Direct Search Methods for Nonsmooth Problems Using Global Optimization Techniques
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Direct Search Methods for Nonsmooth Problems using Global Optimization Techniques A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN MATHEMATICS by Blair Lennon Robertson University of Canterbury 2010 To Peter i Abstract This thesis considers the practical problem of constrained and unconstrained local optimization. This subject has been well studied when the objective function f is assumed to smooth. However, nonsmooth problems occur naturally and frequently in practice. Here f is assumed to be nonsmooth or discontinuous without forcing smoothness assumptions near, or at, a potential solution. Various methods have been presented by others to solve nonsmooth optimization problems, however only partial convergence results are possible for these methods. In this thesis, an optimization method which use a series of local and localized global optimization phases is proposed. The local phase searches for a local minimum and gives the methods numerical performance on parts of f which are smooth. The localized global phase exhaustively searches for points of descent in a neighborhood of cluster points. It is the localized global phase which provides strong theoretical convergence results on nonsmooth problems. Algorithms are presented for solving bound constrained, unconstrained and con- strained nonlinear nonsmooth optimization problems. These algorithms use direct search methods in the local phase as they can be applied directly to nonsmooth prob- lems because gradients are not explicitly required. The localized global optimization phase uses a new partitioning random search algorithm to direct random sampling into promising subsets of Rn. The partition is formed using classification and regression trees (CART) from statistical pattern recognition. The CART partition defines desir- able subsets where f is relatively low, based on previous sampling, from which further samples are drawn directly. For each algorithm, convergence to an essential local min- imizer of f is demonstrated under mild conditions. That is, a point x∗ for which the set of all feasible points with lower f values has Lebesgue measure zero for all suffi- ciently small neighborhoods of x∗. Stopping rules are derived for each algorithm giving practical convergence to estimates of essential local minimizers. Numerical results are presented on a range of nonsmooth test problems for 2 to 10 dimensions showing the methods are effective in practice. ii Acknowledgements It has been a pleasure working with Dr Chris Price over the years, learning from his extensive knowledge of mathematics. On many occasions Chris stepped above and beyond the role of primary supervisor and offered advice to me as a friend which I am very grateful for. I also thank my secondary supervisor Dr Marco Reale for his encouragement, advice and enthusiasm in my research. I thoroughly enjoyed our supervisory team discussions, both mathematical and non-mathematical. There are too many friends to thank for whom I bounced ideas off, or bored to tears as the case may be, but I will mention Jason Bentley with whom I shared an office and many interesting discussions. I would also like to thank my parents for their support and financial understanding over the years. I dedicate this thesis to my father Peter who is sadly no longer with us. Finally, I would like to thank the Department for all the financial support including a Departmental PhD Scholarship and conference funds which allowed me to present some of my research at the 17th International Conference on Computing in Economics and Finance in Cyprus and present a seminar at the Third University of Rome. iii iv Contents Abstract i Acknowledgements iii Notation ix 1 Introduction 1 1.1 Local Optimization . 2 1.2 Direct Search Local Optimization . 4 1.2.1 Simplex Based Methods . 5 1.3 Directional Direct Search Methods . 8 1.3.1 Positive Bases . 8 1.3.2 Grid Based Methods . 10 1.3.3 Partial Nonsmooth Convergence Results . 13 1.4 Trajectory Following Optimization . 17 1.4.1 Physical Analog to Function Minimization . 18 1.4.2 Connections with Classical Direct Search . 19 1.5 Global Optimization . 20 1.6 Nonsmooth Local Optimization vs Global Optimization . 21 1.7 Optimization Method . 24 1.8 Thesis Overview . 25 2 Localized Global Phase 27 2.1 Random Search Optimization . 28 2.2 Partitioning Algorithms . 30 2.3 Classification and Pattern Recognition . 33 2.3.1 Classification Trees . 33 2.3.2 Induced Partitions on Ω . 35 v vi 2.4 CART . 36 2.4.1 Locating Potential Splitting Hyperplanes . 37 2.4.2 Choosing a Potential Split and Node Impurity . 40 2.4.3 When to Stop Node Splitting . 41 2.4.4 Defining CART Sub-Regions . 42 2.4.5 CART Algorithm . 43 2.4.6 Classification Tree and Partition on Ω . 44 2.5 Global Optimization Algorithmic Framework . 45 2.5.1 Analogs with Pure Adaptive Search . 48 2.5.2 Convergence . 48 3 CARTopt: A Random Search Nonsmooth Optimization Method 51 3.1 Bound Constrained CARTopt . 52 3.2 Training Data and Classification . 54 3.3 Transforming Training Data Sets . 56 3.4 Partitioning the Optimization Region . 59 3.4.1 Defining Low Sub-Regions Only . 60 3.4.2 Minimum Sub-Region Radius . 60 3.4.3 Updating Problematic Sub-Region Bounds . 61 3.4.4 Singleton Low Sub-Regions . 65 3.5 Generating The Next Batch . 67 3.5.1 Batch Size . 67 3.5.2 Delivery Method . 69 3.5.3 Effectiveness of Distribution Method . 71 3.6 A Deterministic CARTopt Instance . 72 3.6.1 Halton Sequence . 73 3.6.2 Implementation . 74 3.6.3 Notes on Convergence . 78 3.7 An Unconstrained CARTopt Instance . 78 3.7.1 Initialization . 79 3.7.2 Training Data Transformation . 79 3.7.3 CART Partition . 79 3.7.4 Sampling Phase . 80 3.8 Convergence Analysis . 80 3.8.1 Bound Constrained Nonsmooth Result . 83 3.8.2 Unconstrained Result . 84 vii 4 Sequential Stopping Rule 85 4.1 Resource Based Stopping Rules . 87 4.2 Sequential Stopping Rules . 87 4.2.1 Existing Stopping Rules . 88 4.3 New Sequential Stopping Rule . 91 4.3.1 Empirical Data . 92 4.3.2 Expected κ Values . 93 4.4 Fitting the Empirical Data . 96 4.4.1 Approximating f∗ .......................... 96 4.4.2 Optimal Power Fit . 96 4.5 Solving the 1-norm Fit . 99 4.5.1 Initial Bracket . 100 4.5.2 Choosing the Optimal κ∗ ...................... 101 4.5.3 Computational Efficiency . 102 4.6 Goodness of Fit . 102 4.7 Deterministic Instances of CARTopt . 104 4.8 Stopping Rule for the CARTopt Method . 104 5 Hooke and Jeeves / CARTopt Hybrid Method 107 5.1 The Algorithm . 107 5.1.1 Grid Structure . 108 5.1.2 Algorithm Details . 109 5.2 Exploratory Phase . 109 5.3 Sinking Lid . 111 5.4 Pattern Move . 113 5.4.1 Uphill Steps . 113 5.5 Localized Global Phase . 114 5.5.1 Recycling Points . 115 5.6 Generating the Next Grid . 117 5.6.1 Perturbation . 117 5.6.2 Rotation . 117 5.6.3 Scaling . 119 5.7 Convergence . 120 5.7.1 Smooth Results . 120 5.7.2 Nonsmooth Result . 123 viii 6 A CARTopt Filter Method for Nonlinear Programming 125 6.1 Filter Algorithms . 127 6.1.1 Filter . 127 6.1.2 Constraint Violation Function . 128 6.2 A CARTopt Filter Algorithm . 129 6.2.1 Sloping Filter . 132 6.2.2 Training Data Set . 134 6.2.3 Classification . 136 6.2.4 Partition and Sampling Phase . 138 6.2.5 Penalty Parameter . 139 6.3 Stopping Rule . 140 6.4 Convergence Analysis . 142 7 Empirical Testing of Algorithms 145 7.1 Bound Constrained Optimization using the CARTopt Method . 146 7.2 Unconstrained Optimization . 147 7.3 Nonlinear Programming using the CARTopt Filter Method . 151 7.3.1 Inequality Constrained Problems . 152 7.3.2 Equality Constrained Problems . 153 7.4 Concluding Remarks . 156 8 Summary and Conclusions 159 8.1 Where to Next? . 161 A Test Functions 171 A.1 Unconstrained Test Problems . 171 A.2 Nonlinear Programming Test Problems . 173 B Matlab Code 177 B.1 CARTopt Algorithm . ..