to lectures, although I ve had to do these notes in black and white. 2. Some introductory examples Graphs are a very useful way of recording information about objects and relationships between pairs of objects. A graph consists of a set V of vertices or points and a collection E of edges joining some of the pairs of points. In this introduction we ll look at a range of examples to indicate the flavour and variety of the topic. Graph theory goes back to ancient Greek times, with the study of the 5 regular platonic solids, but it really started with the following problem. Example 2.1. Königsberg Bridge problem. Consider this town plan of Königsberg (now Kaliningrad) in East Prussia. The river Pregel runs through the middle of town, and a set of 7 bridges links the banks and two islands in the river. The problem is to find a walk through the town that begins and ends at the same place, crossing each bridge exactly once. 3

4 4 Colva M. Roney-Dougal Euler in 1736 realised that we can simplify the problem by representing it with this graph, with one edge for each bridge, and vertices representing each island and each bank. The problem now is to find a path around the graph which begins and ends at the same vertex and follows each edge exactly once. Looking at this graph, Euler was able to show that the problem has no solution. Example 2.2. Knight s tour. Given an n m chessboard, we ask whether it s possible to find a route for a knight that visits each square on the board exactly once. We represent the 3 4 chessboard with a graph like this, with one vertex for each square, and edges linking squares that a knight can move between in a single go: A knight s tour corresponds to finding a path on the graph that visits each vertex exactly once. Euler considered the problem of the 8 8 board. Example 2.3. Logical games. Consider a game such as noughts and crosses, where there is no element of chance. We can draw a graph whose vertices represent all possible positions during a game. The edges of the graph connect each position to those positions which can be reached by a single move. A strategic analysis of the game is possible by looking at the paths in the graph. Example 2.4. Paths on a die. To find a circuit on a die visiting each face exactly once we consider this graph:

5 Graph Theory 5 There is one vertex for each face, and an edge connecting adjacent faces. We want to find a closed circuit on the graph visiting each vertex exactly once. One solution is 1, 3, 6, 5, 4, 2, 1. Problem: How many such paths are there? Example 2.5. Maps. Information on maps can often be presented with a graph, where edges may also be labelled with distances. An example with no distances is the London underground map. Here we give a more local map with distances: A famous problem connected with graphs representing maps is the Travelling Salesman Problem: find the shortest path that visits each town at least once and returns to the start. Example 2.6. People at a party. Given a group of six people, we can always find either three who all know each other or three who don t know each other. We can represent this with this graph, with one vertex for each person. We put a red edge between two vertices if the two people know each other, and a green edge if they do not. The previous example is equivalent to the following: Theorem 2.7. Let (V, E) be a graph with six vertices and an edge between every pair of vertices that is either red or green. Then (V, E) contains either a red triangle or a green triangle. Proof. Let v V be any vertex. Then v has five adjacent edges, so either at least three red edges or at least three green edges leave v. Without loss of generality we may assume that at least three red edges leave v, and that they join v to vertices v 1, v 2, v 3 :

6 6 Colva M. Roney-Dougal Now consider the edges v 1 v 2, v 1 v 3 and v 2 v 3. If any of them, say v i v j, is red then we have found a red triangle vv i v j. If none of them is red then we have found a green triangle v 1 v 2 v 3. Similarly, in a group of 18 people there are 4 mutual acquaintances or nonacquaintances, and we know that in a group of 17 people there may not be such a group of four people. In a group of 49 people there are 5 mutual acquaintances or non-acquaintances, but we do not know if 49 is the smallest number that guarantees the existence of 5 acquaintances or non-acquaintances: the smallest such number may be as low as 43. Example 2.8. A puzzle: missionaries and cannibals. Two missionaries and two cannibals need to cross a river from west to east in a boat holding at most two people. To avoid being eaten, if there are any missionaries on a bank then there must be at least as many missionaries as cannibals. We make a graph whose vertices are pairs (M, C) describing how many missionaries and cannibals are on the west bank of the river. We put arrows of one type indicating boat trips from west to east, and of another type to indicate boat trips from east to west. To find a solution we need to find a path of alternating types of arrows (as we must always bring the boat back from the other side), that starts with the vertex (2, 2) and ends with the vertex (0, 0). A possible solution is: (2, 2) (1, 1) (2, 1) (0, 1) (0, 2) (0, 0). That is, one missionary and one cannibal cross, the missionary comes back, then two missionaries cross, then the cannibal comes back, then both cannibals cross. Example 2.9. Further examples include The internet: vertices for computers and edges for connections, maybe labelled by their bandwidth. Graphs of groups, for example the automorphism group of the following graph is A 4 :

7 Graph Theory 7 Chemistry: diagrams of saturated hydrocarbons. Management structures. Network flows, such as the national grid, the water supply.

8 Chapter 2 Basic definitions and isomorphism In this chapter we give some of the formal definitions of graph theory, and establish some elementary properties of graphs. 1. Some definitions Definition 1.1. A simple graph G consists of a finite set V of vertices (also called points or nodes) and a set E of edges such that each edge joins a pair of vertices in V. More formally, a simple graph is a pair (V, E) where V is a finite set and E is a set of subsets of V, each of size 2. When V and E are finite sets we write G or V to denote the number of vertices of G. The graph G is finite when V and E are finite. We denote the edge joining v i and v j by v i v j. Note that v i v j = v j v i as the edges of a graph do not have a direction. Example 1.2. Here are some finite and infinite graphs: Definition 1.3. A loop is an edge joining a vertex to itself. A graph has multiple edges if there exist two vertices with more than one edge between them. In general, a graph may have loops and multiple edges: a simple graph is a finite graph with no loops and no multiple edges. Often we will simply say graph for finite graph when the context is clear. Definition 1.4. A graph is connected if it is possible to go between any pair of vertices by traversing a sequence of edges. If a graph G is not connected then we may consider G as a disjoint union of connected graphs, known as the connected components of G. Example 1.5. The graph G is connected, the graph H is unconnected and has two connected components. 8

9 Graph Theory 9 Definition 1.6. Two vertices v 1, v 2 V are adjacent if there is an edge joining v 1 to v 2. The degree or valency of a vertex v is the number of edges that have one end at v: each loop at a vertex v contributes 2 to the valency of v. The graph is regular if all vertices have the same degree. Example 1.7. This graph has vertices labelled by their valencies. Lemma 1.8 (Handshaking lemma) The sum of all of the degrees of all vertices of a finite graph is even. Proof. This is clear since each edge contributes 1 to the degree of two vertices, or 2 to a single one. Definition 1.9. A graph G is planar if G can be drawn in the plane (i.e. on a flat piece of paper) without the edges crossing. Example G is a planar graph, H is not. 2. Isomorphism The way that we draw a graph, or the nature of the set V, does not matter when we are studying graphs. Consider the bijection φ between the following graphs, which shows that they are essentially the same:

10 10 Colva M. Roney-Dougal Definition 2.1. The simple graphs G = (V, E) and G = (V, E ) are isomorphic if there exists a bijection φ : V V such that v 1 v 2 is an edge of E if and only if φ(v 1 )φ(v 2 ) is an edge of E. If an isomorphism exists then we write G = G. Two graphs that are isomorphic share many properties in an obvious way: They have the same number of vertices. They have the same number of edges. Corresponding vertices have equal valencies. Example 2.2. Here are three nonisomorphic graphs on 7 vertices. To see that they are pairwise nonisomorphic, note that the graph A has more edges than B or C. The graph B has a vertex of degree 6 whereas the graph C does not. Example 2.3. Problem: Find a complete set of non-isomorphic simple graphs with a given number of vertices. For instance with 3 vertices we find: The following information is known: G # of simple graphs Example 2.4. Two isomorphic graphs: Two non-isomorphic graphs: These graphs both have 9 vertices, 12 edges, 1 vertex of degree 4, 4 vertices of degree 3 and 4 vertices of degree 2. However, they are not isomorphic as the vertex of degree 4 is joined to vertices of degree 3 in the first graph, and vertices of degree 2 in the second graph.

11 3. Some families of graphs Graph Theory 11 Definition 3.1. The complete graph on n vertices is denoted K n, and is the graph on n vertices where every pair of distinct vertices is joined by an edge. Every vertex of K n has degree (n 1) so there are n(n 1)/2 edges. Definition 3.2. A bipartite graph is one where the vertices are divided into two classes, R and B say, with the property that every edge has one end in R and the other end in B. For example: We denote by K m,n the complete bipartite graph with R = m and B = n and every vertex of R joined to every vertex of B. Here are some other kinds of graph. A tree is a connected graph which contains no circuits. 4. Degree sequences Definition 4.1. Let G be a simple graph. The degree sequence of G is the list of degrees of the vertices of G, usually in decreasing order. For instance the following graph has degree sequence 4, 3, 3, 2, 2:

12 12 Colva M. Roney-Dougal Given a sequence of non-negative integers, how can we tell whether there is a graph with that as a degree sequence? For example, 6, 6, 5, 5, 3, 3, 3, 3 is the degree sequence of a simple graph, but 6, 6, 5, 5, 2, 2, 2, 2 is not. Theorem 4.2. The following are necessary conditions for d 1,..., d v to be a degree sequence. 1. The sum of the d i is even. 2. We have d i v 1 for 1 i v. 3. At least two vertices have equal degree. Proof. 1. The sum of the degrees is twice the number of edges (recall the Handshaking Lemma, Chapter 1, 1.8). 2. In a simple graph, each vertex can be joined to at most v 1 others. 3. Suppose not, then by (2) the degrees must be v 1, v 2,..., 1, 0. Therefore the vertex of degree v 1 must be joined to all other vertices, including that of degree 0. Remark 4.3. Nonisomorphic graphs can have the same degree sequences. example consider the following two graphs: For They both have degree sequence 3, 3, 2, 2, 2 but are nonisomorphic since the first contains a triangle but the other does not. Lemma 4.4. Let a graph G have the degree sequence s v 1 v 2 v s d 1 d k and vertices S, V i and D j of corresponding degrees. Then there exists a graph H with the same degree sequence such that S is joined to V 1, V 2,..., V s. Proof. Suppose that in G, the vertex S is not joined to at least one of the V i. Then S must be joined to at least one D j. Recall that v i d j. If v i = d j then we may swap the labels V i and D j on the graph to get a graph with S joined to one more of the V i and one less of the D j vertices. Otherwise v i > d j. Then there is some vertex W joined to V i but not to D j. Consider the graph G obtained from G by removing edges SD j and W V i and replacing them by edges SV i and W D j :

13 Graph Theory 13 Then none of the vertex degrees have changed, but S is joined to one more of the V i and one fewer of the D j vertices. In particular, G has degree sequence (1), so we may replace G by G. We may then repeat these two processes to get a graph H with S joined to all of the V i, proving the lemma. Theorem 4.5. Consider the sequences s, v 1, v 2,..., v s, d 1,..., d k (1) and v 1 1, v 2 1,..., v s 1, d 1, d 2,..., d k (2), where (1) is written in decreasing order, v i 1, and v i, d i N for all i. Then (1) is a degree sequence of a simple graph if and only if (2) is the degree sequence of a simple graph. For example 6, 6, 5, 5, 2, 2, 2, 2 is a degree sequence if and only if 5, 4, 4, 2, 1, 1, 1 is a degree sequence, if and only if 3, 3, 1, 1, 0, 0 is a degree sequence, if and only if 2, 0, 0, 0, 0 is a degree sequence. Since this final sequence is clearly not the degree sequence of a simple graph, none of the preceding ones are. Proof. : If (2) is the degree sequence of some graph then by adding one further vertex S joined to the vertices of degree v 1 1, v 2 1,..., v s 1 we get a graph with degree sequence (1). : Let G have degree sequence (1) and write S, V i, D j for the vertices with degrees s, v i and d j respectively. The graph H given by Lemma 4.4 has the same degree sequence as G, and in H the vertex S is joined to V 1,..., V s. By removing the vertex S and edges SV 1, SV 2,..., SV s we get a graph with degree sequence (2). Example 4.6. sum is odd. 1. The sequence 4, 3, 3, 2, 1 is not a degree sequence since the 2. Consider the sequence 6, 5, 4, 3, 2, 2, 2. Applying the theorem we get a sequence 4, 3, 2, 1, 1, 1, and hence a sequence 2, 1, 1, 0, 0. From this we obtain 0, 0, 0, 0 which is obviously a degree sequence. We use the theorem in reverse to construct a graph with the original degree sequence:

14 14 Colva M. Roney-Dougal 3. Consider the sequence 7, 7, 7, 6, 6, 5, 2, 2, 1, 1. Applying the theorem we get a sequence 6, 6, 5, 5, 4, 1, 1, 1, 1, then 5, 4, 4, 3, 1, 1, 0, 0, and then 3, 3, 2, 0, 0, 0, 0 ( ). There is no graph with degree sequence, as in order for a vertex to have degree 3 there must be at least four vertices of nonzero degree. Hence by the theorem there is no graph with the original degree sequence. 4. Consider the sequence 7, 7, 6, 5, 4, 4, 4, 2, 1. Applying the theorem we get a sequence 6, 5, 4, 3, 3, 3, 1, 1, which yields 4, 3, 2, 2, 2, 1, 0. In turn we now get 2, 1, 1, 1, 1, 0 and then 1, 1, 0, 0, 0. This last sequence is the degree sequence of a graph with 5 vertices and a single edge, so all of these sequences are degree sequences of simple graphs.

15 Chapter 3 Paths on graphs 1. Definitions Throughout this chapter, G will be a finite connected graph. Definition 1.1. A path on G is a sequence of vertices v 1, v 2,..., v n such that v i v i+1 is an edge of G for 1 i n 1. A circuit is a path v 1,..., v n such that v n v 1 is also an edge. Example 1.2. In the first graph a circuit is marked, in the second a path is marked. Definition 1.3. A connected graph is Eulerian if there exists a circuit using every edge exactly once. It is semi-eulerian if there is a path including every edge exactly once. It is Hamiltonian if there exists a circuit including every vertex exactly once, and semi-hamiltonian if there exists a path including every vertex exactly once. Example 1.4. Graph 1 is semi-eulerian and semi-hamiltonian. Graph 2 is Eulerian and Hamiltonian (follow the arrows). Graph 3 is Hamiltonian but not Eulerian or semi-eulerian. Do you recognise graph 3? 2. Eulerian circuits Theorem 2.1 (Euler s Theorem) A connected graph is Eulerian if and only if every vertex has even degree. Proof. : A circuit goes into and then out of each vertex an unknown number of times, each time using two edges (one to get in and then one to get out). Thus there are an even number of edges incident to each vertex. 15

16 16 Colva M. Roney-Dougal : Start at any vertex v and walk around the graph using no edge more than once. Since each vertex has even degree, we will always have a way out of each vertex until we return to vertex v. Call this circuit C 1. If C 1 contains all of the edges then we are done. If not, the remaining edges are such that an even number of then are incident to each vertex, so we may repeat our initial process to get further circuits C 2, C 3,..., C k, until all of the edges are used in exactly one of the C i. If k > 1 we now join up the circuits to form a single circuit. Since the graph is connected, C 1 must have a vertex in common with at least one C j for j > 1. We may amalgamate C 1 and C j to form a single circuit by rerouting at the common vertex: Repeating this we may combine C 1,..., C k into a single path including each edge exactly once. Example 2.2. In the following graph, we note that all vertices have even degree, so we may find an Eulerian circuit. In the final graph we amalgamate C 1, C 2, C 3 to make a single circuit. Example 2.3. It is immediate from Euler s theorem that the Bridges of Königsberg graph from Chapter 1, Example 2.1 has no Eulerian path. Corollary 2.4. A connected graph is semi-eulerian but not Eulerian if and only if it has exactly two vertices of odd degree. Proof. We start by noting that any Eulerian path must begin at one odd degree vertex and end at another. : Except for the vertices at the start and finish of the path, every time we go into a vertex along one edge, we go out along another edge. Therefore each time that we visit an intermediate vertex we account for two edges. Thus every vertex other than the first and last must have even degree. : Let v 0, v 1 be the two vertices of odd degree in a graph G. Add an edge joining v 0 and v 1 to get a graph G such that every vertex of G has even degree. By Euler s theorem, G has an Eulerian circuit. Removing the edge v 0 v 1 leaves an Eulerian path on G which begins at v 0 and ends at v 1, as required. Example 2.5. The first of these graphs is Eulerian. The second, with one extra edge, is semi-eulerian.

17 Graph Theory 17 Algorithm 2.6. Fleury s algorithm This is an algorithm for finding Eulerian paths or circuits in a graph. Given a graph with either two or no vertices of odd degree, start at any vertex in the Eulerian case and at one of the two odd degree vertices in the semi-eulerian case. Walk around the graph choosing new edges with the proviso that you never take an edge if it would split the remaining edges into two disconnected graphs, and that you don t use the last edge into the start vertex too soon. Example 2.7. Consider the first graph of Example 2.5:

18 18 Colva M. Roney-Dougal Remark 2.8. Fleury s algorithm is common sense, in that if we end up with two disconnected blocks of edges we clearly cannot complete a circuit. 3. Hamiltonian Circuits Unlike the Eulerian case, where Euler s Theorem 2.1 completely classifies the Eulerian graphs, there is no simple criterion which guarantees the existence of a Hamiltonian circuit. Many puzzles and problems require one to find a Hamiltonian path, for instance the Knight s Tour of Chapter 1, Example 2.2 and the Travelling Salesman problem. Example 3.1. Hamilton s Round the World Problem Can you find a Hamiltonian circuit visiting all vertices of a dodecahedron? First we sketch a dodecahedron, which has 20 vertices, 12 pentagonal faces, and 30 edges. Then we give an isomorphic graph to consider: One can see a Hamiltonian circuit for this graph relatively easily. However, a small change gives the following graph, which has no Hamiltonian path or circuit:

19 Graph Theory 19 To see this, note that although this graph has the same number of vertices and edges as the previous one, to visit the vertices of degree two we must go around the middle ring of edges. But now any circuit cannot include both the inner and outer rings. Remark 3.2. In general, one has to argue in a systematic way to show that a graph is not Hamiltonian, or to find a Hamiltonian circuit. Example 3.3. The Petersen Graph The Petersen graph is not Hamiltonian. To prove this, note that up to symmetry there are three ways of passing from the inner to the outer pentagon. In picture (a) we pass at adjacent vertices, in (b) we pass at non-adjacent vertices, and in (c) we pass twice from the inner to the outer pentagon. The passing edges are the non-dotted ones. In each case, we are forced to traverse certain other edges to guarantee visiting each vertex. These are the dotted edges. In each case it follows that we cannot complete the circuit. Intuitively, given a graph, if it has many edges then it is more likely to have a Hamiltonian circuit. We formalise this as follows: Theorem 3.4 (Dirac s theorem) Let G be a simple graph with n 3 vertices such that every vertex has degree at least n/2. Then G has a Hamiltonian circuit. Proof. We assume that G is a graph with every vertex of degree at least n/2 that is not Hamiltonian, and derive a contradiction. By adding as many edges as possible without making the graph Hamiltonian, we may assume that G is maximal, in the sense that it is non-hamiltonian but the addition of any further edges will make it Hamiltonian. Since adding edges doesn t alter the number of vertices, and the complete graph is Hamiltonian, this assumption is safe. Let v, w be any pair of non-adjacent vertices of G. There must be a path from v to w which visits all vertices once, since adding the edge vw would create a Hamiltonian circuit, from which we can remove the edge vw. Label the vertices in this path v = v 1, v 2,..., v n = w.

20 20 Colva M. Roney-Dougal Let R be the set of vertices joined to v 1, so R n/2, and let B be the set of vertices that are one before the vertices in R as we go along the path v 1, v 2,..., v n. Note that some vertices may be in both B and R. Then B = R n/2 and v 1 B (as v 1 is one before v 2, and v 1 v 2 is an edge in the path). The vertex v n is joined to at least n/2 of the n 2 vertices v 2, v 3,..., v n 1, as v n is not joined to v 1 or itself. Since v 1 B and v n B, at least n/2 1 of the vertices v 2, v 3,..., v n 1 are in B. Since n/2 + (n/2 1) > n 2 there exists at least one vertex v i in v 2, v 3,..., v n 1 that is both in B and joined to v n. Therefore we have a Hamiltonian circuit v 1, v 2,..., v i, v n, v n 1, v n 2,..., v i+1, v 1, a contradiction. Remark 3.5. Dirac s theorem does not give a necessary condition. For instance the cycle with 6 vertices has all vertices of degree 2 (which is less than 6/2) and yet has a Hamiltonian circuit. A minor modification of the proof of Dirac s theorem allows one to prove the following: Theorem 3.6 (Ore s Theorem) Let G be a simple graph with n 3 vertices such that for all non-adjacent vertices v and w we have Degree(v)+Degree(w) n. Then G has a Hamiltonian circuit.

21 Chapter 4 Planarity and Colouring 1. Euler s Formula and Kuratowski s Theorem Definition 1.1. A graph G is planar if it can be drawn in the plane without edges crossing. Example 1.2. The graph K 4 (a) is planar because it is isomorphic to the graph (b) and this second graph has no crossing edges. We call both graphs planar. Definition 1.3. The faces of a planar graph are the regions into which the edges divide the plane, when the graph is drawn with no crossing edges. We always include the unbounded or exterior face. Example 1.4. The following graph has six faces: Given a planar graph, we write v, e, f for the number of vertices, edges and faces respectively. So this graph has v = 6, e = 10 and f = 6. Theorem 1.5 (Euler s formula) In every connected planar graph v e + f = 2. Proof. We prove this by induction on e. Base case: The connected graph with no edges consists of a single vertex. It has v = 1, e = 0 and f = 1 so the theorem holds. Inductive step: Assume that the theorem holds for every planar connected graph with at most n edges. Let G have n+1 edges. Then either G is a tree or G contains a circuit. We consider the two cases separately. If G is a tree then by removing any edge at the end of a branch, along with its terminal vertex, we get a connected graph with n edges. 21

22 22 Colva M. Roney-Dougal The value of v is decreased by 1, the value of e is decreased by 1, and the value of f is unchanged. So v e + f is unchanged, and so v e + f = 2 by the inductive hypothesis and the theorem holds for G. If G is not a tree, and so contains a circuit, we consider the effect of removing an edge that lies in a circuit, to get a connected graph with n edges. The value of v is unchanged, e is decreased by 1, and f is decreased by 1 (since two faces have been merged). So v e + f is unchanged, and so v e + f = 2 by the inductive hypothesis and the theorem holds for G. Therefore the result follows for all e by induction. Remark 1.6. Euler s formula is also valid for graphs drawn on a sphere. Corollary 1.7. The complete graph K 5 is not planar. Proof. The complete graph K 5 has v = 5 and e = ( 5 2 ) = 10. Therefore by Theorem 1.5 we must have f = 2 v + e = 7 if K 5 is planar. The boundary of any face is a circuit. The circuit cannot have a single vertex as K 5 is simple so has no loops. It also cannot have precisely two vertices as K 5 is simple so has no multiple edges. Therefore the boundary of each face contains at least three vertices, and hence at least three edges. Since each edge separates two faces we have e 3f/2, that is 10 21/2, a contradiction. Corollary 1.8. The complete bipartite graph K 3,3 is not planar. Proof. The graph K 3,3 has v = 6 and e = 3 3 = 9. Therefore if K 3,3 can be drawn in the plane then Theorem 1.5 gives f = 2 v+e = 5. However, since K 3,3 is bipartite each face is bounded by at least four edges. Each edge separates 2 faces, giving e 4f/2, which gives 9 10, a contradiction. Remark 1.9. The graphs K 3,3 and K 5 are the two basic non-planar graphs. Corollary Every planar, connected simple graph has at least one vertex of degree less than 6. Proof. Suppose all vertices have degree at least 6. Each edge ends in two vertices, so e 6v/2 = 3v. However, by Corollary 1.16 we have e 3v 6 so 3v 3v 6, a contradiction.

23 Graph Theory 23 Exercise In fact, every such graph with at least four vertices has at least four vertices of degree at most 5. Prove this. Definition A subgraph of a graph G is any graph that can be produced from G by removing edges and vertices. Two graphs G 1 and G 2 are homeomorphic if there exists a graph G such that both G 1 and G 2 may be produced from G by putting additional vertices of degree 2 along the edges of G. Note that any graph is homeomorphic to itself. Example The first graph is homeomorphic to K 3,3, the second is a subgraph of K 3,3. Theorem 1.14 (Kuratowski s Theorem) A finite connected graph is planar if and only if it contains no subgraph that is homeomorphic to K 3,3 or K 5. Less formally: A finite connected graph G is non-planar if and only if we can find 5 or 6 vertices in G that are connected together, possibly via other vertices, as K 3,3 or K 5. Non proof: One direction follows from Corollaries 1.7 and 1.8. The other direction is too long to be included in the course see Chapter 11 of the book by Harary. Example The Petersen graph is non-planar because it has a subgraph that is homeomorphic to K 3,3. Corollary Every planar connected simple graph with v 3 satisfies e 3v 6. Proof. Apart from the line of length three (for which v = 3, e = 2 so e 3v 6 = 3), every face of such a graph drawn in the plane is bounded by at least three edges, and every edge is adjacent to at most 2 faces (there is only 1 face around an edge at the end of a branch). Therefore e 3f/2. We have 2 = v e + f v e + 2e/3 = v e/3 e 3v Regular Polyhedra We may use Euler s formula 1.5 to show that there are just 5 regular polyhedra, or platonic solids. This method using Euler s formula is due to Cauchy in Definition 2.1. A polyhedron is convex if it has no inward-bending faces. A convex polyhedron is regular if every face is a congruent regular polygon and the arrangement of faces at each vertex is the same.

24 24 Colva M. Roney-Dougal Theorem 2.2. There are exactly 5 regular polyhedra. Proof. Let P be a regular polyhedron with all faces regular p-gons (that is, regular p-sided shapes). Assume that d edges meet at each vertex. We can project the edges and vertices of P on to the plane to get a corresponding planar graph G. Each vertex of G has degree d and each face of G has p edges. Then e = vd/2 and e = fp/2, as each edge bounds two faces. Therefore v = 2e/d and f = 2e/p. By Euler s formula we have Dividing by 2e we get 2 = v e + f = 2e d e + 2e p 2 + e = 2e d + 2e p e = 1 d + 1 p. Therefore, 1/2 < 1/d + 1/p, where d and p are integers with d 3, p 3. We have 1/2 < 1/d + 1/p 1/3 + 1/p and so 1/6 < 1/p and so p < 6. Similarly, d < 6. Thus (d, p) {(3, 3), (3, 4), (4, 3), (3, 5), (5, 3)}. Case 1 d = p = 3. Then we have 1/e = 1/3 + 1/3 1/2, so e = 6. Hence v = 2e/d = 4 and f = 2e/p = 4. This is the tetrahedron, or triangle-based pyramid. Case 2 d = 3, p = 4. We have 1/e = 1/3 + 1/4 1/2 so e = 12. Hence v = 2e/d = 8 and f = 2e/p = 6. This is the cube. Case 3 d = 4, p = 3. This time we have e = 12, v = 6 and f = 8. This is the octahedron, made by gluing the bottoms of two square-based pyramids together. Case 4 d = 3, p = 5. Now we have e = 30, v = 20 and f = 12. This is the dodecahedron; a solid whose surface is made by gluing 12 identical regular pentagons together. Case 5 d = 5, p = 3. This gives e = 30, v = 12, f = 20, corresponding to the icosahedron; a solid whose surface is made by gluing 20 identical equilateral triangles together. 3. Colouring Planar Graphs In 1852 Guthrie asked whether it is possible to colour every map in the plane with at most four colours so that no two countries with a common boundary are the same colour. Remark 3.1. Some maps cannot be coloured with less than four colours, for instance

25 Graph Theory 25 The theorem was first announced as proved true by Kempe in However, Heawood found an error in the proof in The theorem was finally proved true in 1976 by Appel and Haken, by one of the first lengthy proofs to use a computer. The problem can easily be rephrased as a problem of colouring the vertices of a graph. Definition 3.2. A graph G is k-colourable if we may label its vertices with k colours so that no two adjacent vertices are of the same colour. Definition 3.3. The chromatic number of a graph G is the least k for which G is k-colourable. We write χ(g) for the chromatic number of G. Example 3.4. The following graph is 3-colourable: It is not 2-colourable because it contains triangles. Example 3.5. A nontrivial bipartite graph G has χ(g) = 2. Colouring maps is directly related to colouring graphs. Definition 3.6. Given a map in the plane, the graph of the map is a graph with a vertex in each region, and an edge joining a pair of vertices if and only if the regions have a common boundary. Property: Every map in the plane has a simple planar graph. The four colour map theorem is therefore equivalent to: Theorem 3.7 (The four colour theorem) Every planar simple graph is fourcolourable. The proof of this theorem is very long, instead we prove the following. Theorem 3.8 (The five colour theorem) Every planar simple graph is five-colourable.

26 26 Colva M. Roney-Dougal Proof. We prove this by induction on the number n of vertices. For n 5 the result is trivial as we can simply colour each vertex a distinct colour. So assume that the result holds for all graphs with fewer than n vertices. Let G be a planar simple graph with n vertices. Since n > 5 it follows from Corollary 1.10 that G has a vertex A of degree at most 5. Remove A (and all edges ending at A) to get a new graph G. Since G has n 1 vertices, we can colour G with at most 5 colours by induction. If the degree of A is at most 4, then there is a colour not assigned to any vertex that is adjacent to A in G, so we can take the colouring for G and use this remaining colour on A to finish the colouring of G. Therefore we need only consider the case where A has degree 5. Let the neighbours of A be a 1, a 2, a 3, a 4, a 5, numbered to go in a clockwise direction around A. If any two of the a i have the same colour, then there is a spare colour available for A and we are done. Let i denote the colour of the vertex a i, and write G ij for the subgraph of G\{A} consisting of the vertices with colours i and j, and any edges between them that are in G. We divide into two cases: Case 1. a 1 and a 3 are not joined within G 1,3. Then we may interchange colours 1 and 3 on the part of G 1,3 connected to a 1 to give a 5-colouring of G \ {A} such that a 1 now has colour 3. We now can colour A with colour 1, as none of its neighbours is coloured 1. Case 2. a 1 and a 3 are joined by a path in G 1,3. Then a 2 and a 4 cannot be joined by a path in G 2,4 as G is planar so G 2,4 cannot cross G 1,3 (and cannot share a vertex with G 1,3 ). Therefore we may proceed as in case 1 to recolour a 2 with colour 4, and then colour A with colour 2. Remark Since K 4 is planar there are planar graphs which require 4 colours. 2. Colouring graphs on more complicated surfaces is easier. Suppose that we start with a sphere, and glue n handles onto it. That is, gluing one handle produces a torus, gluing two produces a double torus and so on. Then any graph that can be drawn on a surface with n handles can be coloured with at most N colours, where N = n, 2

27 Graph Theory 27 and x is the largest integer that is less than or equal to x. Thus for a torus we require N = 7 colours. This is best possible as K 7 can be drawn on the surface of the torus without crossings. 4. Colouring non-planar graphs Colouring questions are also of interest for non-planar graphs, even though these do not correspond to maps. We define a k-colouring of a graph G and its chromatic number χ(g) as in Definitions 3.2 and 3.3. Example 4.1. We show how to solve a timetabling problem using graph-colouring. Suppose we have the following requirements: Student Courses 1 A, B 2 A, C 3 C, D 4 B, E 5 D, E We wish to schedule classes so that no student has to be in two places at once. To do so we draw the following graph G, with one node per course and edges joining two courses if they are taken by the same student. We see that χ(g) = 3. Colouring the graph G with 3 colours gives a timetabling arrangement with each colour corresponding to a timetabling slot. We now consider some more general estimates on χ. Proposition 4.2. Let G be a graph with every vertex of degree at most n for some n, then χ(g) n + 1. Proof. Label the vertices of G with v 1,..., v k in any order. Let the colours be c 1,..., c n+1. We then use the greedy algorithm to colour the graph: consider the vertices in order v 1,..., v k, and for each vertex v i use the smallest value of j such that c j is not the colour of a neighbouring vertex. Since no vertex has more than n neighbours, there will always be at least one colour available. Example 4.3. Here s a worked example: Here is a more powerful theorem, whose proof is too hard for this course: Theorem 4.4 (Brooke s Theorem) Let G be a graph with all vertices of degree at most n. Then χ(g) n unless one of the following holds

28 28 Colva M. Roney-Dougal 1. A connected component of G is K n+1 where χ(g) = n We have n = 2 and a connected component of G is a circuit with an odd number of vertices, where χ(g) = The chromatic polynomial Definition 5.1. Let G be a graph with no loops. We write P G (k) for the number of different ways of colouring G using k colours so that adjacent vertices are different colours. Example 5.2. Consider the following graph: We have P G (k) = k k (k 1) = k 2 (k 1). Definition 5.3. By amalgamating two vertices v and w of a graph G, we mean removing the edge vw and bringing together all of the other edges that were originally incident to either v or w to meet at a new vertex that is both v and w. Sometimes we remove any multiple edges created, as these do not affect the colouring of the resulting graph. Example 5.4. Graph (a) has two labelled vertices v and w. Graph (b) has been produced from graph (a) by amalgamating v and w. We use amalgamation to show that P G (k) is a polynomial with integer coefficients. Lemma 5.5 (The Deletion/Contraction Lemma) Let vw be an edge of the simple graph G. Let G 1 be the graph obtained by deleting the edge vw and let G 2 be the graph obtained by amalgamating the vertices v and w. Then Consequently, P G (k) is a polynomial in k. P G (k) = P G1 (k) P G2 (k). (4.1) Proof. Consider the possible ways of colouring G 1 with k colours. In a given colouring, one of the following holds: 1. v and w are the same colour, in which case there are P G2 (k) ways of colouring G 1 ; 2. v and w are different colours, in which case the colouring of G 1 is a colouring of G, so there are P G (k) ways of doing this. Therefore we have P G1 (k) = P G2 (k) + P G (k), so P G (k) = P G1 (k) P G (k), as required. Since G 1 and G 2 have strictly fewer edges than G we may apply these deletion and amalgamation processes repeatedly until we reach graphs with no edges (which we must do eventually since there are only finitely many edges in G). If there are n vertices and no edges in a graph H, then P H (k) = k n. Substituting these values back into equation 4.1 shows that P G (k) is a polynomial in k.

29 Graph Theory 29 Remark 5.6. The chromatic number χ(g) of a graph is the least positive integer k such that P G (k) 0. The deletion/contraction lemma 5.5 gives an algorithm for chromatic polynomials. We usually just draw diagrams rather than writing equations in P G (k). Example 5.7. Consider the following graph G, with labelled vertex v. There are (k 1) ways of colouring v once the other vertices have been coloured, so we write By the Deletion/Contraction Lemma 5.5 we have Once again, considering the isolated vertex we have There are k(k 1)(k 2) ways of colouring a triangle since all vertices are joined. Similarly, there are k(k 1)(k 2) ways of colouring since all vertices are joined. So the number of ways of k-colouring is (k 1) 2 k(k 2) k(k 1)(k 2) = k(k 1)(k 2)[k 1 1] = k(k 1)(k 2) 2 Therefore the number of ways of colouring G is and hence χ(g) = 3. k(k 1) 2 (k 2) 2 Proposition 5.8. For a simple graph G with v vertices and e edges, P G (k) = k v ek v 1 + lower order terms. Moreover, the signs of the terms alternate. Exercise 5.9. Prove this using the Deletion/Contraction Lemma 5.5

30 30 Colva M. Roney-Dougal Example Consider the following graph G, and let us use the deletion/contraction lemma to find P k (G). We have So the number of ways of colouring G is k(k 1)(k 2)(k 1) 2 k(k 1)(k 2) 2 = k(k 1)(k 2)[(k 1) 2 (k 2)] = k(k 1)(k 2)[k 2 2k + 1 k + 2] = k(k 1)(k 2)[k 2 3k + 3] Therefore χ(g) = 3. Definition A polygon is a planar circuit satisfying v = e. Lemma Let G be a graph consisting of the vertices and edges of some polygon, together with a set of non-crossing diagonals of the polygon as further edges. Then G is 3-colourable. Proof. We prove this by induction on the number of vertices. It clearly holds for n = 3 vertices: We assume inductively that the result holds for all graphs of this type with less than n vertices. Let G be such a graph with n vertices. If G is itself a polygon then G is clearly 3-colourable, so assume that G contains at least one diagonal v 1 v 2. Split G into two smaller graphs along v 1 v 2. Each of the two graphs to either side of the diagonal satisfies the hypotheses of the lemma, and has less than n vertices. Therefore each part may be 3-coloured by induction. By permuting the colours on one of the parts so that v 1 and v 2 are assigned the same colour on both parts, we get a 3-colouring of G. Example Guarding an Art Gallery Let an art gallery be in the shape of a polygon with n sides. What is the least number of guards that can be placed in the gallery so that they can see every point in the gallery? The first of these graphs requires at least two guards, the second requires at least one guard in each shaded region. Proposition A polygonal art gallery with n sides may be guarded by n/3 guards.

31 Graph Theory 31 Proof. Break the polygon up into triangles by adding diagonals. A guard standing at the vertex of a triangle can see all of the triangle. By Lemma 5.12 we may 3-colour the graph. One of the colours is used at most n/3 times. Put a guard at each vertex of that colour. Since each triangle has one vertex of each colour, these guards cover the whole region. Example Consider the following art gallery: We triangulate it, then colour it as as follows: We have eight vertices, and used colours 1 and 3 both no more than 8/3 = 2 times. So placing guards at vertices coloured 1 will do, a total of 2 guards. 6. The chromatic number of the line and plane Question 6.1. Let G be a graph whose vertices are the points of either R or R 2, with a pair of points x, y joined if and only if x y = 1. What is χ(g)? We may rephrase the question by asking how many colours are needed to colour R (or R 2 ) such that no pair of points which are distance 1 apart are of the same colour? Here is a 2-colouring of R, where each interval is [n, n + 1) for n an integer: Therefore, χ(r) = 2: since some points are distance 1 apart we cannot 1-colour R. The chromatic number of R 2 is harder. We prove some bounds. Lemma 6.2. With this adjacency relation we have χ(r 2 ) 7. Proof. Tile the plane with hexagons of diameter slightly less than 1.

32 32 Colva M. Roney-Dougal Colour them in a periodic manner to get a 7-colouring. It is clear that any two tiles of the same colour are distance more than 1 apart. Lemma 6.3. With these edges we have χ(r 2 ) 4. Proof. We may draw the following graph in the plane with all edges of length 1. In any 3-colouring of this graph, the bottom two vertices have the same colour as the top one because of the red and green triangles, contradicting the fact that the bottom two vertices are distance 1 apart. Hence this graph has chromatic number 4 and so χ(r 2 ) 4. It is unknown whether χ(r 2 ) is 4, 5, 6 or 7.

33 Chapter 5 Marriage, Matchings and Connectivity 1. Hall s Marriage Theorem Many problems involve matching objects from one set with objects in another. Problem 1.1. [Hall s Marriage Problem] Given a set B of boys and a set G of girls, some of whom know each other, under what conditions can we marry off the boys so that each boy marries a girl he knows? (We assume that knowing is symmetric, so that if a boy knows a girl then the girl knows him too!) We can represent this problem with a bipartite graph with vertex sets B and G, where we put an edge joining a boy and a girl if they know each other. Example 1.2. In this graph a matching is possible. In this graph a matching is not possible, as boys 1, 3 and 5 only know 2 girls between them. Definition 1.3. Two edges of a (not necessarily bipartite) graph are called independent if they do not share a vertex. A perfect matching of a bipartite graph G is a set of independent edges such that every vertex is incident with one of them. Let G be bipartite, with parts V 1 and V 2. A matching from V 1 to V 2 is a set of independent edges incident to every vertex in V 1. Thus Hall s Marriage Problem asks if there is a perfect matching from the girls to the boys. Clearly, if there is some set of k boys that know fewer than k girls, then no complete matching is possible. Hall s Marriage Theorem says (surprisingly) that this is the only situation where a matching is not possible. Theorem 1.4 (Hall s Marriage Theorem) 1. There is a solution to Hall s Marriage Problem 1.1 if and only if every set of k boys knows at least k girls, for 1 k B. 33

34 34 Colva M. Roney-Dougal 2. Equivalently, let G be a bipartite graph with parts V 1 and V 2. There is a matching from V 1 to V 2 if and only if every set S of vertices in V 1 is joined to at least S vertices of V 2. Proof. We prove the marriage version of the theorem. ( ): If some set of k boys knows fewer than k girls then we clearly cannot marry them off. ( ): We prove the converse by induction on b = B. The base case b = 1 is trivial: if there is at least one girl then he can marry. Suppose inductively that the result holds whenever there are fewer than b boys. We divide the proof into two cases. Case 1: Every set of k boys (where 1 k < b) knows at least k + 1 girls between them. Take any of the boys, and marry him to a girl that he knows. Now the remaining b 1 boys still satisfy the condition, as every set of k of them knows at least k of the remaining girls. Since there are now only b 1 boys, the result follows by induction. Case 2: There exists a set B 1 of k boys (where 1 k < b) that know exactly k girls between them. By induction we can marry the boys in B 1 off to the k girls that they know, since k < b. For the remaining b k boys, every set S of i of them must know at least i girls, otherwise S B 1 is a set of i + k boys that together know less than i + k girls. Therefore the remaining b k boys satisfy Hall s condition, and so by induction they can be married off to the remaining girls. Everyone is happy, completing the proof. Corollary If for some r > 0 each boy knows eactly r girls and each girl knows exactly r boys, then all of the boys and girls can be married, so B = G. 2. Equivalently, if G is a regular bipartite graph with parts V 1 and V 2, and degree r, then V 1 = V 2 and G has a perfect matching. Proof. We prove the graph version of the corollary. Let A be a set of k vertices, and let F be the set of their neighbours. There are kr edges that are incident with A, and each of them is also incident with F. Since each vertex of F has degree r, there are at least kr/r = k vertices in F. Thus Hall s condition is satisfied, and there is a matching from V 1 to V 2. By symmetry, there is also a matching from V 2 to V 1, so V 1 = V 2 and the matching is perfect. We now consider the possibility that Hall s condition is not satisfied, but we would like to marry off as many of the boys as possible. When can we marry all but d of them? We certainly require that any k of them know at least k d girls. It turns out that this necessary condition is sufficient. Corollary 1.6. Suppose the bipartite graph G with vertex sets (V 1, V 2 ), with V 1 = m, satisfies the condition that any set S of vertices in V 1 has at least S d neighbours. Then G contains m d independent edges. Proof. Add d vertices to V 2, and join them to each vertex in V 1. Then the new graph G satisfies Hall s condition, and so has a perfect matching. At least m d of the edges in this matching belong to G. 2. Edge colourings of graphs Definition 2.1. An edge colouring of a graph is an assignment of colours to edges such that edges that meet at a common vertex are of different colours. A graph is

35 Graph Theory 35 k-edge-colourable if it has an edge colouring with k colours. If G is k-edge-colourable but not (k 1)-edge colourable, then the edge-chromatic number of G is k, and we write χ e (G) = k. Here is a result about edge colourings that uses Hall s Marriage Theorem 1.4. Theorem 2.2. A bipartite graph with all vertices of degree at most r can be edge coloured with r colours. Proof. We prove this by induction. If r = 1 then no edges ever meet at a vertex, and the graph can be edge coloured with a single colour. Assume that the result holds for all bipartite graphs with maximum vertex degree less than r, and let G be a graph with maximum vertex degree r. If any of the vertices of G have degree less than r, add new vertices and edges to make a regular graph G 1 of degree r. If we can edge colour G 1 then we can edge colour G. By Corollary 1.5, the graph G 1 has a perfect matching, which is a set F of edges. Colour all of the edges in F with colour r, and then delete them. This produces a bipartite graph with maximum vertex degree r 1, which can be coloured with colours {1,..., r 1} by induction. The following result, whose proof is omitted, shows that something similar is true in general. Theorem 2.3 (Vizing s theorem) Let G be a graph without loops, with all vertices of degree at most r. Then r χ e (G) r + 1. For graphs it is straightforward to decide whether the edge-chromatic number is r or r + 1. For example, the cycle of length n has edge-chromatic number 2 if n is even and 3 if n is odd. Theorem 2.4. For n > 1 the number χ e (K n ) = n if n is odd, and χ e (K n ) = n 1 if n is even. Proof. If n is odd, then the edges of K n can be n-coloured by placing the vertices of K n in the form of a regular n-gon, colouring the edges around the outside with a different colour for each edge. Then colour each remaining edge with the same colour as the one that you used on the outside edge that is parallel to it. The largest possible number of edges of the same colour is (n 1)/2, as each edge touches two vertices and they must all be distinct. There are n(n 1)/2 edges in K n, so n colours are definitely required and hence χ e (K n ) = n. If n is even then think of K n as the sum of a complete graph K n 1 and a single vertex. Colour the edges of K n 1 using the method above, using n 1 colours. Since each vertex has n 2 neighbours within the K n 1, there will be one colour missing at it. This colour will be the colour of the outside edge that is opposite it, so all of the missing colours are distinct. Thus the colouring of the edges of K n can therefore be completed by colouring the remaining edges with these missing colours. It is clear that this colouring is minimal, so χ e (K n ) = n 1. Example 2.5. Here is an edge colouring of K 5 using the method of Theorem 2.4.

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