Classification of commutative association schemes with almost commutative Terwilliger algebras

We classify the commutative association schemes such that all non-primary irreducible modules of their Terwilliger algebras are one-dimensional.     

On triangular algebras with noncommutative diagonals

We construct a triangular algebra whose diagonals form a noncommutative algebra and its lattice of invariant projections contains only two nontrivial projections. Moreover we prove that our triangular algebra is maximal.     

On Fréchet–Hilbert algebras

We consider Hilbert algebras with a supplementary Fréchet topology and get various extensions of the algebraic structure by using duality techniques. In particular we obtain optimal multiplier-type involutive algebras which in applications are large enough to be of significant practical use. The setting covers many situations arising from quantization rules, as those involving square-integrable families of bounded operators     

Krall–Laguerre commutative algebras of ordinary differential operators

In 1999, Grünbaum, Haine and Horozov defined a large family of commutative algebras of ordinary differential operators, which have orthogonal polynomials as eigenfunctions. These polynomials are mutually orthogonal with respect to a Laguerre-type weight distribution, thus providing solutions to Krall’s problem. In the present paper, we give a new proof of their result, which establishes a conjecture, concerning the explicit characterization of the dual commutative algebra of eigenvalues. In particular, for the Koornwinder’s generalization of Laguerre polynomials, our approach yields an explicit set of generators for the whole algebra of differential operators. We also illustrate how more general Sobolev-type orthogonal polynomials fit within this theory.     

When does the top homology of a random simplicial complex vanish?

Several years ago Linial and Meshulam (Combinatorica 26 (2006) 457–487) introduced a model called of random n‐vertex d‐dimensional simplicial complexes. The following question suggests itself very naturally: What is the threshold probability at which the d‐dimensional homology of such a random d‐complex is, almost surely, nonzero? Here we derive an upper bound on this threshold. Computer experiments that we have conducted suggest that this bound may coincide with the actual threshold, but this remains an open question. © 2013 Wiley Periodicals, Inc. Random Struct. Alg., 46, 26–35, 2015     

On normal τ-measurable operators affiliated with semifinite von Neumann algebras

Let τ be a faithful normal semifinite trace on the von Neumann algebra M, 1 ≥ q > 0. The following generalizations of problems 163 and 139 from the book [1] to τ-measurable operators are obtained; it is established that: 1) each τ-compact q-hyponormal operator is normal; 2) if a τ-measurable operator A is normal and, for some natural number n, the operator A ⁿ is τ-compact, then the operator A is also τ-compact. It is proved that if a τ-measurable operator A is hyponormal and the operator A ² is τ-compact, then the operator A is also τ-compact. A new property of a nonincreasing rearrangement of the product of hyponormal and cohyponormal τ-measurable operators is established. For normal τ-measurable operators A and B, it is shown that the nonincreasing rearrangements of the operators AB and BA coincide. Applications of the results obtained to F-normed symmetric spaces on (M, τ) are considered.     

Constructive Néron Desingularization of algebras with big smooth locus

An algorithmic proof of the General Néron Desingularization theorem and its uniform version is given for morphisms with big smooth locus. This generalizes the results for the one-dimensional case (cf. [10 Pfister, G., Popescu, D. (2017). Constructive General Neron Desingularization for one dimensional local rings. J. Symbolic Comput. 80:570–580.[Crossref], [Web of Science ®] , [Google Scholar]], [7 Khalid, A., Pfister, G., Popescu, D. (2018). A uniform General Neron Desingularization in dimension one. J. Algebra Appli. 16. arXiv:AC/1612.03416. [Google Scholar]]).     

On the Stanley–Reisner ideal of an expanded simplicial complex

Let Δ be a simplicial complex. We study the expansions of Δ mainly to see how the algebraic and combinatorial properties of Δ and its expansions are related to each other. It is shown that Δ is Cohen–Macaulay, sequentially Cohen–Macaulay, Buchsbaum or k-decomposable, if and only if an arbitrary expansion of Δ has the same property. Moreover, some homological invariants like the regularity and the projective dimension of the Stanley–Reisner ideals of Δ and those of their expansions are compared.     

On the category 𝒪 for rational Cherednik algebras

We study the category of representations of the rational Cherednik algebra A_W attached to a complex reflection group W. We construct an exact functor, called Knizhnik-Zamolodchikov functor: _W-mod, where _W is the (finite) Iwahori-Hecke algebra associated to W. We prove that the Knizhnik-Zamolodchikov functor induces an equivalence between /_tor, the quotient of by the subcategory of A_W-modules supported on the discriminant, and the category of finite-dimensional _W-modules. The standard A_W-modules go, under this equivalence, to certain modules arising in Kazhdan-Lusztig theory of cells, provided W is a Weyl group and the Hecke algebra _W has equal parameters. We prove that the category is equivalent to the module category over a finite dimensional algebra, a generalized q-Schur algebra associated to W.     

Lévy processes and quasi-shuffle algebras

We investigate the algebra of repeated integrals of semimartingales. We prove that a minimal family of semimartingales generates a quasi-shuffle algebra. In essence, to fulfil the minimality criterion, first, the family must be a minimal generator of the algebra of repeated integrals generated by its elements and by quadratic covariation processes recursively constructed from the elements of the family. Second, recursively constructed quadratic covariation processes may lie in the linear span of previously constructed quadratic covariation processes and of the family, but may not lie in the linear span of repeated integrals of these. We prove that a finite family of independent Lévy processes that have finite moments generates a minimal family. Key to the proof are the Teugels martingales and a strong orthogonalization of them. We conclude that a finite family of independent Lévy processes forms a quasi-shuffle algebra. We discuss important potential applications to constructing efficient numerical methods for the strong approximation of stochastic differential equations driven by Lévy processes.     

On interpolation for Hrmander’s algebras

We discuss some recent results on interpolation problems for weighted Hrmander’s algebras of holomorphic functions in several complex variables, and also give a sharp estimate on counting functions of interpolating varieties.