# 2.1 You need to know how graphs and networks can be used to create mathematical

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1 fter completing this chapter you should: 1 know how graphs and networks can be used to create mathematical models 2 know some basic terminology used in graph theory 3 know some special types of graph 4 understand how graphs and networks can be represented using matrices 2 Graphs and networks Graph theory is a branch of mathematics which is used to model, analyse and solve many real-life problems. You will have met the word graph before, in connection with drawing curved lines on squared paper, but in graph theory the word graph has a much broader meaning. his map is an example of a graph. 25

2 HP You need to know how graphs and networks can be used to create mathematical models. graph consists of points (called vertices or nodes) which are connected by lines (edges or arcs). If a graph has a number associated with each edge (usually called its weight), then the graph is known as a weighted graph or network. xample 1 a Why is this a graph? b What are i the vertices, ii the edges? c What is the purpose of this graph? a b It has vertices connected by edges. i Stations on the London Underground ii Underground railway lines c o make route planning on the Underground easier. It does not show any distances or the correct geographical position of the stations. opy of London Underground map (PLS SUPPLY) xample 2 a Why is this a graph? lf etty arol ylan dward ay George Hazel Loan Julie Kate Lauren Michael Neil Oscar b What does it represent? c What do i the vertices, ii the edges represent? a b c It has vertices connected by edges. family tree. i he people in the family ii he relationships between the people 26

3 Graphs and networks xample 3 he editor of a newspaper has to assign five reporters, mro, avika, hi, avid and rin, to cover five events, hospital opening, movie premiere, celebrity wedding, royal visit and 110th birthday party. mro could be assigned to the movie premier or the celebrity wedding. avika could be assigned to the hospital opening, movie premiere or 110th birthday. hi could be assigned to the royal visit, celebrity wedding or 110th birthday. avid could be assigned to the hospital opening, movie premiere or royal visit. rin could be assigned to the celebrity wedding, royal visit or 110th birthday. omplete this graph to model this information. mro avika hi avid Hospital openings Movie premiere elebrity weddings oyal list rin 110 th birthday party mro avika hi Hospital openings Movie premiere elebrity weddings Here the vertices represent the reporters and the events, and the edges indicate the possible assignments. avid oyal list rin 110 th birthday party xample 4 he manager of a camp site needs to provide a water supply to stand pipes located at various points on the site. a What do the vertices represent? b What do the edges represent? c What do the numbers represent? a b c the stand pipes the possible supply routes the lengths of each supply route main supply 30 m 60 m 100 m 150 m 80 m 50 m 80 m 100 m 90 m 60 m his is a network showing a simplified view of the situation. 27

4 HP 2 xample 5 his network represents a project with six activities,,,,, and. Interpret the network. ctivities and can start straightaway. ctivity cannot start until is completed. ctivity cannot start until is completed. ctivity cannot start until both and are completed. ctivity cannot start until is completed. 2.2 You need to be familiar with some basic terminology used in graph theory. ecision mathematics is a relatively new branch of mathematics which has grown out of several fields of study. here are often two different notations in use. You need to be familiar with both sets. and are nodes or vertices is a nodes or vertices is an arc or edge. In the graph G, above the vertices (or nodes) are:,,,, and (this list of vertices is sometimes called the vertex set), the edges (or arc) are:,,,,, and (this list of edges is sometimes called the edge set). Note: the intersection of and is not a vertex. 28

5 Graphs and networks subgraph of G is a graph, each of whose vertices belongs to G and each of whose edges belongs to G. It is simply a part of the original graph. Graph, G some possible subgraphs Subgraph 1 Subgraph 2 he degree or valency or order of a vertex is the number of edges incident to it. J K 2 degree because 2 lines meet here M Vertex egree J 2 K 2 L 1 M 4 N 1 If the degree of a vertex is even, we say it has even degree, so J, K and M have even degree. Similarly vertices L and N have odd degree. L N ll three words mean the same thing. In any graph the sum of the degrees will be precisely equal to 2 the number of edges. his is because, in finding the sum of degrees, we are counting each end of each edge. his is known as the Handshaking Lemma. (See Section 4.1.) path is a finite sequence of edges, such that the end vertex of one edge in the sequence is the start vertex of the next, and in which no vertex appears more than once. S V U W or example, in the graph above, one possible path is S U W and another is U V W S. walk is a path in which you are permitted to return to vertices more than once. or example in the graph above, a walk could be U V S W U V. cycle (or circuit) is a closed path, i.e. the end vertex of the last edge is the start vertex of the first edge. or example in the graph above, a possible cycle is U V W. 29

6 HP 2 wo vertices are connected if there is a path between them. graph is connected if all its vertices are connected. G H his is a connected graph. path can be found between any two vertices. his shows a graph that is not connected. here is no path from to V, for example. loop is an edge that starts and finishes at the same vertex. S V U loop his contains a loop from to. simple graph is one in which there are no loops and not more than one edge connecting any pair of vertices. multiple edges he graph above is not a simple graph because it contains a loop. his is not a simple graph because it has two edges from to. If the edges of a graph have a direction associated with them they are known as directed edges and the graph is known as a digraph. Q P S xercise 2 1 raw a connected graph with a one vertex of degree 4 and 4 vertices of degree 1, b three vertices of degree 2, on of degree 3 and one of degree 1, c two vertices of degree 2, two of degree 3 and one of degree 4. 30

7 Graphs and networks 2 Which of the graphs below are not simple? a b G K c P d I U H V J Q W S Z X 3 In question 2, which graphs are not connected? 4 rom the graph, state here are many correct answers to these questions. a four paths from to, b a cycle passing through and, c the degree of each vertex. Use the graph to d draw a subgraph, e confirm the handshaking lemma (that the sum of the degrees is equal to twice the number of edges). lemma is a mathematical fact used a stepping stone to more important results. 5 a epeat question 4 parts c, d and e using this graph. K L N P Q J M S b onfirm that there is only one path between any two vertices. 6 Show that it is possible to draw a graph with a an even number of vertices of even degree, b an odd number of vertices of even degree. It is not possible to draw a graph with an odd number of vertices or odd degree. xplain why not. Use the handshaking lemma. 31

8 HP 2 7 ive volunteers, nn, rian, onor, ave and un Jung are going to run a help desk from Monday to riday next week. One person is required each day. nn is available on uesday and riday, rian is available on Wednesday, onor is available on Monday, hursday and riday, ave is available on Monday, Wednesday and hursday, un Jung is available on uesday, Wednesday and hursday. raw a graph to model this situation. Look at xample 3. 8 project consists of six activities 1, 2, 3, 4, 5 and 6. 1 and 2 can start immediately, but 3 cannot start until 1 is completed. 4 cannot start until both 2 and 3 are complete, 4 cannot start until 4 is complete and 6 cannot start until 5 is complete. raw a digraph to model this situation. Look at xample You need to know some special types of graph. tree is a connected graph with no cycles. S tree G spanning tree of a graph, G, is a subgraph which includes all the vertices of G and is also a tree. or example, starting with this graph, G: V U W his is not a tree. It contains a cycle U S. Possible spanning trees are here are many other spanning trees 32

9 Graphs and networks bipartite graph consists of two sets of vertices, X and Y. he edges only join vertices in X to vertices in Y, not vertices within a set. You saw an example of a bipartite graph in xample 3. nother one is drawn below Notice that each edge connects a left hand vertex set to a right hand vertex set. here are no edges that connect vertices in the same set. complete graph is a graph in which every vertex is directly connected by an edge to each of the other vertices. If the graph has n vertices the connected graph is denoted by k n. k 4 k 3 k 6 complete bipartite graph is denoted k r,s, where there are r vertices in set X and s vertices in set Y. J U K G V L k 3,3 Isomorphic graphs are graphs that show the same information but are drawn differently. or example k 2,3 is isomophicto J K L G U V and W M N P Q S X or two graphs to be isomorphic, they must have the same number of vertices of the same degree, but these vertices must also be connected together in the same way. If graphs are isomorphic it is possible to pair equivalent vertices in this case. can be paired with J in the first graph and in the second can be paired with M in the first graph and in the second can be paired with N in the first graph and S in the second can be paired with L in the first graph and V in the second can be paired with Q in the first graph and W in the second can be paired with P in the first graph and U in the second G can be paired with K in the first graph and X in the second 33

10 HP You should understand how graphs and network can be represented using matrices. n adjacency matrix records the number of direct links between vertices. distance matrix records the weights on the edges. Where there is no edge, we write. xample 6 Use an adjacency matrix to represent this graph You should be able to write down the adjacency matrix given the graph, and draw a graph given the adjacency matrix. his indicates that there are 2 direct connections between and. his indicates a loop from to. It could be travelled in either direction, and hence counts as 2. xample 7 Use a distance matrix to represent this network You should be able to write down the distance matrix given the network and draw the network given the distance matrix. You will not have to construct matrices for nondirected networks with loops or multiple edges. Notice that the matrix is symmetrical about the leading diagonal (top left to bottom right). 34

11 Graphs and networks xample 8 Use a distance matrix to represent this directed network. S U V W 4 18 S U V 7 W S U 10 W he matrix will not be symmetrical about the leading diagonal for a directed network. his indicates a direct link of weight 6 from U to V. 7 9 V his indicates a direct link of 7 from V to U. his shows a direct link of 9 from W to V. xercise 2 1 State which of the following graphs are trees. a b c d 2 here are 11 spanning trees for the graph above. See how many you can find. 3 raw k 8. What is the degree of each vertex in the graph k n? 4 raw k 3,4 How many edges would be in k n,m? 35

12 HP 2 5 Which of these graphs are isomorphic? a b c d e f g h i 6 G Use an adjacency matrix to represent the graph above. 7 raw a graph corresponding to each adjacency matrix. a c b d Which graphs in Question 5 could be described by the adjacency matrices in Question 7c and d? 36

13 Graphs and networks 9 raw the network corresponding to each distance matrix b Use a distance matrix to represent the directed network above. Mixed exercise 2 1 group of 6 children, bmed, ronwen, i, ddie, iona and Gary, were asked which of 6 sports, football, swimming, cricket, tennis, hockey and rugby, they enjoyed playing. he results are shown in the table below. bmed football cricket rugby ronwen swimming basketball hockey i football tennis hockey ddie football cricket basketball hockey rugby iona tennis swimming hockey Gary football swimming hockey raw a bipartite graph to show this information. 2 project involves eight activities,,,,,, G and H, some of which cannot be started until others are completed. he table shows the tasks that need to be completed before the activity can start. or example, activity cannot start until both and are completed. ctivity ctivity that must be completed G H G raw a digraph to represent this information. 37

14 HP 2 3 a raw a graph with eight vertices, all of degree 1. b raw a graph with eight vertices, all of degree 2, so that the graph is i connected and simple ii not connected and simple iii not connected and not simple. 4 Use a distance matrix to represent the network below ind two spanning trees for the graph in Question 4. 6 Write down a formula connecting the number of edges,, in a spanning tree with V vertices. Summary of key points 1 graph consists of vertices or (nodes) which are connected by edges (arcs). 2 If a graph has a number associated with each edge (its weight) then the graph is a weighted graph (network). 3 he degree or valency (order) of a vertex is the number of edges incident to it. 4 path is a finite sequence of edges, such that the end vertex of one edge in the sequence is the start vertex of the next, and in which no vertex appears more than once. 5 cycle (circuit) is a closed path, i.e. the end vertex of the last edge is the start vertex of the first edge. 6 wo vertices are connected if there is a path between them. graph is connected of all its vertices are connected. 7 loop is an edge that starts and finishes at the same vertex. 8 simple graph is one in which there are no loops and not more than one edge connecting any pair of vertices. 38

15 Graphs and networks 9 If the edges of a graph have a direction associated with them they are known as a digraph. 10 tree is a connected graph with no cycles. 11 spanning tree of a graph, G, is a subgraph which includes all the vertices of G and is also a tree. 12 bipartite graph consists of two sets of vertices, X and Y. he edges only join vertices in X to vertices in Y, not vertices within a set. 13 complete graph is a graph in which every vertex is directly connected by an edge to each of the other vertices. If the graph has n vertices the connected graph is denoted k n. 14 complete bipartite graph is denoted k r,s, where there are r vertices in set X and s vertices in set Y. 15 Isomorphic graphs show the same information but are drawn differently. 16 n adjacency matrix records the number of direct links between vertices. 17 distance matrix records the weights on the edges. Where there is no weight, this is indicated by. 39

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