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math 262

# math 262 - Lecture 1 Basic Structures and Definition of...

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Unformatted text preview: Lecture 1: Basic Structures and Definition of PageRank Transcriber: Mary Radcliffe 1 A Glimpse of Spectral Graph Theory Let G = ( V,E ) be a graph, where n = | V | . Define the adjacency matrix of G , denoted by A , to be a matrix indexed by V , with A ( u,v ) = 1 u ∼ v else where u ∼ v denotes adjacency (i.e., u ∼ v iff { u,v } ∈ E ). We generally think of the graph G as being fairly large, so we are interested in ways to simplify the computations required to glean information from the graph. Because A is a real-valued symmetric matrix, we know from linear algebra that has real eigenvalues ρ ≥ ρ 1 ≥ ··· ≥ ρ n- 1 . Moreover, A is diagonalizable, so A = T- 1 Γ T , where Γ is the diagonal matrix with ρ i in the ( i,i ) position and the columns of T form an orthogonal eigenbasis corresponding to A . Diagonalizing the matrix allows multiplication with the adjacency matrix to be done more simply, as A k = T- 1 Γ k T . Let 1 denote the length n vector 1 of all 1’s. Then 1 A k 1 * is the number of length k walks in G . Exercise 1. ρ = lim k →∞ 1 A k 1 * 11 * 1 /k 2 Introduction to Random Walks Given G as above, define the degree matrix of G , denoted by D , to be a diagonal matrix indexed by V with D ( v,v ) = d v , where d v denotes the degree of v . We define a random walk on G to be a Markov chain with transition probability matrix P , where P ( u,v ) denotes the probability that, given the random walk is...
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math 262 - Lecture 1 Basic Structures and Definition of...

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