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Abstract

We provide exposition into the field of projection theory, which lies at the intersection of incidence geometry and geometric measure theory. We first give the necessary preliminaries in Chapter 2, focusing on incidences between points and lines and the definition of Hausdorff dimension. With this background in tow, in Chapter 3 we dive into thorough surveys on three topics in projection theory: orthogonal projections, Furstenberg sets, and radial projections. We particularly highlight the interconnectedness of these topics in both the discrete and continuum settings. Through these surveys, we also give the necessary background for discussing applications of projection theory in Chapter 4. The first application is on Beck-type problems, first studied in the discrete setting by József Beck in 1983 and in the continuum setting by Orponen, Shmerkin, and Wang in 2022. Given X ⊂ Rⁿ, these problems seek to understand how large the set of lines that contain at least 2 points of X, L(X), can be. To this end, we present a continuum Erdős–Beck theorem due to myself and Marshall in 2024, which motivates and makes use of a dual Furstenberg set estimate due to myself, Fu, and Ren from the same year. The second application is on Falconer-type (distance) problems which have been a prominent topic in both the discrete and continuum settings. Given X ⊂ Rⁿ, these problems seek to understand how large X must be until the set of distinct "distances" between points of X is large (for a reasonable notion of "distance"). To this end, we present a Falconer-type distance problem for dot products due to myself, Marshall, and Senger in 2024, making use of both standard and modern results for orthogonal and radial projections.