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Damaris Schindler, University of Göttingen

Density of rational points near manifolds

Given a bounded submanifold M in R^n, how many rational points with common bounded denominator are there in a small thickening of M? How does this counting function behave if we let the size of the denominator go to infinity? The study of the density of rational points near manifolds has seen significant progress in the last couple of years. In this talk I will explain why we might be interested in this question, focusing on applications in Diophantine approximation and the (quantitative) arithmetic of projective varieties.

MC 5403

Luke Postle, University of Waterloo

A New Proof of the Existence Conjecture and its Applications to Extremal and Probabilistic Design Theory

We discuss the recently developed method of refined absorption and how it is used to provide a new proof of the Existence Conjecture for combinatorial designs. This method can also be applied to resolve open problems in extremal and probabilistic design theory while providing a unified framework for these problems. Crucially, the main absorption theorem can be used as a ``black-box'' in these applications obviating the need to reprove the absorption step for each different setup.

MC 5403

Beining Mu, University of Waterloo

Algorithmic randomness and Turing degrees 3

In this seminar we talk about coding strategies to encode an arbitrary set into a 1-random set in a sense that every set is wtt-reducible to a 1-random set. We will also have a review of the jump operator and lowness of Turing degrees to explore the distribution of 1-random sets in terms of Turing degrees.

MC 5403

Damaris Schindler, University of Göttingen

Quantitative weak approximation and quantitative strong approximation for certain quadratic forms

In this talk we discuss recent results on optimal quantitative weak approximation for certain projective quadrics over the rational numbers as well as quantitative results on strong approximation for quaternary quadratic forms over the integers. We will also discuss results on the growth of integral points on the three-dimensional punctured affine cone and strong approximation with Brauer-Manin obstruction for this quasi-affine variety. This is joint work with Zhizhong Huang and Alec Shute.

MC 5479

Erica Liu, University of Waterloo

Toric Compactifications and Critical Points at Infinity in Analytic Combinatorics

The field of Analytic Combinatorics in Several Variables (ACSV) provides powerful tools for deriving asymptotic information from multivariate generating functions. A key challenge arises when standard saddle-point techniques fail due to the presence of critical points at infinity (CPAI), obstructing local analyses near singularities. Recent work has shown that Morse-theoretic decompositions remain valid under the absence of CPAI, traditionally verified using projective compactifications. In this talk, I will present a toric approach to compactification that leverages the Newton polytope of a generating function to construct a toric variety tailored to the function’s combinatorial structure. This refinement not only tightens classification of CPAI but also enhances computational efficiency. Through concrete examples and an introduction to tropical and toric techniques, I will demonstrate how these methods clarify the asymptotic landscape of ACSV problems, especially in combinatorially meaningful settings. This talk draws on joint work studying toric compactifications as a bridge between algebraic geometry and analytic combinatorics.

MC 5479

Michael Gregory  University of Waterloo,

Computability Relative to Random Sets (2)

Now that we have covered the required background, we begin discussion on 1-random sets and how randomness interacts with computable reducibility. Several fundamental results are discussed including Kučera's Theorem which states that if a 1 random set is Turing reducible to a c.e. Set, then that set is Turing Equivalent to 0'. We then cover the Space Lemma which is used in the proof of Kučera Gác's Theorem which establishes that every set is weak truth-table reducible to a 1-random set.

MC 5403

Stephen Melczer University of Waterloo,

Analytic Combinatorics in One and Several Variables

The field of analytic combinatorics develops effective methods to compute the asymptotic behaviour of combinatorial sequences from analytic properties of their generating functions. This talk surveys the classical methods of analytic combinatorics and details the newer area of analytic combinatorics in several variables (ACSV), which handles multivariate sequences and their multivariate generating functions. Applications to several areas of mathematics and computer science, including number theory, will be discussed. This talk will be complemented by a presentation of Erica Liu on January 27 describing some recent progress on new approaches to ACSV.

MC 5479

Jashan Bal University of Waterloo,

Strong convergence of random permutations

We will start proving that i.i.d random permutations strongly converge to Haar unitaries.

MC 5479

Spencer Kelly, University of Waterloo

Proper Group Actions and the Slice Theorem in Finite Dimensions

In this talk we will begin by reviewing important properties of group actions on manifolds, and characteristics of proper actions. We then define isotropy and orbit types, discuss the slice theorem (on finite dimensional manifolds), and go over non-trivial examples of slice bundles. This will set us up to conclude with the principal orbit theorem and the stratification of the orbit space.

MC 5403

Zhihao Zhang, University of Waterloo

Spectra of Beurling Algebras of Discrete Abelian Groups

We will discuss a variant of the group algebra, called the Beurling algebra. These algebras differ from their classical counterpart through the addition of a weight function modifying the norm. The Gelfand spectrum of the group algebra of absolutely integrable functions on an abelian group, G, is well known to be the Pontryagin dual of G. In the case of a Beurling algebra, the Gelfand spectrum can be much larger for suitable weights. We will focus on the Beurling algebra of a discrete abelian group, G, and give a description of its Gelfand spectrum in terms of a seminorm constructed from a symmetric weight.

MC 5417 or Join on Zoom