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Spectral Graph Theory

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Adjacency Matrix

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A square matrix AA where AijA_{ij} represents the presence (often with a 1) or absence (with a 0) of an edge between vertices i and j in a graph. Used to analyze graph properties and compute eigenvalues that relate to the graph's structure.

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Algebraic Connectivity

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The second smallest eigenvalue of the Laplacian matrix of a graph, denoted by λ2\lambda_2. It measures how well-connected a graph is. A value greater than 0 indicates the graph is connected.

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Spectral Clustering

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A technique that uses eigenvectors of a graph's Laplacian matrix to cluster nodes into groups that minimize the number of inter-cluster edges. It is widely used in machine learning for data classification and image segmentation.

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Cheeger Inequality

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An inequality which provides a bound relating the algebraic connectivity (second smallest eigenvalue) of a graph and its edge expansion (Cheeger constant). It is a tool used in understanding the efficiency of network flow and bottleneck issues.

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Spectrum of a Graph

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The set of eigenvalues of a graph's adjacency matrix, which provides insights into the graph’s structure. Can be used to determine properties like connectivity, bipartiteness, and graph isomorphisms.

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Eigenvector Centrality

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A measure of the influence of a node in a network. It assigns relative scores to all nodes based on the concept that connections to high-scoring nodes contribute more to the score of a node than equal connections to low-scoring nodes.

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Rayleigh Quotient

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A value expressing the tightness of a graph's eigenvalues, defined as R(x)=xTAxxTxR(x) = \frac{x^T A x}{x^T x} for a vector xx. It characterizes various properties of the graph and is used in optimization problems involving eigenvalues.

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Laplacian Matrix

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A matrix L=DAL = D - A where DD is the degree matrix (diagonal matrix with vertex degrees) and AA is the adjacency matrix of the graph. It is used to study the graph's connectivity, compute the number of spanning trees, and analyze random walks.

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