Studies on Kernel Learning and Independent Component Analysis

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Nima Reyhani Name of the doctoral dissertation Studies on Kernel Learning and Independent Component Analysis Publisher School of Science Unit Department of Information and Computer Science Series Aalto University publication series DOCTORAL DISSERTATIONS 47/2013 Field of research Computer and Information Science Manuscript submitted 9 October 2012 Date of the defence 19 March 2013 Permission to publish granted (date) 21 February 2013 Language English Monograph Article dissertation (summary + original articles) Abstract A crucial step in kernel-based learning is the selection of a proper kernel function or kernel matrix. Multiple kernel learning (MKL), in which a set of kernels are assessed during the learning time, was recently proposed to solve the kernel selection problem. The goal is to estimate a suitable kernel matrix by adjusting a linear combination of the given kernels so that the empirical risk is minimized. MKL is usually a memory demanding optimization problem, which becomes a barrier for large samples. This study proposes an efficient method for kernel learning by using the low rank property of large kernel matrices which is often observed in applications. The proposed method involves selecting a few eigenvectors of kernel bases and taking a sparse combination of them by minimizing the empirical risk. Empirical results show that the computational demands decrease significantly without compromising classification accuracy, when compared with previous MKL methods. Computing an upper bound for complexity of the hypothesis set generated by the learned kernel as above is challenging. Here, a novel bound is presented which shows that the Gaussian complexity of such hypothesis set is controlled by the logarithm of the number of involved eigenvectors and their maximum distance, i.e. the geometry of the basis set. This geometric bound sheds more light on the selection of kernel bases, which could not be obtained from previous results. The rest of this study is a step toward utilizing the statistical learning theory to analyze independent component analysis estimators such as FastICA. This thesis provides a sample convergence analysis for FastICA estimator and shows that the estimations converge in distribution as the number of samples increase. Additionally, similar results for the bootstrap FastICA are established. A direct application of these results is to design a hypothesis testing to study the convergence of the estimates.A crucial step in kernel-based learning is the selection of a proper kernel function or kernel matrix. Multiple kernel learning (MKL), in which a set of kernels are assessed during the learning time, was recently proposed to solve the kernel selection problem. The goal is to estimate a suitable kernel matrix by adjusting a linear combination of the given kernels so that the empirical risk is minimized. MKL is usually a memory demanding optimization problem, which becomes a barrier for large samples. This study proposes an efficient method for kernel learning by using the low rank property of large kernel matrices which is often observed in applications. The proposed method involves selecting a few eigenvectors of kernel bases and taking a sparse combination of them by minimizing the empirical risk. Empirical results show that the computational demands decrease significantly without compromising classification accuracy, when compared with previous MKL methods. Computing an upper bound for complexity of the hypothesis set generated by the learned kernel as above is challenging. Here, a novel bound is presented which shows that the Gaussian complexity of such hypothesis set is controlled by the logarithm of the number of involved eigenvectors and their maximum distance, i.e. the geometry of the basis set. This geometric bound sheds more light on the selection of kernel bases, which could not be obtained from previous results. The rest of this study is a step toward utilizing the statistical learning theory to analyze independent component analysis estimators such as FastICA. This thesis provides a sample convergence analysis for FastICA estimator and shows that the estimations converge in distribution as the number of samples increase. Additionally, similar results for the bootstrap FastICA are established. A direct application of these results is to design a hypothesis testing to study the convergence of the estimates.