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Weight Determination by Manifold Regularization
Linköping University, Department of Electrical Engineering, Automatic Control. Linköping University, The Institute of Technology.
Linköping University, Department of Electrical Engineering, Automatic Control. Linköping University, The Institute of Technology.
2010 (English)In: Distributed Decision-Making and Control / [ed] Rolf Johansson and Anders Rantzer, Springer London, 2010, 195-214 p.Chapter in book (Refereed)
Abstract [en]

A new type of linear kernel smoother is derived and studied. The smoother, referred to as weight determination by manifold regularization, is the solution to a regularized least squares problem. The regularization avoids overfitting and can be used to express prior knowledge of an underlying smooth function. An interesting property ofthe kernel smoother is that it is well suited for systems govern by the semi-supervised smoothness assumption. Several examples are given to illustrate this property. We also discuss why these types of techniques can have a potential interest for the system identification community.

Place, publisher, year, edition, pages
Springer London, 2010. 195-214 p.
Series
Lecture Notes in Control and Information Sciences, ISSN 0170-8643 ; 417
Keyword [en]
Regularization, Weight determination
National Category
Control Engineering
Identifiers
URN: urn:nbn:se:liu:diva-60987DOI: 10.1007/978-1-4471-2265-4_9ISBN: 978-1-4471-2264-7 (print)ISBN: 978-1-4471-2265-4 (print)OAI: oai:DiVA.org:liu-60987DiVA: diva2:360031
Available from: 2010-11-01 Created: 2010-11-01 Last updated: 2013-09-29
In thesis
1. Regularization for Sparseness and Smoothness: Applications in System Identification and Signal Processing
Open this publication in new window or tab >>Regularization for Sparseness and Smoothness: Applications in System Identification and Signal Processing
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In system identification, the Akaike Information Criterion (AIC) is a well known method to balance the model fit against model complexity. Regularization here acts as a price on model complexity. In statistics and machine learning, regularization has gained popularity due to modeling methods such as Support Vector Machines (SVM), ridge regression and lasso. But also when using a Bayesian approach to modeling, regularization often implicitly shows up and can be associated with the prior knowledge. Regularization has also had a great impact on many applications, and very much so in clinical imaging. In e.g., breast cancer imaging, the number of sensors is physically restricted which leads to long scantimes. Regularization and sparsity can be used to reduce that. In Magnetic Resonance Imaging (MRI), the number of scans is physically limited and to obtain high resolution images, regularization plays an important role.

Regularization shows-up in a variety of different situations and is a well known technique to handle ill-posed problems and to control for overfit. We focus on the use of regularization to obtain sparseness and smoothness and discuss novel developments relevant to system identification and signal processing.

In regularization for sparsity a quantity is forced to contain elements equal to zero, or to be sparse. The quantity could e.g., be the regression parameter vectorof a linear regression model and regularization would then result in a tool for variable selection. Sparsity has had a huge impact on neighboring disciplines, such as machine learning and signal processing, but rather limited effect on system identification. One of the major contributions of this thesis is therefore the new developments in system identification using sparsity. In particular, a novel method for the estimation of segmented ARX models using regularization for sparsity is presented. A technique for piecewise-affine system identification is also elaborated on as well as several novel applications in signal processing. Another property that regularization can be used to impose is smoothness. To require the relation between regressors and predictions to be a smooth function is a way to control for overfit. We are here particularly interested in regression problems with regressors constrained to limited regions in the regressor-space e.g., a manifold. For this type of systems we develop a new regression technique, Weight Determination by Manifold Regularization (WDMR). WDMR is inspired byapplications in biology and developments in manifold learning and uses regularization for smoothness to obtain smooth estimates. The use of regularization for smoothness in linear system identification is also discussed.

The thesis also presents a real-time functional Magnetic Resonance Imaging (fMRI) bio-feedback setup. The setup has served as proof of concept and been the foundation for several real-time fMRI studies.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2010. 89 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1351
Keyword
Regularization, sparsity, smothness, lasso, l1, fMRI, bio-feedback
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-60531 (URN)978-91-7393-287-5 (ISBN)
Public defence
2010-11-26, I101, Hus I, Campus Valla, Linköping University, Linköping, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2010-11-03 Created: 2010-10-16 Last updated: 2010-11-03Bibliographically approved

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Ohlsson, HenrikLjung, Lennart

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