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Essays in Econometrics and Political Economy
Get PDFThis dissertation comprises three essays in Econometrics and Political Economy offering both methodological and substantive contributions to the study of electoral coalitions (Chapter 2), the effectiveness of campaign expenditures (Chapter 3), and the general practice of experimentation (Chapter 4).
Chapter 2 presents an empirical investigation of coalition formation in elections. Despite its prevalence in most democracies, there is little evidence documenting the impact of electoral coalition formation on election outcomes. To address this imbalance, I develop and estimate a structural model of electoral competition that enables me to conduct counterfactual analyses of election outcomes under alternative coalitional scenarios. The results uncover substantial equilibrium savings in campaign expenditures from coalition formation, as well as significant electoral gains benefitting electorally weaker partners.
Chapter 3, co-authored with Benjamin J. Gillen, Hyungsik Roger Moon, and Matthew Shum, proposes a novel data-driven approach to the problem of variable selection in econometric models of discrete choice estimated using aggregate data. Our approach applies penalized estimation algorithms imported from the machine learning literature along with confidence intervals that are robust to variable selection. We illustrate our approach with an application that explores the effect of campaign expenditures on candidate vote shares in data from Mexican elections.
Chapter 4, co-authored with Abhijit Banerjee, Sylvain Chassang, and Erik Snowberg, provides a decision-theoretic framework in which to study the question of optimal experiment design. We model experimenters as ambiguity-averse decision makers who trade off their own subjective expected payoff against that of an adversarial audience. We establish that ambiguity aversion is required for randomized controlled trials to be optimal. We also use this framework to shed light on the important practical questions of rerandomization and resampling.</p
Convex Programming-Based Phase Retrieval: Theory and Applications
Get PDFPhase retrieval is the problem of recovering a signal from its Fourier magnitude. This inverse problem arises in many areas of engineering and applied physics, and has been studied for nearly a century. Due to the absence of Fourier phase, the available information is incomplete in general. Classic identifiability results state that phase retrieval of one-dimensional signals is impossible, and that phase retrieval of higher-dimensional signals is almost surely possible under mild conditions. However, there are no efficient recovery algorithms with theoretical guarantees. Classic algorithms are based on the method of alternating projections. These algorithms do not have theoretical guarantees, and have limited recovery abilities due to the issue of convergence to local optima.
Recently, there has been a renewed interest in phase retrieval due to technological advances in measurement systems and theoretical developments in structured signal recovery. In particular, it is now possible to obtain specific kinds of additional magnitude-only information about the signal, depending on the application. The premise is that, by carefully redesigning the measurement process, one could potentially overcome the issues of phase retrieval. To this end, another approach could be to impose certain kinds of prior on the signal, depending on the application. On the algorithmic side, convex programming based approaches have played a key role in modern phase retrieval, inspired by their success in provably solving several quadratic constrained problems.
In this work, we study several variants of phase retrieval using modern tools, with focus on applications like X-ray crystallography, diffraction imaging, optics, astronomy and radar. In the one-dimensional setup, we first develop conditions, which when satisfied, allow unique reconstruction. Then, we develop efficient recovery algorithms based on convex programming, and provide theoretical guarantees. The theory and algorithms we develop are independent of the dimension of the signal, and hence can be used in all the aforementioned applications. We also perform a comparative numerical study of the convex programming and the alternating projection based algorithms. Numerical simulations clearly demonstrate the superior ability of the convex programming based methods, both in terms of successful recovery in the noiseless setting and stable reconstruction in the noisy setting.</p
Probabilistic Imaging and Dynamic Modeling of Earthquake Source Processes
Get PDFInvestigation of large, destructive earthquakes is challenged by their infrequent occurrence and the remote nature of geophysical observations. This thesis sheds light on the source processes of large earthquakes from two perspectives: robust and quantitative observational constraints through Bayesian inference for earthquake source models, and physical insights on the interconnections of seismic and aseismic fault behavior from elastodynamic modeling of earthquake ruptures and aseismic processes.
To constrain the shallow deformation during megathrust events, we develop semi-analytical and numerical Bayesian approaches to explore the maximum resolution of the tsunami data, with a focus on incorporating the uncertainty in the forward modeling. These methodologies are then applied to invert for the coseismic seafloor displacement field in the 2011 Mw 9.0 Tohoku-Oki earthquake using near-field tsunami waveforms and for the coseismic fault slip models in the 2010 Mw 8.8 Maule earthquake with complementary tsunami and geodetic observations. From posterior estimates of model parameters and their uncertainties, we are able to quantitatively constrain the near-trench profiles of seafloor displacement and fault slip. Similar characteristic patterns emerge during both events, featuring the peak of uplift near the edge of the accretionary wedge with a decay toward the trench axis, with implications for fault failure and tsunamigenic mechanisms of megathrust earthquakes.
To understand the behavior of earthquakes at the base of the seismogenic zone on continental strike-slip faults, we simulate the interactions of dynamic earthquake rupture, aseismic slip, and heterogeneity in rate-and-state fault models coupled with shear heating. Our study explains the long-standing enigma of seismic quiescence on major fault segments known to have hosted large earthquakes by deeper penetration of large earthquakes below the seismogenic zone, where mature faults have well-localized creeping extensions. This conclusion is supported by the simulated relationship between seismicity and large earthquakes as well as by observations from recent large events. We also use the modeling to connect the geodetic observables of fault locking with the behavior of seismicity in numerical models, investigating how a combination of interseismic geodetic and seismological estimates could constrain the locked-creeping transition of faults and potentially their co- and post-seismic behavior.</p
A Can of Worms
No full textIn "The Incompatibility of Free Will and Determinism" [1], Peter van Inwagen argues that if the universe
is deterministic, then free will does not exist. (He is silent about whether the universe is in fact deterministic
and about whether free will in fact exists.) This is in contrast to the compatibilist position, which holds that
free will and determinism are not contradictory. Briefly, van Inwagen's argument is that when an agent with
free will performs some action, she (by definition of "free will") could have performed a different action. But
in a deterministic universe, acting a different way requires either altering the past or violating the laws of
physics. So van Inwagen concludes that the free agent could have either altered the past or violated the laws
of physics. Finally, van Inwagen says that it is obvious that nobody can alter the past, and by definition of
the phrase "law of physics", nobody can violate the laws of physics either. So our hypothetical free agent in
a deterministic universe cannot exist
Applications of Nickel-Catalyzed Cross-Coupling Methods in the Synthesis of Organofluorine Compounds
Get PDFThe applications of nickel-catalyzed cross-coupling strategy to the synthesis of organofluorine compounds are explored in this thesis.
Chapter 2 describes the development of the first enantioselective cross-coupling method using secondary geminal dihalides as electrophiles. This method provides a unique approach for the generation of enantioenriched tertiary alkyl fluorides. These cross-coupling products can be further transformed into a variety of potentially valuable chiral building blocks.
Chapter 3 describes the development of a practical and versatile Negishi alkylation method employing α-halo-α-perfluoroalkyl secondary electrophiles. Target molecules bearing perfluoroalkyl-substituted (including trifluoromethyl-substituted) tertiary carbons can be easily generated from fluorinated electrophiles. Competition experiments and mechanistic studies have been performed to reveal the unique properties of these electrophiles and also prove the existence of alkyl radicals.
Chapter 4 describes the development of an asymmetric Negishi arylation protocol with α-halo-α-trifluoromethyl secondary electrophiles. This study provides a unique approach to construct trifluoromethyl-substituted tertiary stereocenters. The optimized condition can also be directly applied to substrates bearing an array of fluoroalkyl groups.
Chapter 5 details the progress towards the development of an asymmetric alkynylation method employing α-halo-α-trifluoromethyl secondary electrophiles. Preliminary studies also demonstrate that the protocol we developed has the potential to be used for other non-fluorinated secondary electrophiles.</p
Constructing Self-Dual Automorphic Representations on General Linear Groups
Get PDFWe prove a globalization theorem for self-dual representations of GLN over a totally real number field F, which gives a positive existence criterion for self-dual cuspidal automorphic representations of GLN(AF) with prescribed local components at a finite set of finite places. A byproduct of our argument is that the automorphic representations that we construct are cohomological (equivalently, regular algebraic) and so fall into the class of automorphic representations on GLN for which there is a well-established theory for how to attach Galois representations, using the etale cohomology of certain Shimura varieties. The primary motivation is to give a sort of "bare-handed" or "low tech" proof of a result that is implied by the philosophy of twisted endoscopy in the Langlands program. While we are guided by this overarching picture, in the argument itself, we obtain all our results by working directly on GLN and the group obtained by twisting it under the "inverse-transpose" involution. In particular, we do not appeal to any general results on twisted endoscopic transfer or assume any big "black box" results like the (conjectured) stabilization of the twisted trace formula. Hence, such results are unconditional as stated, and we remark throughout on why the particular assumptions that we impose turn out to be necessary, indicating the (often substantial amount of) additional work required to generalize the stated results.
In an appendix, in stark contrast to our approach above, we give an abridged argument for proving a globalization theorem on GLN in great generality, assuming a couple of major technical hypotheses (albeit, ones that are widely believed to be true) and yielding to Arthur's endoscopic classiffication of representations of symplectic and special orthogonal groups. Our hope is for such an argument to provide an outline for how we might ultimately prove results like generalizations of the globalization criterion above in the future.</p
Electrocatalysis in Solid Acid Fuel Cell Electrodes
Get PDFFuel cells are appealing alternatives to combustion engines for efficient conversion of chemical energy to electrical energy, with the potential to meet substantial energy demands with a small carbon footprint. Intermediate temperature fuel cells (200-300 °C) combine the kinetic benefits and fuel flexibility of higher operating temperatures along with the flexibility in material choices that lower operating temperatures allow. Solid acid fuel cells (SAFCs) offer the unique benefit amongst intermediate temperature fuel cells of a truly solid electrolyte, specifically, CsH2PO4, which in turn, provides significant system simplifications relative to phosphoric acid or alkaline fuel cells. However, the power output of even the most advanced SAFCs has not yet reached levels typical of conventional polymer electrolyte or solid oxide fuel cells. This is largely due to poor activity of the cathodes. That is, while it has been possible to limit electrolyte voltage losses in SAFCs through fabrication of thin-membrane fuel cells (with electrolyte thicknesses of 25–50 μm), it has not been possible to attain high activity cathodes or to limit Pt loadings to competitive levels. In this thesis, the efficacy of non-precious metal catalysts in the solid acid electrochemical system is evaluated. In addition, an attractive synthesis route (specifically, the electrospray method) to fabricating high surface area electrodes with high catalyst utilization is presented.
Elimination of Pt was pursued by the evaluation of carbon nanostructures as potential oxygen reduction reaction (ORR) catalysts in the solid acid electrochemical system. Multi-walled carbon nanotubes were the most consistently catalytically active in comparison with nano-graphite. It is demonstrated that the a) precursor partial pressure, b) seed catalyst size, c) growth temperature and d) chemical functionalization can be used to control the defect density and atomic composition of multi-walled carbon nanotubes (MWCNTs), all of which play a significant role on the measured ORR activity. Increasing the precursor partial pressure, decreasing the seed catalyst size, and decreasing the growth temperature increases the density of ORR active defects. In addition, the oxygen reduction reaction (ORR) electrochemical activity evaluated by symmetric cell AC impedance spectroscopy and fuel cell measurements, were significantly enhanced by chemical functionalization with oxygen containing functional groups. Area normalized impedance responses as low as 7 Ω cm2 were measured on symmetric MWCNT/ CsH2PO4 cells. However, it was discovered that these reactive MWCNTs also catalyze and are slightly consumed by steam reforming. Moreover, the orders of magnitude improvement with functionalization measured in impedance measurements is not replicated in fuel cell power output as a result of a decrease in open circuit voltage relative to standard cells. It is proposed that the loss in voltage results from hydrogen production at the cathode via the steam reforming reaction, although formation of hydrogen peroxide rather than water as the oxygen reduction product cannot be ruled out. This work has a significant contribution to catalysis, it demonstrates how carbon nanostructures can be designed by synthesis routes and chemical functionalization processes, to create active precious-metal-free ORR catalysts. It is also important that we have demonstrated potential ORR catalysts in acidic media. These catalysts have potential applications in phosphoric acid fuel cells and PEMFCs.
In addition to the study of carbon nanostructures, oxides were evaluated as potential ORR catalysts. Specifically, TiOx nanoparticles were studied. Analysis shows that the activity is controlled by the oxidation state of Ti. The active site seems to be on or near slightly reduced Ti sites. In this study we have outlined synthesis routes to tune the oxidation state of Ti and enhance ORR activity in the solid acid fuel cell.
Finally, the fundamentals of the electrospray process are explored to understand how the particle size ultimately resulting from electrospray synthesis depends on both solution properties and process parameters. This analysis presents a systematic way to control the fabrication of high surface area SAFC electrodes with increased throughput, catalyst utilization and consequently power density.</p
A Total Synthesis of (+)-Ryanodol
Get PDFHighly oxygenated, architecturally complex terpenoids constitute a biologically important class of natural products, yet their development into medicinally relevant analogs and effective biological probes are obstructed by their synthetic accessibility. Ryanodine is a unique diterpenoid that exhibits high affinity to a class of intracellular calcium ion channels bearing its name: ryanodine receptors. Structure-activity relationship studies have demonstrated how peripheral structural modifications affect binding affinity and selectivity among receptor isoforms, but to date have been limited to analogs prepared via chemical derivatization of natural material due to the intractability of total chemical synthesis.
This thesis details synthetic efforts culminating in a total synthesis of ryanodol that proceeds in only 15-steps from commercially available (–)-pulegone. Early stage oxygen atom incorporation is strategically implemented to facilitate key, stereoselective carbon-carbon bond formation. In particular, a rhodium-catalyzed, intramolecular Pauson–Khand reaction is utilized to rapidly assemble the tetracyclic ABCD-ring system that constitutes the anhydroryanodol core. A novel, selenium-dioxide mediated oxidation to install three oxidation states and three oxygen atoms was discovered, enabling the rapid oxidative functionalization of the ryanodol A-ring. The modular route described herein allows for the preparation of synthetic structural analogs not readily accessible via chemical degradation, and is anticipated to enable rapid construction and evaluation of biologically active ryanodine analogs.</p
Biochemical and Biophysical Characterization of Huntingtin
Get PDFHuntington’s disease (HD) is a fatal autosomal dominant neurodegenerative disease. HD has no cure, and patients pass away 10-20 years after the onset of symptoms. The causal mutation for HD is a trinucleotide repeat expansion in exon 1 of the huntingtin gene that leads to a polyglutamine (polyQ) repeat expansion in the N-terminal region of the huntingtin protein. Interestingly, there is a threshold of 37 polyQ repeats under which little or no disease exists; and above which, patients invariably show symptoms of HD. The huntingtin protein is a 350 kDa protein with unclear function. As the polyQ stretch expands, its propensity to aggregate increases with polyQ length. Models for polyQ toxicity include formation of aggregates that recruit and sequester essential cellular proteins, or altered function producing improper interactions between mutant huntingtin and other proteins. In both models, soluble expanded polyQ may be an intermediate state that can be targeted by potential therapeutics.
In the first study described herein, the conformation of soluble, expanded polyQ was determined to be linear and extended using equilibrium gel filtration and small-angle X-ray scattering. While attempts to purify and crystallize domains of the huntingtin protein were unsuccessful, the aggregation of huntingtin exon 1 was investigated using other biochemical techniques including dynamic light scattering, turbidity analysis, Congo red staining, and thioflavin T fluorescence. Chapter 4 describes crystallization experiments sent to the International Space Station and determination of the X-ray crystal structure of the anti-polyQ Fab MW1. In the final study, multimeric fibronectin type III (FN3) domain proteins were engineered to bind with high avidity to expanded polyQ tracts in mutant huntingtin exon 1. Surface plasmon resonance was used to observe binding of monomeric and multimeric FN3 proteins with huntingtin.</p
Kinematics and Local Motion Planning for Quasi-static Whole-body Mobile Manipulation
Get PDFThis thesis studies mobile robotic manipulators, where one or more robot manipulator arms are
integrated with a mobile robotic base. The base could be a wheeled or tracked vehicle, or it might be a
multi-limbed locomotor. As robots are increasingly deployed in complex and unstructured environments,
the need for mobile manipulation increases. Mobile robotic assistants have the potential to revolutionize human
lives in a large variety of settings including home, industrial and outdoor environments.
Mobile Manipulation is the use or study of such mobile robots as they interact with physical
objects in their environment. As compared to fixed base manipulators, mobile manipulators can take
advantage of the base mechanism’s added degrees of freedom in the task planning and execution process.
But their use also poses new problems in the analysis and control of base system stability, and the
planning of coordinated base and arm motions. For mobile manipulators to be successfully and
efficiently used, a thorough understanding of their kinematics, stability, and capabilities is required.
Moreover, because mobile manipulators typically possess a large number of actuators, new and efficient
methods to coordinate their large numbers of degrees of freedom are needed to make them practically
deployable. This thesis develops new kinematic and stability analyses of mobile manipulation, and new
algorithms to efficiently plan their motions.
I first develop detailed and novel descriptions of the kinematics governing the operation of multi-
limbed legged robots working in the presence of gravity, and whose limbs may also be simultaneously
used for manipulation. The fundamental stance constraint that arises from simple assumptions about
friction and the ground contact and feasible motions is derived. Thereafter, a local relationship between
joint motions and motions of the robot abdomen and reaching limbs is developed. Baseeon these
relationships, one can define and analyze local kinematic qualities including limberness, wrench
resistance and local dexterity. While previous researchers have noted the similarity between multi-
fingered grasping and quasi-static manipulation, this thesis makes explicit connections between these two
problems.
The kinematic expressions form the basis for a local motion planning problem that that
determines the joint motions to achieve several simultaneous objectives while maintaining stance stability
in the presence of gravity. This problem is translated into a convex quadratic program entitled the
balanced priority solution, whose existence and uniqueness properties are developed. This problem is
related in spirit to the classical redundancy resoxlution and task-priority approaches. With some simple
modifications, this local planning and optimization problem can be extended to handle a large variety of
goals and constraints that arise in mobile-manipulation. This local planning problem applies readily to
other mobile bases including wheeled and articulated bases. This thesis describes the use of the local
planning techniques to generate global plans, as well as for use within a feedback loop. The work in this
thesis is motivated in part by many practical tasks involving the Surrogate and RoboSimian robots at
NASA/JPL, and a large number of examples involving the two robots, both real and simulated, are
provided.
Finally, this thesis provides an analysis of simultaneous force and motion control for multi-
limbed legged robots. Starting with a classical linear stiffness relationship, an analysis of this problem for
multiple point contacts is described. The local velocity planning problem is extended to include
generation of forces, as well as to maintain stability using force-feedback. This thesis also provides a
concise, novel definition of static stability, and proves some conditions under which it is satisfied.</p