Laplacian Operator

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Leandro S Tavares - One of the best experts on this subject based on the ideXlab platform.

Claudianor O Alves - One of the best experts on this subject based on the ideXlab platform.

Anand Louis - One of the best experts on this subject based on the ideXlab platform.

  • hypergraph markov Operators eigenvalues and approximation algorithms
    Symposium on the Theory of Computing, 2015
    Co-Authors: Anand Louis
    Abstract:

    The celebrated Cheeger's Inequality [AM85,a86] establishes a bound on the expansion of a graph via its spectrum. This inequality is central to a rich spectral theory of graphs, based on studying the eigenvalues and eigenvectors of the adjacency matrix (and other related matrices) of graphs. It has remained open to define a suitable spectral model for hypergraphs whose spectra can be used to estimate various combinatorial properties of the hypergraph. In this paper we introduce a new hypergraph Laplacian Operator generalizing the Laplacian matrix of graphs. Our Operator can be viewed as the gradient Operator applied to a certain natural quadratic form for hypergraphs. We show that various hypergraph parameters (for e.g. expansion, diameter, etc) can be bounded using this Operator's eigenvalues. We study the heat diffusion process associated with this Laplacian Operator, and bound its parameters in terms of its spectra. All our results are generalizations of the corresponding results for graphs. We show that there can be no linear Operator for hypergraphs whose spectra captures hypergraph expansion in a Cheeger-like manner. Our Laplacian Operator is non-linear, and thus computing its eigenvalues exactly is intractable. For any k, we give a polynomial time algorithm to compute an approximation to the kth smallest eigenvalue of the Operator. We show that this approximation factor is optimal under the SSE hypothesis (introduced by [RS10]) for constant values of k. Finally, using the factor preserving reduction from vertex expansion in graphs to hypergraph expansion, we show that all our results for hypergraphs extend to vertex expansion in graphs.

  • hypergraph markov Operators eigenvalues and approximation algorithms
    arXiv: Discrete Mathematics, 2014
    Co-Authors: Anand Louis
    Abstract:

    The celebrated Cheeger's Inequality \cite{am85,a86} establishes a bound on the expansion of a graph via its spectrum. This inequality is central to a rich spectral theory of graphs, based on studying the eigenvalues and eigenvectors of the adjacency matrix (and other related matrices) of graphs. It has remained open to define a suitable spectral model for hypergraphs whose spectra can be used to estimate various combinatorial properties of the hypergraph. In this paper we introduce a new hypergraph Laplacian Operator (generalizing the Laplacian matrix of graphs)and study its spectra. We prove a Cheeger-type inequality for hypergraphs, relating the second smallest eigenvalue of this Operator to the expansion of the hypergraph. We bound other hypergraph expansion parameters via higher eigenvalues of this Operator. We give bounds on the diameter of the hypergraph as a function of the second smallest eigenvalue of the Laplacian Operator. The Markov process underlying the Laplacian Operator can be viewed as a dispersion process on the vertices of the hypergraph that might be of independent interest. We bound the {\em Mixing-time} of this process as a function of the second smallest eigenvalue of the Laplacian Operator. All these results are generalizations of the corresponding results for graphs. We show that there can be no linear Operator for hypergraphs whose spectra captures hypergraph expansion in a Cheeger-like manner. For any $k$, we give a polynomial time algorithm to compute an approximation to the $k^{th}$ smallest eigenvalue of the Operator. We show that this approximation factor is optimal under the SSE hypothesis (introduced by \cite{rs10}) for constant values of $k$. Finally, using the factor preserving reduction from vertex expansion in graphs to hypergraph expansion, we show that all our results for hypergraphs extend to vertex expansion in graphs.

Lishan Liu - One of the best experts on this subject based on the ideXlab platform.

Xin Wang - One of the best experts on this subject based on the ideXlab platform.

  • Laplacian Operator based edge detectors
    IEEE Transactions on Pattern Analysis and Machine Intelligence, 2007
    Co-Authors: Xin Wang
    Abstract:

    Laplacian Operator is a second derivative Operator often used in edge detection. Compared with the first derivative-based edge detectors such as Sobel Operator, the Laplacian Operator may yield better results in edge localization. Unfortunately, the Laplacian Operator is very sensitive to noise. In this paper, based on the Laplacian Operator, a model is introduced for making some edge detectors. Also, the optimal threshold is introduced for obtaining a maximum a posteriori (MAP) estimate of edges