We consider the problem of optimization and decision-making in various settings involving complex systems. In particular, we consider specific problems in decentralized finance which we address employing insights from mathematical finance, in course-mode selection that we solve by applying mixed-integer programming, and in social networks that we approach using tools from graphical game theory.In the first part of the thesis, we model and analyze fixed spread liquidation lending in DeFi as implemented by popular pooled lending protocols such as AAVE, JustLend, and Compound. Empirically, we observe that over 70% of liquidations occur in the absence of any downward price jumps. Then, assuming the borrowers monitor their loans with exponentially distributed horizons, we compute the expected liquidation cost incurred by the borrowers in closed form as a function of the monitoring frequency. We compare this cost against liquidation data obtained from AAVE protocol V2, and observe a match with our model assuming the borrowers monitor their loans five to six times more often than they interact with the pool. Such borrowers must balance the financing cost against the likelihood of liquidation. We compute the optimal health factor in this situation assuming a financing rate for the collateral. Empirically, we observe that borrowers are often more conservative compared to model predictions, though on average, model predictions match with empirical observations. In the second part of the thesis, we consider the problem of hybrid scheduling that was faced by Columbia Business School during the Covid-19 pandemic and describe the system that we implemented to address it. The system allows some students to attend in-person classes with social distancing, while their peers attend online, and schedules vary by day. We consider two variations of this problem: one where students have unique, individualized class enrollments, and one where they are grouped in teams that are enrolled in identical classes. We formulate both problems as mixed-integer programs. In the first setting, students who are scheduled to attend all classes in person on a given day may, at times, be required to attend a particular class on that day online due to social distancing constraints. We count these instances as “excess.” We minimize excess and related objectives, and analyze and solve the relaxed linear program. In the second setting, we schedule the teams so that each team’s in-person attendance is balanced over days of week and spread out over the entire term. Our objective is to maximize interaction between different teams. Our program was used to schedule over 2,500 students in student-level scheduling and about 790 students in team-level scheduling from the Fall 2020 through Summer 2021 terms at Columbia Business School. In the third part of the thesis, we consider a social network, where individuals choose actions which optimize utility which is a function of their neighbors’ actions. We assume that a central authority aiming to maximize social welfare at equilibrium can intervene by paying some cost to shift individual incentives, and that the cost is upper bounded by a budget. The intervention that maximizes the social welfare can be computed using the spectral decomposition of the adjacency matrix of the graph, yet this is infeasible in practice if the adjacency matrix is unknown. We study the question of designing intervention strategies for graphs where the adjacency matrix is unknown and is drawn from some distribution. For several commonly studied random graph models, we show that the competitive ratio of in intervention proportional to the first eigenvector of the expected adjacency matrix, approaches 1 in probability as the graph size increases. We also provide several efficient sampling-based approaches for approximately recovering the first eigenvector when we do not know the distribution. On the whole, our analysis compares three categories of interventions: those which use no data about the network, those which use some data (such as distributional knowledge or queries to the graph), and those which are fully optimal. We evaluate these intervention strategies on synthetic and real-world network data, and our results suggest that analysis of random graph models can be useful for determining when certain heuristics may perform well in practice.

Dopamin und Acetylcholin sind wichtige neuromodulatorische Substanzen im menschlichen zentralen Nervensystem mir einem starken Einfluß auf Neuroplastizität. Der spezifische Einfluß dieser Substanzen auf Neuroplastizität wird durch verschiedene Faktoren, unter anderem Dosisabhängigkeit, Hintergrundaktivität neuronaler Netze und Subrezeptorspezifität determiniert. In dieser Dissertation haben wir den dosis- und subrezeptorabhängigen Effekt des cholinergen und dopaminergen Systems auf Neuroplastizität des menschlichen motorischen Kortex untersucht. Transkranielle Gleichstromstimulation (tDCS) und gepaarte assoziative Stimulation (PAS) stellen nicht-invasive Hirnstimulationstechniken dar, die die Erzeugung von Neuroplastizität beim Menschen ermöglichen. Wir haben in unseren Studien tDCS zur Erzeugung nicht-fokaler und PAS zur Erzeugung fokaler synapsenspezifischer Plastizität im motorischen Kortex von gesunden Probanden eingesetzt. In der ersten Studie konnten wir zeigen, daß die Aktivierung nikotinischer Rezeptoren einen fokussierenden Effekt auf fazilitatorische Plastizität hatte, inhibitorische Plastizität aber unabhängig von ihrer Fokalität verhinderte. Somit hat die Aktivierung nikotinerger Rezeptoren ähnliche Effekte wie globale cholinerge Aktivierung auf fazilitatorische, aber differente Effekte auf inhibitorische Plastizität. In der zweiten Studie untersuchten wir die dosisabhängigen Auswirkungen der Dopamin-Vorläufersubstanz l-Dopa auf Neuroplastizität. Bei mittlerer Dosis von l-Dopa war fokale Plastizität unverändert, wohingegen niedrige und hohe Dosen von l-Dopa die Induktion von Plastizität verhinderten. Die Ergebnisse zeigen somit einen klaren nicht-linearen dosisabhängigen Effekt. Aus den Ergebnissen dieser Studien kann geschlossen werden, daß Neuromodulatoren Plastizität im motorischen Kortex des Menschen relevant beeinflussen. Diese Effekte sind Subrezeptor- und Dosis-abhängig.

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