Department of

# Mathematics

Seminar Calendar
for events the day of Tuesday, April 27, 2021.

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events for the
events containing

Questions regarding events or the calendar should be directed to Tori Corkery.
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Tuesday, April 27, 2021

11:00 am in via Zoom,Tuesday, April 27, 2021

#### Equivariant quotients and localizations of norms of $BP_{\mathbb{R}}$

###### Agnès Beaudry (University of Colorado Boulder)

Abstract: Quotients, localizations and completions of $BP$ play a central role in chromatic homotopy theory. For example, the Johnson-Wilson spectra $E(h)$ obtained by a quotient and localization of $BP$ are key players in the chromatic story at height $h$. However, working only with $E(h)$, the equivariance inherent to the chromatic story coming from the Morava stablizer group is obscured. A first step is to instead consider $E_{\mathbb{R}}(h)$, the Real Johnson-Wilson $C_2$-spectrum. However, for many heights $h$ there are bigger subgroups of the Morava stabilizer group lurking and $E_{\mathbb{R}}(h)$ does not capture their action. Indeed, restricting to finite cyclic 2-groups and for $h=2^{n-1}m$, the stabilizer group contains a subgroup isomorphic to $C_{2^n}$. In this talk, I will explain how one can instead consider quotients of norms of $N_{C_2}^{C_{2^n}}BP_{\mathbb{R}}$ to construct height $h$, $C_{2^n}$-equivariant analogues of $E(h)$.

11:00 am in Zoom (contact Olivia Beckwith at obeck@illinois.edu for details),Tuesday, April 27, 2021

#### Weighted Diophantine Approximation on Manifolds

###### Demi Allen (University of Bristol)

Abstract: Diophantine Approximation is a branch of Number Theory in which the central theme is understanding how well real numbers can be approximated by rationals. Consequently, studying the size of sets of numbers that can be approximated to within a given precision is very important. In the most classical setting, a $\psi$-well-approximable number is one which can be approximated by rationals to a given degree of accuracy specified by an approximating function $\psi$. From a measure-theoretic point of view, this classical set of $\psi$-well-approximable numbers in Euclidean space is well understood. However, it is far less clear what happens when this set, or its various generalisations or higher dimensional analogues, is intersected with other natural sets such as curves, manifolds, or fractals. Studying such questions has become a hugely popular topic of interest in Diophantine Approximation in recent years. In this talk I will discuss recent joint work with Baowei Wang (HUST, China) concerning the particular topic of weighted Diophantine Approximation on manifolds.

1:00 pm in Zoom (email Philipp Hieronymi),Tuesday, April 27, 2021

#### Decidability bounds for Presburger arithmetic extended by sine

###### Eion Blanchard (Illinois)

Abstract: Since 1929, when Presburger formalized his eponymous arithmetic $(\mathbb{Z}, <, +)$—integers with order and addition, but not multiplication—various extensions of this theory have been studied. Presburger arithmetic (PA) is decidable, as are some of its extensions, such as that by base-2 exponentiation. In joint work with Philipp Hieronymi, we extend PA by the sine function. Let sine-Presburger arithmetic ($\sin$-PA) be the theory of $(\mathbb{R}, <, +, \mathbb{Z}, \sin)$ with quantification only over integers. We show that the $\sin$-PA fragment with 4 alternating quantifier blocks is undecidable by encoding the halting problem. On the other hand, we present a decision procedure for the existential fragment of $\sin$-PA, under assumption of Schanuel’s Conjecture from transcendental number theory.

2:00 pm in Zoom,Tuesday, April 27, 2021

#### An improvement on Łuczak's connected matching method

###### Shoham Letzter (University College London)

Abstract: A connected matching in a graph $G$ is a matching contained in a connected component of $G$. A well-known method due to Łuczak reduces Ramsey-type problems about paths and cycles in complete graphs to Ramsey-type problems about connected matchings in almost complete graphs. We show that these can be further reduced to Ramsey-type problems about connected matchings in complete graphs.

2:00 pm in Zoom Meeting (email daesungk@illinois.edu for info),Tuesday, April 27, 2021

#### Wave front propagation for FKPP reaction-diffusion equation on a class of infinite random trees

###### Wenqing Hu (Missouri University of Science and Technology)

Abstract: The asymptotic wave speed for FKPP type reaction-diffusion equations on a class of infinite random metric trees is considered in this talk. We show that a travelling wave front emerges, provided that the reaction rate is large enough. The wave travels at a speed that can be quantified via a variational formula involving the random branching degrees and the random branch lengths of the tree. This speed is slower than that of the same equation on the real line, and we can estimate this slow-down in terms of the structure of the tree. Our key idea is to project the Brownian motion on the tree onto a one-dimensional axis along the direction of the wave propagation. This idea, combined with the Feynman-Kac formula, connect our analysis of the wave front propagation to the Large Deviations Principle (LDP) of the multi-skewed Brownian motion with random skewness and random interface set. Our LDP analysis for this multi-skewed Brownian motion involves delicate estimates for an infinite product of 2 by 2 random matrices parametrized by the structure of the tree and for hitting times of a random walk in random environment. Joint work with Wai-Tong (Louis) Fan (Indiana University) and Grigory Terlov (UIUC).

4:00 pm in Zoom,Tuesday, April 27, 2021

#### Compositions as Trans-Scalar Identity

###### Gualtiero Piccinini (University of Missouri, St. Louis)

Abstract: We define mereologically invariant composition as the relation between a whole object and its parts when the object retains the same parts during a time interval. We argue that mereologically invariant composition is identity between a whole and its parts taken collectively. Our reason is that parts and wholes are equivalent measurements of a portion of reality at different scales in the precise sense employed by measurement theory. The purpose of these scales is the numerical representation of primitive relations between quantities of objects. To show this, we prove representation and uniqueness theorems for composition. Thus, mereologically invariant composition is trans-scalar identity.

Zoom Info: Please email Kay Kirkpatrick (kkirkpat@illinois.edu).