# Graph Theory. Introduction. Distance in Graphs. Trees. Isabela Drămnesc UVT. Computer Science Department, West University of Timişoara, Romania

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1 Graph Theory Introduction. Distance in Graphs. Trees Isabela Drămnesc UVT Computer Science Department, West University of Timişoara, Romania November 2016 Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 1 / 33

2 What is Graph Theory? The study of graphs as mathematical structures G = (V, E) used to model pairwise relations (a.k.a. edges) between objects of a collection V. The objects are modeled as nodes (or vertices) of a set V The pairwise relations are modeled as edges, which are elements of a set E. Graphs differ mainly by the types of edges between nodes. Most common types of graphs are: Undirected: there is no distinction between the nodes associated with each edge. Directed: edges are arcs from one node to another. Weighted: every edge has a weight which is typically a real number. Labeled: every edge has its own label.... Graphs are among the most frequently used models in problem solving. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 2 / 33

3 History of graph theory 1736: L. Euler publishes Seven Bridges of Königsberg first paper on graph theory. Later: Euler s formula relating the number of edges, vertices, and faces of a convex polyhedron generalizations by Cauchy and L Huillier study of topology and special classes of graphs. 1852: De Morgan introduces the Four Color Map Conjecture : four is the minimum number of colors required to color any map where bordering regions are colored differently. 1969: Heesch publishes a solving method 1976: computer-generated proof of K. Appel and W. Haken. 1878: The term graph was first used by Sylvester in a publication in Nature. 1936: D. König publishes first textbook on graph theory. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 3 / 33

4 Introductory concepts Vocabulary Assumption: G = (V, E) is a simple graph or digraph. The order of G is V, the number of its nodes. The size of G is E, the number of its edges. The neighborhood of v V is N(v) = {x V (v, x) E}. The closed neighborhood of v V is N[v] = {v} N(v). The degree of v V is the number of edges incident with V : deg(v) = {e E e = (v, x) or e = (x, v) for some x V } The maximum degree of G is (G) = max{deg(v) v V }. The minimum degree of G is δ(g) = min{deg(v) v V }. The degree sequence of G with order n is the n-term sequence (usually written in descending order) of the vertex degrees of its nodes. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 4 / 33

5 Introductory concepts Example a b c d e f g h G = (V, E) where V = {a, b, c, d, e, f, g, h}, E = {{a, d}, {a, e}, {b, c}, {b, e}, {b, g}, {c, f }, {d, f }, {d, g}, {g, h}} N(d) = {a, f, g}, N[d] = {a, d, f, g}, (G) = deg(b) = 3 δ(g) = deg(h) = 1, The degree sequence is 3, 3, 3, 2, 2, 2, 2, 1 Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 5 / 33

6 First Theorem of Graph Theory Theorem In a graph G, the sum of the degrees of the vertices is equal to twice the number of edges. Consequently, the number of vertices with odd degree is even. Combinatorial proof. Let S = v V deg(v). Notice that in counting S, we count each edge exactly twice. Thus, S = 2 E (the sum of the degrees is twice the number of edges). Since S = v V deg(v) even deg(v) + v V deg(v) odd deg(v) and S is even, the second sum must be even, thus the number of vertices with odd degree is even. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 6 / 33

7 Introductory concepts Perambulation and Connectivity Assumption: G = (V, E) is a simple graph or digraph. A walk or path in G is a sequence of (not necessarily distinct) nodes v 1, v 2,..., v k such that (v i, v i+1 ) E for i = 1, 2,..., k 1. Such a walk is sometimes called a v 1 v k walk. v 1 and v k are the end vertices of the walk. If the vertices in a walk are distinct, then the walk is called a simple path. If the edges in a walk are distinct, then the walk is called a trail. A cycle is a simple path v 1,..., v k (where k 3) together with the edge (v k, v 1 ). A circuit or closed trail is a trail that begins and ends at the same node. The length of a walk (or simple path, trail, cycle, circuit) is its number of edges, counting repetitions. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 7 / 33

8 Introductory concepts Perambulation and Connectivity Example a c b d e f g a, c, f, c, b, d is a walk of length 5. b, a, c, b, d is a trail of length 4. d, g, b, a, c, f, e is a simple path of length 6. g, d, b, c, a, b, g is a circuit. e, d, b, a, c, f, e is a cycle. Note that walks, trails and simple paths can have length 0. The minimum length of a cycle or circuit is 3. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 8 / 33

9 Second Theorem of Graph Theory Theorem In a graph G with vertices u and v, every u v walk contains a u v simple path. Proof. Let W be a u v walk in G. We prove this theorem by induction on the length of the walk W. If W has length 1 or 2, then it is easy to see that W must be a simple path. For the induction hypothesis, suppose the result is true for all walks of length < k and suppose W has length k. Say that W is u = w 0, w 1,..., w k 1, w k = v. If the nodes are distinct, then W itself is the desired u v simple path. If not, then let j be the smallest integer such that w j = w r for some r > j. Let W 1 be the walk u = w 0,..., w j, w r+1..., w k = v. This walk has length strictly less than k, and thus W 1 contains a u v simple path by induction hypothesis. Thus W contains a simple u v path. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 9 / 33

10 Operations on graphs Assumptions: G = (V, E) is a simple graph, v V, S V, e E, T E Vertex deletion: G v is the graph obtained by removing v and all edges incident with v from G. G S is the graph obtained by removing each node of S and each edge incident with a node of S from G. Edge deletion: G e is the graph obtained by removing only the edge e from G (its end nodes stay). G T is the graph obtained by removing each edge of T from G. G is connected if every pair of nodes can be joined by a path. Otherwise, G is disconnected. A component of G is a maximal connected piece of G. v is a cut vertex if G v has more components than G. e is a bridge if G e has more components than G. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 10 / 33

11 Operations on graphs and properties related to connectivity Example (Deletion operations) a a a a b c b c b c b c d d d e f e f e f e f g G d g G g G (c, d) g G {(e, g), (f, g)} d is a cut node in G. (a, b) is a bridge in G. Example (Connected and disconnected graphs) Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 11 / 33

12 Graph-related notions Node cut sets, connectivity, complete graphs Assumption: G = (V, E) is a graph. S V is a node cut set of G if G S is disconnected. G is complete if every node is adjacent to every other node. We write K n for the complete graph with n nodes. The complete graphs K n have no node cut sets because K n S is connected for all proper subsets S of the set of nodes. If G is not complete then the connectivity of G, denoted by κ(g), is the minimum size of a node cut set of G. If G is a connected and incomplete graph of order n, then 1 κ(g) n 2. If G is disconnected, then κ(g) = 0. If G = K n then we say that κ(g) = n 1. If k > 0, we say that G is k-connected if k κ(g). Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 12 / 33

13 Consequences of the definitions 1 A graph is connected if and only if κ(g) 1. 2 κ(g) 2 if and only if G is connected and has no cycles. 3 Every 2-connected graph contains at least one cycle. 4 For every graph G, κ(g) δ(g). Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 13 / 33

14 Exercises (1) 1. If G is a graph of order n, what is the maximum number of edges in G? 2. Prove that for any graph G of order at least 2, the degree sequence has at least one pair of repeated entries. 3. Consider the complete graph K 5 shown in the following figure. c b a d e a. How many different simple paths have c as an end vertex? b. How many different simple paths avoid vertex c altogether? c. What is the maximum length of a circuit in this graph? Give an example of such a circuit. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 14 / 33

15 Exercises (2) 4. Let G be a graph where δ(g) k. a. Prove that G has a simple path of length at least k. b. If k 2, prove that G has a cycle of length at least k Prove that every closed odd walk in a graph contains an odd cycle. 6. Let P 1 and P 2 be two paths of maximum length in a connected graph G. Prove that P 1 and P 2 have a common vertex. 7. Prove that every 2-connected graph contains at least one cycle. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 15 / 33

16 Special types of graphs Complete graphs K n and empty graphs E n 1 The complete graphs K n. The graph K n has order n and a connection between every two nodes. Examples: 2 Empty graphs E n. The graph E n has order n and no edges. Example: Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 16 / 33

17 Special types of graphs Complements and regular graphs Assumption: G = (V, E) is a graph. The complement of G is the graph G whose node set is the same as that of G and whose edge set consists of all the edges that are not in E. For example G is regular if all its nodes have the same degree. G is r-regular if deg(v) = r for all nodes v in G. K n are (n 1)-regular graphs; E n are 0-regular graphs. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 17 / 33

18 Special types of graphs Cycles, paths, and subgraphs The cycle C n is simply a cycle on n vertices. Example: The graph C 7 looks as follows: The graph P n is a simple path on n vertices. For example, the graph P 6 looks as follows: Given a graph G = (V, E) and a subset S V, the subgraph of G induced by S, denoted S G, is the subgraph with vertex set S and with edge set {(u, v) u, v S and (u, v) E}. So, S G contains all vertices of S and all edges of G whose end vertices are both in S. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 18 / 33

19 Special types of graphs Example: a graph and two of its induced subgraphs A graph G = (V, E) is bipartite if V can be partitioned into two sets X and Y such that every edge of G has one end vertex in X and the other in Y. In this case, X and Y are called the partite sets. A complete bipartite graph is a bipartite graph with partite sets X and Y such that its edge set is E = {(x, y) x X, y Y }. Such a graph is complete bipartite graph denoted by K X, Y. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 19 / 33

20 Special types of graphs: Examples Bipartite graphs The first two graphs in the following figure are bipartite, whereas the third graph is not bipartite. Complete bipartite graphs Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 20 / 33

21 Bipartite graphs Characterization Theorem Theorem A graph with at least 2 nodes is bipartite if and only if it contains no odd cycles. Proof. : Let G = (V, E) be a bipartite graph with partite sets X and Y, and let C = v 1,..., v k, v 1 be a cycle in G. We can assume v 1 X w.l.o.g. Then v i X for all even i and v i Y for all odd i. Since (v k, v 1 ) E, we must have k even we can not have an odd cycle in G. : We can assume w.l.o.g. that G is connected, for otherwise we could treat each of its components separately. Let v V and define X = {x V the shortest path from x to v has even length}, Y = V \ X. It is easy to verify that G is a bipartite graph with partite sets X and Y. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 21 / 33

22 Isomorphic graphs Note that the following graphs are the same: a c d b g e f h This is so because one graph can be redrawn to look like the other. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 22 / 33

23 Isomorphic graphs Note that the following graphs are the same: a c d b g e f h This is so because one graph can be redrawn to look like the other. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 22 / 33

24 Isomorphic graphs Note that the following graphs are the same: a c d b g e f h This is so because one graph can be redrawn to look like the other. The idea of isomorphism formalizes this phenomenon. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 22 / 33

25 Isomorphic graphs Note that the following graphs are the same: a c d b g e f h This is so because one graph can be redrawn to look like the other. The idea of isomorphism formalizes this phenomenon. Isomorphic graphs Two graphs G = (V 1, E 1 ) and H = (V 2, E 2 ) are isomorphic if there is a bijective mapping f : V 1 V 2 such that (x, y) E 1 if and only if (f (x), f (y)) E 2. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 22 / 33

26 Isomorphic graphs When two graphs G and H are isomorphic, it is not uncommon to simply say that G = H or that G is H. If G and H are isomorphic then they have the same order and size. The converse of this statement is not true, as seen in Figure 1 below. Figure: Two graphs G and H with same order and size, which are not isomorphic. If G and H are isomorphic then their degree sequences coincide. The converse of this statement is not true. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 23 / 33

27 Exercises 1. For n 2 prove that K n has n(n 1)/2 edges. 2. Determine whether K 4 is a subgraph of K 4,4. If yes, then exhibit it. If no, then explain why not. 3. The line graph L(G) of a graph G is defined in the following way: the vertices of L(G) are the edges of G, V (L(G)) = E(G), and two vertices in L(G) are adjacent if and only if the corresponding edges in G share a vertex. a. Find L(G) for the graph b. Find the complement of L(K 5 ). Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 24 / 33

28 Distance in Graphs Definitions Assumption: G = (V, E) is a connected graph. Example The distance d(u, v) from node u to node v in G is the length of the shortest path u v from u to v in G. The eccentricity ecc(v) of v in G is the greatest distance from v to any other node. d(b, k) = 4, d(c, m) = 6. ecc(a) = 5 since the farthest nodes from a are k, m, n, and they are a distance 5 from a. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 25 / 33

29 Distance in Graphs More definitions Assumption: G = (V, E) is a connected graph. Example The radius rad(g) of G is the value of the smallest eccentricity. The diameter diam(g) of G is the value of the greatest eccentricity. The center of G is the set of nodes v such that ecc(v) = rad(g). The periphery of G is the set of nodes v such that ecc(v) = diam(g). rad(g) = 3 and diam(g) = 6. The center of G is {e, f, g}. The periphery of G is {c, k, m, n}. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 26 / 33

30 Distance in graphs Properties Theorem 1 For any connected graph G, rad(g) diam(g) 2 rad(g). Proof. By definition, rad(g) diam(g), so we just need to prove the second inequality. Let u, v be nodes in G such that d(u, v) = diam(g). Let c be a node in the center of G. Then diam(g) = d(u, v) d(u, c) + d(c, v) 2 ecc(c) = 2 rad(g). Theorem 2 Every graph G = (V, E) is isomorphic to the center of some graph. Proof. We construct a new graph H by adding 4 nodes w, x, y, z to G along with the following edges: {(w, x), (y, z)} {(x, a) a V } {(b, y) b V }. The newly constructed graph H looks as shown in the figure below. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 27 / 33

31 Distance in graphs Properties Proof of Theorem 2 continued. ecc(w) = ecc(z) = 4, ecc(y) = ecc(x) = 3, and for any node v of G: ecc(v) = 2. Therefore G is the center of H. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 28 / 33

32 Exercises 1. Find the radius, diameter and center of the following graph: 2. Find the radius and diameter of each of the following graphs: P 2k, P 2k+1, C 2k, C 2k+1, K n, and K m,n. 3. Given a connected graph G = (V, E) and a positive integer k, the k-th power of G, denoted G k, is the graph whose set of nodes os V and where vertices u and v are adjacent in G k if and only if d(u, v) k in G. a. Draw the 2-nd and 3-rd powers of P 8 and C 10. b. For a graph G of order n, what is G diam(g)? Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 29 / 33

33 Trees A tree is a connected graph which contains no cycles. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 30 / 33

34 Trees A tree is a connected graph which contains no cycles. Quiz: Which ones are trees? Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 30 / 33

35 Trees A tree is a connected graph which contains no cycles. Quiz: Which ones are trees? A forest is a graph whose connected components are trees. E.g., the graph D is a forest. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 30 / 33

36 Trees A tree is a connected graph which contains no cycles. Quiz: Which ones are trees? A forest is a graph whose connected components are trees. E.g., the graph D is a forest. A leaf in a tree is a node with degree 1. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 30 / 33

37 Trees A tree is a connected graph which contains no cycles. Quiz: Which ones are trees? A forest is a graph whose connected components are trees. E.g., the graph D is a forest. A leaf in a tree is a node with degree 1. Note that K 1 and K 2 are the only trees of order 1 and 2, respectively. P 3 is the only tree of order 3. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 30 / 33

38 Trees and forests Properties If T is a tree of order n, then T has n 1 edges. If F is a forest of order n containing k connected components, then F contains n k edges. A graph of order n is a tree if and only if it is connected and contains n 1 edges. A graph of order n is a tree if and only if it is acyclic and contains n 1 edges. Let T be the tree of order n 2. Then T has at least two leaves. In any tree, the center is either a single vertex or a pair of adjacent vertices. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 31 / 33

39 Trees Exercises 1 Show that every edge in a tree is a bridge. 2 Show that every nonleaf in a tree is a cut vertex. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 32 / 33

40 References J. M. Harris, J. L. Hirst, M. J. Mossinghoff. Combinatorics and Graph Theory, Second Edition. Springer Chapter 1: Graph Theory. Sections 1.1, 1.2 and 1.4. Isabela Drămnesc UVT Graph Theory and Combinatorics Lecture 7 33 / 33

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