L18cs2110fa09-6up - 10/20/2009 Announcements 2 Prelim 2:...

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10/20/2009 1 GRAPHS Lecture 18 CS2110 – Fall 2009 Announcements 2 ± Prelim 2: Two and a half weeks from now ± Tuesday, Nov 17, 7:30-9pm ± Uris G01 Auditorium ± Exam conflicts ± Email Ken or Maria soon so that we can plan ahead ± Old exams are available for review on the course website These are not Graphs 3 ...not the kind we mean, anyway These are Graphs 4 K 5 K 3,3 = Applications of Graphs 5 ± Communication networks ± Routing and shortest path problems ± Commodity distribution (flow) ± Traffic control ± Resource allocation ± Geometric modeling ± ... Graph Definitions 6 ± A directed graph (or digraph ) is a pair (V, E) where ± V is a set ± E is a set of ordered pairs (u,v) where u,v ± V ² Usually require u v (i.e., no self-loops) ± An element of V is called a vertex (pl. vertices ) or node ± An element of E is called an edge or arc ± |V| = size of V, often denoted n ± |E| = size of E, often denoted m
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10/20/2009 2 Example Directed Graph (Digraph) 7 b a c d e f V = {a,b,c,d,e,f } E = {(a,b), (a,c), (a,e), (b,c), (b,d), (b,e), (c,d), (c,f), (d,e), (d,f), (e,f)} |V| = 6, |E| = 11 Example Undirected Graph 8 An undirected graph is just like a directed graph, except the edges are unordered pairs ( sets ) {u,v} Example: b d a c e f V = {a,b,c,d,e,f } E = {{a,b}, {a,c}, {a,e}, {b,c}, {b,d}, {b,e}, {c,d}, {c,f}, {d,e}, {d,f}, {e,f}} Some Graph Terminology 9 ± Vertices u and v are called the source and sink of the directed edge (u,v), respectively ± Vertices u and v are called the endpoints of (u,v) ± Two vertices are adjacent if they are connected by an edge ± The outdegree of a vertex u in a directed graph is the number of edges for which u is the source ± The indegree of a vertex v in a directed graph is the number of edges for which v is the sink ± The degree of a vertex u in an undirected graph is the number of edges of which u is an endpoint b a c e d f b a c d e f More Graph Terminology 10 ± A path is a sequence v 0 ,v 1 ,v 2 ,...,v p of vertices such that (v i ,v i+1 ) א E, 0 i p – 1 ± The length of a path is its number of edges ± In this example, the length is 5 v 0 v 5 ± A path is simple if it does not repeat any vertices ± A cycle is a path v 0 ,v 1 ,v 2 ,...,v p such that v 0 = v p ± A cycle is simple if it does not repeat any vertices except the first and last ± A graph is acyclic if it has no cycles ± A directed acyclic graph is called a dag b a c d e f Is This a Dag? 11 b a c d e f ± Intuition: ± If it’s a dag, there must be a vertex with indegree zero –why? ± This idea leads to an algorithm ± A digraph is a dag if and only if we can iteratively delete indegree-0 vertices until the graph disappears Is This a Dag? 12 b a c d e f ± Intuition: ± If it’s a dag, there must be a vertex with indegree zero ± This idea leads to an algorithm ± A digraph is a dag if and only if we can iteratively delete indegree-0 vertices until the graph disappears
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10/20/2009 3 Is This a Dag?
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L18cs2110fa09-6up - 10/20/2009 Announcements 2 Prelim 2:...

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