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UBC Theses and Dissertations

A binary relation inference network for constrained optimization Su, Crystal J.


Constrained optimization is an essential problem in artificial intelligence, operations research, robotics and very large scale integration hardware design, etc. Many constrained optimization problems are computation-intensive, and yet require fast solutions. This thesis presents a novel parallel computing network designed for some optimization problems which can be represented by binary relations and the calculation upon them. Traditional graph search techniques provide sequential solutions to constraint satisfaction, but their speed is limited by the computational ability of a single processor. Parallel processing offers faster solutions by dividing its work load among many processors. A parallel algorithm can be developed from its sequential counterpart by decomposing the unfinished task dynamically according to the availability of processors. These kind of algorithms usually have difficulty in distributing work load evenly among processors [69]. Many distributed parallel computing models have been proposed for various types of problems. A distributed computation model usually embodies the data structure for a specific type of problem [16]. Many of them work in the discrete time domain [71, etc]. Neural network approaches have been proposed for optimization problems [28], however, optimal solutions are often not guaranteed when the gradient descent scheme is used. The determination of the link weights in a Hopfield-type neural network can be elaborate and convergence for large problems remains a problem [2, 90]. This thesis proposes a new parallel computing network — a binary relation inference network. The network topology is set up to match conventional optimization procedures. The network transforms some optimization problems (shortest path, transitive closure, assignment, etc.) into network convergence problems and divides the work load evenly among all processors. Link weights are known. It can operate in synchronous discrete-time, asynchronous discrete-time, and continuous-time domains. Inference network algorithms are derived directly from problem formulation, and are effective for a large range of problem sizes. The inference network consists of interconnected components; each component is composed of a unit, a number of sites attached to the unit, and links from the unit to sites on other units. Each unit is a simple computational engine. The topology of the inference network matches naturally to many optimization problems, since its deduction sites produce candidates to compete for the optimum and its decision-making units select the optimum from the site values. Either directly or through a transformation mapping to a systolic structure, discrete inference network algorithms can be implemented on a commercially available parallel processing facility, such as the Connection Machine. Analog inference network can be built using integrated circuits which solve problems efficiently in the continuous-time domain. In this thesis, mathematical analysis and numerical simulation were conducted for the synchronous, asynchronous, and analog inference network. Various applications has been discussed. The inference network has shown to solve the all-pair shortest path problem efficiently in various updating schemes. The inference network algorithm for the transitive closure problem was derived straightforwardly from the formulation of the problem and the topology of the inference network. Efficient continuous-time solution was obtained from a non-synchronized logical circuit. The convergence of the network for the shortest path or transitive closure problems was shown to be independent of the problem size. It was demonstrated that the inference network can solve the assignment problem in a way similar to the Hopfield net's solution to the traveling salesman problem. However, the former was shown to converge for all test cases with problem size up to 128.

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