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Unformatted text preview: Lecture Note 1: Jan 17, 2007 Dr. Jeff ChakFu WONG Department of Mathematics Chinese University of Hong Kong jwong@math.cuhk.edu.hk MAT 2310C Linear Algebra and Its Applications Spring, 2007 Produced by Jeff ChakFu WONG 1 L INEAR E QUATIONS AND M ATRICES 1. Linear Systems 2. Matrices 3. Dot Product and Matrix Multiplication 4. Properties of Matrix Operations 5. Solutions of Linear Systems of Equations 6. The Inverse of A Matrix 7. Applications to Graph Theory 8. LUFactorization LINEAR EQUATIONS AND MATRICES 2 A PPLICATION TO G RAPH T HEORY 1. Matrix Multiplication 2. Matrix Addition APPLICATION TO GRAPH THEORY 3 B ASIC CONCEPTS OF GRAPH THEORY Before we make the connection between graph theory and linear algebra, we start with some basic definitions in graph theory for those of you who are not familiar with the topic: A graph is a collection of points called vertices , joined by lines called edges . BASIC CONCEPTS OF GRAPH THEORY 4 cf. Steven J. Leon’s book, page 52, Figure 1.3.2 BASIC CONCEPTS OF GRAPH THEORY 5 S O HOW IS LINEAR A LGEBRA RELATED TO GRAPH THEORY ? As you can expect, graphs can be sometimes very complicated. So one needs to find more practical ways to represent them. Matrices are a very useful way of studying graphs, since they turn the picture into numbers, and then one can use techniques from linear algebra. Given a graph G with n vertices V 1 , ··· , V n , we define the adjacency matrix of G with respect to the enumeration V 1 , ··· , V n of the vertices as being the n × n matrix A = [ a ij ] defined by a ij = 1 if { V i ,V j } is an edge of the graph 0 if there is no edge joining V i and V j (1) SO HOW IS LINEAR ALGEBRA RELATED TO GRAPH THEORY? 6 The line segment joining the vertices correspond to the edges: { V 1 ,V 2 } , { V 2 ,V 5 } , { V 3 ,V 4 } , { V 3 ,V 5 } , { V 4 ,V 5 } For example, for the given graph G , the adjacency matrix (with respect to the enumeration V 1 ··· , V 5 of its vertices) is A = 0 1 0 0 0 1 0 0 0 1 0 0 0 1 1 0 0 1 0 1 0 1 1 1 0 Interpret V 1 V 2 V 3 V 4 V 5 V 1 1 V 2 1 1 V 3 1 1 V 4 1 1 V 5 1 1 1 SO HOW IS LINEAR ALGEBRA RELATED TO GRAPH THEORY? 7 A = 0 1 0 0 0 1 0 0 0 1 0 0 0 1 1 0 0 1 0 1 0 1 1 1 0 Note that the matrix A is symmetric (i.e., it is equal to its transpose). In fact, any adjacent matrix must be symmetric, for if { V i ,V j } is an edge of the graph, then a ij = a ji = 1 and a ij = a ji = 0 if there is no edge joining V i and V j . In either case, a ij = a ji . SO HOW IS LINEAR ALGEBRA RELATED TO GRAPH THEORY? 8 A WALK A walk in a graph as a sequence of edges linking one vertex to another....
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This note was uploaded on 06/05/2011 for the course MATH 3333 taught by Professor Jeffwong during the Spring '11 term at CUHK.
 Spring '11
 JeffWong
 Linear Algebra, Algebra

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