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1. Mean values of multiplicative functions and applications to residue-class distribution

   https://doi.org/10.1017/S0013091524000890  

   Pollack, Paul; Singha_Roy, Akash (February 2025, Proceedings of the Edinburgh Mathematical Society)

   Abstract We provide a uniform bound on the partial sums of multiplicative functions under very general hypotheses. As an application, we give a nearly optimal estimate for the count of$$n \le x$$for which the Alladi–Erdős function$$A(n) = \sum_{p^k \parallel n} k p$$takes values in a given residue class moduloq, whereqvaries uniformly up to a fixed power of$$\log x$$. We establish a similar result for the equidistribution of the Euler totient function$$\phi(n)$$among the coprime residues to the ‘correct’ moduliqthat vary uniformly in a similar range and also quantify the failure of equidistribution of the values of$$\phi(n)$$among the coprime residue classes to the ‘incorrect’ moduli. 

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2. Reflective centers of module categories and quantum K-matrices

   https://doi.org/10.1017/fms.2025.10055  

   Laugwitz, Robert; Walton, Chelsea; Yakimov, Milen (January 2025, Forum of Mathematics, Sigma)

   Abstract Our work is motivated by obtaining solutions to the quantum reflection equation (qRE) by categorical methods. To start, given a braided monoidal category$${\mathcal {C}}$$and$${\mathcal {C}}$$-module category$${\mathcal {M}}$$, we introduce a version of the Drinfeld center$${\mathcal {Z}}({\mathcal {C}})$$of$${\mathcal {C}}$$adapted for$${\mathcal {M}}$$; we refer to this category as thereflective center$${\mathcal {E}}_{\mathcal {C}}({\mathcal {M}})$$of$${\mathcal {M}}$$. Just like$${\mathcal {Z}}({\mathcal {C}})$$is a canonical braided monoidal category attached to$${\mathcal {C}}$$, we show that$${\mathcal {E}}_{\mathcal {C}}({\mathcal {M}})$$is a canonical braided module category attached to$${\mathcal {M}}$$; its properties are investigated in detail. Our second goal pertains to when$${\mathcal {C}}$$is the category of modules over a quasitriangular Hopf algebraH, and$${\mathcal {M}}$$is the category of modules over anH-comodule algebraA. We show that the reflective center$${\mathcal {E}}_{\mathcal {C}}({\mathcal {M}})$$here is equivalent to a category of modules over an explicit algebra, denoted by$$R_H(A)$$, which we call thereflective algebraofA. This result is akin to$${\mathcal {Z}}({\mathcal {C}})$$being represented by the Drinfeld double$${\operatorname {Drin}}(H)$$ofH. We also study the properties of reflective algebras. Our third set of results is also in the Hopf setting above. We show that reflective algebras are quasitriangularH-comodule algebras, and we examine their corresponding quantumK-matrices; this yields solutions to the qRE. We also establish that the reflective algebra$$R_H(\mathbb {k})$$is an initial object in the category of quasitriangularH-comodule algebras, where$$\mathbb {k}$$is the ground field. The case whenHis the Drinfeld double of a finite group is illustrated. 

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3. Asymptotic freeness in tracial ultraproducts

   https://doi.org/10.1017/fms.2024.93  

   Houdayer, Cyril; Ioana, Adrian (January 2024, Forum of Mathematics, Sigma)

   Abstract We prove novel asymptotic freeness results in tracial ultraproduct von Neumann algebras. In particular, we show that whenever$$M = M_1 \ast M_2$$is a tracial free product von Neumann algebra and$$u_1 \in \mathscr U(M_1)$$,$$u_2 \in \mathscr U(M_2)$$are Haar unitaries, the relative commutants$$\{u_1\}' \cap M^{\mathcal U}$$and$$\{u_2\}' \cap M^{\mathcal U}$$are freely independent in the ultraproduct$$M^{\mathcal U}$$. Our proof relies on Mei–Ricard’s results [MR16] regarding$$\operatorname {L}^p$$-boundedness (for all$$1 < p < +\infty $$) of certain Fourier multipliers in tracial amalgamated free products von Neumann algebras. We derive two applications. Firstly, we obtain a general absorption result in tracial amalgamated free products that recovers several previous maximal amenability/Gamma absorption results. Secondly, we prove a new lifting theorem which we combine with our asymptotic freeness results and Chifan–Ioana–Kunnawalkam Elayavalli’s recent construction [CIKE22] to provide the first example of a$$\mathrm {II_1}$$factor that does not have property Gamma and is not elementary equivalent to any free product of diffuse tracial von Neumann algebras. 

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4. Paucity problems and some relatives of Vinogradov’s mean value theorem

   https://doi.org/10.1017/S0305004123000166  

   WOOLEY, TREVOR D. (September 2023, Mathematical Proceedings of the Cambridge Philosophical Society)

   Abstract When$$k\geqslant 4$$and$$0\leqslant d\leqslant (k-2)/4$$, we consider the system of Diophantine equations\begin{align*}x_1^j+\ldots +x_k^j=y_1^j+\ldots +y_k^j\quad (1\leqslant j\leqslant k,\, j\ne k-d).\end{align*}We show that in this cousin of a Vinogradov system, there is a paucity of non-diagonal positive integral solutions. Our quantitative estimates are particularly sharp when$$d=o\!\left(k^{1/4}\right)$$. 

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5. Degrees of maps and multiscale geometry

   https://doi.org/10.1017/fmp.2023.33  

   Berdnikov, Aleksandr; Guth, Larry; Manin, Fedor (January 2024, Forum of Mathematics, Pi)

   Abstract We study the degree of an L-Lipschitz map between Riemannian manifolds, proving new upper bounds and constructing new examples. For instance, if $$X_k$$ is the connected sum of k copies of $$\mathbb CP^2$$for$$k \ge 4$$, then we prove that the maximum degree of an L-Lipschitz self-map of $$X_k$$ is between $$C_1 L^4 (\log L)^{-4}$$ and $$C_2 L^4 (\log L)^{-1/2}$$. More generally, we divide simply connected manifolds into three topological types with three different behaviors. Each type is defined by purely topological criteria. For scalable simply connected n-manifolds, the maximal degree is $$\sim L^n$$. For formal but nonscalable simply connectedn-manifolds, the maximal degree grows roughly like $$L^n (\log L)^{-\theta (1)}$$. And for nonformal simply connected n-manifolds, the maximal degree is bounded by $$L^\alpha $$ for some $$\alpha < n$$. 

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