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L18cs2110fa08

# L18cs2110fa08 - Graphs Lecture 18 CS2110 Fall 2008...

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Graphs Lecture 18 CS2110 – Fall 2008

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2 Announcements Prelim 2 Tuesday, Nov 18, 7:30-9pm Uris Auditorium Exam conflicts Email Kelly Patwell ASAP Old exams are available for review on the course website
3 These are not Graphs East West North ...not the kind we mean, anyway

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4 These are Graphs K 5 K 3,3 =
5 Applications of Graphs Communication networks Routing and shortest path problems Commodity distribution (flow) Traffic control Resource allocation Geometric modeling ...

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6 Graph Definitions 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 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
7 Example Directed Graph (Digraph) 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 b a c d e f

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8 Example Undirected Graph An undirected graph is just like a directed graph, except the edges are unordered pairs ( sets ) {u,v} Example: b a c e d 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 } }
9 Some Graph Terminology 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

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10 More Graph Terminology A path is a sequence v 0 ,v 1 ,v 2 ,...,v p of vertices such that (v i ,v i+1 ) E, 0 i p – 1 The length of a path is its number of edges In this example, the length is 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 v 0 v 5 b a c d e f
11 Is This a Dag? 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 b a c d e f

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12 Is This a Dag? 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 b a c d e f
13 Is This a Dag?

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