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We consider the problem of structural optimization which has many important applications in the engineering sciences. The goal is to find an optimal distribution of the material within a certain volume that will minimize the mechanical and/or thermal compliance of the structure. The physical system is governed by the standard models of elasticity and heat transfer expressed in terms of boundary-value problems for elliptic systems of partial differential equations (PDEs). The structural optimization problem is then posed as a suitably constrained PDE optimization problem, which can be solved numerically using a gradient approach. As a main contribution to the thesis, we derive expressions for gradients (sensitivities) of different objective functionals. This is done in both the continuous and discrete setting using the Riesz representation theorem and adjoint analysis. The sensitivities derived in this way are then tested computationally using simple minimization algorithms and some standard two-dimensional test problems. / Thesis / Master of Science (MSc)

One of the marvelous phenomena of coordinate geometry is the equivalence of Desargues' Theorem to the presence of an underlying division ring in a projective plane. Supplementing this correspondence is the general theory of intersection theorems, which, restricted to desarguian projective planes P, corresponds precisely to the theory of integral rational identities, restricted to division rings D. The first chapter of this paper introduces projective planes, develops the concept of an intersection theorem, and expounds upon the Theorem of Desargues; the discussion culminates with a proof of the desarguian phenomenon in the second chapter. The third chapter characterizes the automorphisms of P and introduces the theory of polynomial identities; the fourth chapter expands this discussion to rational identities and cements the ``dictionary''. The last section describes a measure of complexity for these intersection theorems, and the paper concludes with a curious spawn of the correspondence. / Master of Science

This dissertation examines equilibria in epidemics and introduces novel approaches to epidemic modeling, consisting of two separate but closely related chapters. In Chapter 2, we develop an optimizing epidemic model within a dynamic urn-SIR framework, accommodating generic offspring distributions, and derive a trajectory convergence theorem. We study the mean-field equilibrium through numerical simulations. Our findings reveal two key features often overlooked in existing literature: substantial variation in epidemic outcomes despite homogeneous individual behavior, and the potential for resurgence in the number of infections. We demonstrate that the offspring distribution of infections significantly impacts epidemic dynamics, with negative binomial distributions leading to more dispersed outcomes and higher probabilities of minor outbreaks compared to geometric distributions. These results highlight the importance of stochastic modeling in epidemic forecasting and public health policy. Chapter 3 proposes and examines static and dynamic urn-SIR models, a novel approach to epidemic modeling that addresses key limitations of traditional stochastic SIR models. We focus on the critical issue of heterogeneity in individual infectiousness, which is not adequately captured by the geometric offspring distribution inherent in the continuous-time Markov chain SIR models. Our urn-SIR models accommodates generic offspring distributions, including the empirically supported negative binomial distribution. We formally formulate the static and dynamic urn-SIR models. The static model focuses on the end of the epidemic, where primary variables are the epidemic size and the total number of contacts, while the dynamic model captures the dynamic process of the disease progression. The cornerstone of our work is a proven threshold limit theorem, characterizing the asymptotic behavior of the epidemic size as the population approaches infinity. This theorem extends beyond early-stage branching process approximations in the existing literature that considers generic offspring distribution. Moreover, we also show that in the dynamic model, the trajectories of epidemic processes converges in probability to a corresponding deterministic system, allowing comprehensive analysis of entire epidemic courses. Our work bridges crucial gaps in existing literature, providing a more realistic representation of disease spread while maintaining analytical tractability. The findings have significant implications for epidemiology, public health, and related fields, informing more effective strategies for disease control and prevention.

Yes / Many studies have examined the extent to which individuals’ probability judgments depart from Bayes’ theorem when revising probability estimates in the light of new information. Generally, these studies have not considered the implications of such departures for decisions involving risk. We identify when such departures will occur in two common types of decisions. We then report on two experiments where people were asked to revise their own prior probabilities of a forthcoming economic recession in the light of new information. When the reliability of the new information was independent of the state of nature, people tended to overreact to it if their prior probability was low and underreact if it was high. When it was not independent, they tended to display conservatism. We identify the circumstances where discrepancies in decisions arising from a failure to use Bayes’ theorem were most likely to occur in the decision context we examined. We found that these discrepancies were relatively rare and, typically, were not serious.

In this thesis we attempt to generalize some of Kummer's work on Fermat's Last Theorem over the rational numbers to quadratic fields. In particular, under certain congruence conditions it is shown that the Fermat equation of exponent p has no solution over Q(√m) when p is a m-regular prime. Completely analogous to the work of Kummer, it is shown that m-regular primes can be described in terms of the generalized Bernoulli numbers. When p = 3,5 and 7, an explicit, easily computable criterion is given for m-regularity. / Ph. D.

A Thurston map whose postcritical set consists of exactly four points and for which the local degree at each of its critical points is 2 is called textit{nearly Euclidean}. These maps were specified to parse Thurston's combinatorial characterization of rational functions. We determine an extension of the half-space theorem which provides an open hyperbolic half-space such that the negative reciprocal of any fixed slope value is excluded from the boundary of the half-space. / Master of Science / Thurston proved necessary and sufficient conditions under which a certain class of mappings defined topologically are equivalent, in a precise sense which can be considered less strict than topological conjugacy, to a rational map. The conditions presented in the proof of this theorem are not ones for which computational algorithms are easily admitted in all settings. Nearly Euclidean Thurston maps are a sub-class of the maps to which this theorem is applicable and for which an abundance of information is algorithmically attainable. We extend a theorem in this setting. One main example which speaks to the utility of this extension is in determining when certain rational maps arise as matings of polynomials.

Connectivity is a fundamental tool for matroid theorists, which has become increasingly important in the eventual solution of many problems in matroid theory. Loosely speaking, connectivity can be used to help describe a matroid's structure. In this thesis, we prove a series of results that further the knowledge and understanding in the field of matroid connectivity. These results fall into two parts. First, we focus on 3-connected matroids. A chain theorem is a result that proves the existence of an element, or elements, whose deletion or contraction preserves a predetermined connectivity property. We prove a series of chain theorems for 3-connected matroids where, after fixing a basis B, the elements in B are only eligible for contraction, while the elements not in B are only eligible for deletion. Moreover, we prove a splitter theorem, where a 3-connected minor is also preserved, resolving a conjecture posed by Whittle and Williams in 2013. Second, we consider k-connected matroids, where k >= 3. A certain tree, known as a k-tree, can be used to describe the structure of a k-connected matroid. We present an algorithm for constructing a k-tree for a k-connected matroid M. Provided that the rank of a subset of E(M) can be found in unit time, the algorithm runs in time polynomial in |E(M)|. This generalises Oxley and Semple's (2013) polynomial-time algorithm for constructing a 3-tree for a 3-connected matroid.

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