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1. Root of unity quantum cluster algebras and discriminants

   https://doi.org/10.1112/jlms.70060  

   Nguyen, Bach; Trampel, Kurt; Yakimov, Milen (January 2025, Journal of the London Mathematical Society)

   Abstract We describe a connection between the subjects of cluster algebras, polynomial identity algebras, and discriminants. For this, we define the notion of root of unity quantum cluster algebras and prove that they are polynomial identity algebras. Inside each such algebra we construct a (large) canonical central subalgebra, which can be viewed as a far reaching generalization of the central subalgebras of big quantum groups constructed by De Concini, Kac, and Procesi and used in representation theory. Each such central subalgebra is proved to be isomorphic to the underlying classical cluster algebra of geometric type. When the root of unity quantum cluster algebra is free over its central subalgebra, we prove that the discriminant of the pair is a product of powers of the frozen variables times an integer. An extension of this result is also proved for the discriminants of all subalgebras generated by the cluster variables of nerves in the exchange graph. These results can be used for the effective computation of discriminants. As an application we obtain an explicit formula for the discriminant of the integral form over of each quantum unipotent cell of De Concini, Kac, and Procesi for arbitrary symmetrizable Kac–Moody algebras, where is a root of unity. 

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2. Marcinkiewicz Averages of Smooth Orthogonal Projections on Sphere

   https://doi.org/10.1007/s00041-022-09966-y  

   Bownik, Marcin; Dziedziul, Karol; Kamont, Anna (October 2022, Journal of Fourier Analysis and Applications)

   Abstract We construct a single smooth orthogonal projection with desired localization whose average under a group action yields the decomposition of the identity operator. For any full rank lattice $$\Gamma \subset \mathbb {R}^d$$ Γ ⊂ R d , a smooth projection is localized in a neighborhood of an arbitrary precompact fundamental domain $$\mathbb {R}^d/\Gamma $$ R d / Γ . We also show the existence of a highly localized smooth orthogonal projection, whose Marcinkiewicz average under the action of SO ( d ), is a multiple of the identity on $$L^2(\mathbb {S}^{d-1})$$ L 2 ( S d - 1 ) . As an application we construct highly localized continuous Parseval frames on the sphere. 

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3. A Measurable Selector in Kadison’s Carpenter’s Theorem

   https://doi.org/10.4153/S0008414X19000373  

   Bownik, Marcin; Szyszkowski, Marcin (January 2019, Canadian Journal of Mathematics)

   Abstract: We show the existence of a measurable selector in Carpenter’s Theorem due to Kadison. This solves a problem posed by Jasper and the first author in an earlier work. As an application we obtain a characterization of all possible spectral functions of shift-invariant subspaces of $$L^{2}(\mathbb{R}^{d})$$ and Carpenter’s Theorem for type $$\text{I}_{\infty }$$ von Neumann algebras. 

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4. Free fermions revisited

   https://doi.org/10.1142/S0219498826502695  

   Milas, Antun; Penn, Michael (June 2025, Journal of Algebra and Its Applications)

   Free fermion vertex superalgebras are discussed from the point of view of Urod vertex algebras [T. Arakawa, T. Creutzig and B. Feigin, Urod algebras and translation of [Formula: see text]-algebras, Forum Mathematics Sigma, Vol. 10 (Cambridge University Press, 2022) and M. Bershtein, B. Feigin and A. Litvinov, Coupling of two conformal field theories and Nakajima–Yoshioka blow-up equations, preprint (2013), arXiv:1310.7281]. We present all finite decompositions of the [Formula: see text]-fermion vertex algebra via Virasoro and [Formula: see text] superconformal vertex algebras. We also present decompositions of higher rank fermion algebras using affine [Formula: see text]-algebras, and a conjecture on the existence of the “square root” of the [Formula: see text] fermion algebra. 

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5. Meromorphic cosets and the classification of three-character CFT

   https://doi.org/10.1007/JHEP03(2023)023  

   Das, Arpit; Gowdigere, Chethan N.; Mukhi, Sunil (March 2023, Journal of High Energy Physics)

   A bstract We investigate the admissible vector-valued modular forms having three independent characters and vanishing Wronskian index and determine which ones correspond to genuine 2d conformal field theories. This is done by finding bilinear coset-type relations that pair them into meromorphic characters with central charges 8, 16, 24, 32 and 40. Such pairings allow us to identify some characters with definite CFTs and rule out others. As a key result we classify all unitary three-character CFT with vanishing Wronskian index, excluding c = 8, 16. The complete list has two infinite affine series B r ,1 , D r ,1 and 45 additional theories. As a by-product, at higher values of the total central charge we also find constraints on the existence or otherwise of meromorphic theories. We separately list several cases that potentially correspond to Intermediate Vertex Operator Algebras. 

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