Graph Sparsification by Edge-Connectivity and Random Spanning Trees

We present new approaches to constructing graph sparsifiers --- weighted subgraphs for which every cut has the same value as the original graph, up to a factor of $(1 \pm \epsilon)$. Our first approach independently samples each edge $uv$ with probability inversely proportional to the edge-connectivity between $u$ and $v$. The fact that this approach produces a sparsifier resolves a question posed by Bencz\'ur and Karger (2002). Concurrent work of Hariharan and Panigrahi also resolves this question. Our second approach constructs a sparsifier by forming the union of several uniformly random spanning trees. Both of our approaches produce sparsifiers with $O(n \log^2(n)/\epsilon^2)$ edges. Our proofs are based on extensions of Karger's contraction algorithm, which may be of independent interest

The Effects of Coupling Delay and Amplitude / Phase Interaction on Large Coupled Oscillator Networks

The interaction of many coupled dynamical units is a theme across many scientific disciplines. A useful framework for beginning to understanding such phenomena is the coupled oscillator network description. In this dissertation, we study a few problems related to this. The first part of the dissertation studies generic effects of heterogeneous interaction delays on the dynamics of large systems of coupled oscillators. Here, we modify the Kuramoto model (phase oscillator model) to incorporate a distribution of interaction delays. Corresponding to the continuum limit, we focus on the reduced dynamics on an invariant manifold of the original system, and derive governing equations for the system, which we use to study stability of the incoherent state and the dynamical transitional behavior from stable incoherent states to stable coherent states. We find that spread in the distribution function of delays can greatly alter the system dynamics. The second part of this dissertation is a sequel to the first part. Here, we consider systems of many spatially distributed phase oscillators that interact with their neighbors, and each oscillator can have a different natural frequency, and a different response time to the signals it receives from other oscillators in its neighborhood. By first reducing the microscopic dynamics to a macroscopic partial-differential-equation description, we then numerically find that finite oscillator response time leads to many interesting spatio-temporal dynamical behaviors, and we study interactions and evolutionary behaviors of these spatio-temporal patterns. The last part of this dissertation addresses the behavior of large systems of heterogeneous, globally coupled oscillators each of which is described by the generic Landau-Stuart equation, which incorporates both phase and amplitude dynamics. Our first goal is to investigate the effect of a spread in the amplitude growth parameter of the oscillators and that of a homogeneous nonlinear frequency shift. Both of these effects are of potential relevance to recently reported experiments. Our second goal is to gain further understanding of the observation that, at large coupling strength, a simple constant-amplitude sinusoidal oscillation is always a solution for the dynamics of the global order parameter when the system has constant nonlinear characteristics

Geometric Aspects of Frame Representations of Abelian Groups

We consider frames arising from the action of a unitary representation of a discrete countable abelian group. We show that the range of the analysis operator can be determined by computing which characters appear in the representation. This allows one to compare the ranges of two such frames, which is useful for determining similarity and also for multiplexing schemes. Our results then partially extend to Bessel sequences arising from the action of the group. We apply the results to sampling on bandlimited functions and to wavelet and Weyl-Heisenberg frames. This yields a sufficient condition for two sampling transforms to have orthogonal ranges, and two analysis operators for wavelet and Weyl-Heisenberg frames to have orthogonal ranges. The sufficient condition is easy to compute in terms of the periodization of the Fourier transform of the frame generators.Comment: 20 pages; contact author: Eric Webe

A study of the structure and performance of the real estate development industry in Hong Kong

Includes bibliographical references (p. 105-110).Thesis (B.Sc)--University of Hong Kong, 2008.published_or_final_versio

Regulating the sale of first-hand residential properties in Hong Kong : a study of policy and administrative dynamics

published_or_final_versionPolitics and Public AdministrationMasterMaster of Public Administratio