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Jan Okniński

Publications and source records attributed to Jan Okniński.

12 recordsLinked to original sources
arXiv2022

Indecomposable solutions of the Yang-Baxter equation of square-free cardinality

Indecomposable involutive non-degenerate set-theoretic solutions $(X,r)$ of the Yang-Baxter equation of cardinality $p_1\cdots p_n$, for different prime numbers $p_1,\ldots, p_n$, are studied. It is proved that they are multipermutation solutions of level $\leq n$. In particular, there is no simple solution of a non-prime square-free cardinality. This solves a problem stated in [F. Cedó, J. Okniński, Constructing finite simple solutions of the Yang-Baxter equation, Adv. Math. 391 (2021), 107968] and provides a far reaching extension of several earlier results on indecomposability of solutions. The proofs are based on a detailed study of the brace structure on the permutation group $\mathcal G(X,r)$ associated to such a solution. It is proved that $p_1,\ldots, p_n$ are the only primes dividing the order of $\mathcal{G}(X,r)$. Moreover, the Sylow $p_i$-subgroups of $\mathcal{G}(X,r)$ are elementary abelian $p_i$-groups and if $P_i$ denotes the Sylow $p_i$-subgroup of the additive group of the left brace $\mathcal{G}(X,r)$, then there exists a permutation $σ\in S_n$ such that $P_{σ(1)}, \, P_{σ(1)}P_{σ(2)}, \dots , P_{σ(1)}P_{σ(2)}\cdots P_{σ(n)}$ are ideals of the left brace $\mathcal{G}(X,r)$ and $\mathcal{G}(X,r)=P_1P_2\cdots P_n$. In addition, indecomposable solutions of cardinality $p_1\cdots p_n$ that are multipermutation of level $n$ are constructed, for every nonnegative integer $n$.

math.QA↗
arXiv2021

New simple solutions of the Yang-Baxter equation and solutions associated to simple left braces

Involutive non-degenerate set theoretic solutions of the Yang-Baxter equation are considered, with a focus on finite solutions. A rich class of indecomposable and irretractable solutions is determined and necessary and sufficient conditions are found in order that these solutions are simple. Then a link between simple solutions and simple left braces is established, that allows us to construct more examples of simple solutions. In particular, the results answer some problems stated in the recent paper [13].

math.QA↗
arXiv2020

Constructing finite simple solutions of the Yang-Baxter equation

We study involutive non-degenerate set-theoretic solutions (X,r) of the Yang-Baxter equation on a finite set X. The emphasis is on the case where (X,r) is indecomposable, so the associated permutation group acts transitively on X. One of the major problems is to determine how such solutions are built from the imprimitivity blocks; and also how to characterize these blocks. We focus on the case of so called simple solutions, which are of key importance. Several infinite families of such solutions are constructed for the first time. In particular, a broad class of simple solutions of order p^2, for any prime p, is completely characterized.

math.QA↗
arXiv2019

On the radical of a Hecke-Kiselman algebra

The Hecke-Kiselman algebra of a finite oriented graph $Θ$ over a field $K$ is studied. If $Θ$ is an oriented cycle, it is shown that the algebra is semiprime and its central localization is a finite direct product of matrix algebras over the field of rational functions $K(x)$. More generally, the radical is described in the case of PI-algebras, and it is shown that it comes from an explicitly described congruence on the underlying Hecke-Kiselman monoid. Moreover, the algebra modulo the radical is again a Hecke-Kiselman algebra and it is a finite module over its center.

math.RA↗
arXiv2019

Combinatorics and structure of Hecke-Kiselman algebras

Hecke-Kiselman monoids $\textrm{HK}_Θ$ and their algebras $K[\textrm{HK}_Θ]$, over a field $K$, associated to finite oriented graphs $Θ$ are studied. In the case $Θ$ is a cycle of length $n\geqslant 3$, a hierarchy of certain unexpected structures of matrix type is discovered within the monoid $C_n=\textrm{HK}_Θ$ and it is used to describe the structure and the properties of the algebra $K[C_n]$. In particular, it is shown that $K[C_n]$ is a right and left Noetherian algebra, while it has been known that it is a PI-algebra of Gelfand-Kirillov dimension one. This is used to characterize all Noetherian algebras $K[\textrm{HK}_Θ]$ in terms of the graphs $Θ$. The strategy of our approach is based on the crucial role played by submonoids of the form $C_n$ in combinatorics and structure of arbitrary Hecke-Kiselman monoids $\textrm{HK}_Θ$.

math.RA↗
arXiv2018

Gröbner basis and the automaton property of Hecke--Kiselman algebras

It is shown that the Hecke-Kiselman algebra associated to a finite directed graph is an automaton algebra in the sense of Ufnarovskii. Consequently, its Gelfand-Kirillov dimension is an integer if it is finite. As a consequence, it is proved that the Hecke-Kiselman algebra associated to an oriented cycle admits a finite Gröbner basis.

math.RA↗
arXiv2018

An abundance of simple left braces with abelian multiplicative Sylow subgroups

Braces were introduced by Rump to study involutive non-degenerate set-theoretic solutions of the Yang-Baxter equation. A constructive method for producing all such finite solutions from a description of all finite left braces has been recently discovered. It is thus a fundamental problem to construct and classify all simple left braces, as they can be considered as building blocks for the general theory. This program recently has been initiated by Bachiller and the authors. In this paper we study the simple finite left braces such that the Sylow subgroups of their multiplicative groups are abelian. We provide several new families of such simple left braces. In particular, they lead to the main, surprising result, that shows that there is an abundance of such simple left braces.

math.QA↗
arXiv2017

Asymmetric product of left braces and simplicity; new solutions of the Yang-Baxter equation

The problem of constructing all the non-degenerate involutive set-theoretic solutions of the Yang-Baxter equation recently has been reduced to the problem of describing all the left braces. In particular, the classification of all finite left braces is fundamental in order to describe all finite such solutions of the Yang-Baxter equation. In this paper we continue the study of finite simple left braces with the emphasis on the application of the asymmetric product of left braces in order to construct new classes of simple left braces. We do not only construct new classes but also we interpret all previously known constructions as asymmetric products. Moreover, a construction is given of finite simple left braces with a multiplicative group that is solvable of arbitrary derived length.

math.QA↗
arXiv2017

A note on identities in plactic monoids and monoids of upper-triangular tropical matrices

This paper uses the combinatorics of Young tableaux to prove the plactic monoid of infinite rank does not satisfy a non-trivial identity, by showing that the plactic monoid of rank $n$ cannot satisfy a non-trivial identity of length less than or equal to $n$. A new identity is then proven to hold for the monoid of $n \times n$ upper-triangular tropical matrices. Finally, a straightforward embedding is exhibited of the plactic monoid of rank $3$ into the direct product of two copies of the monoid of $3\times 3$ upper-triangular tropical matrices, giving a new proof that the plactic monoid of rank $3$ satisfies a non-trivial identity.

math.CO↗
arXiv2016

Irreducible representations of the Chinese monoid

All irreducible representations of the Chinese monoid $C_n$, of any rank $n$, over a nondenumerable algebraically closed field $K$, are constructed. It turns out that they have a remarkably simple form and they can be built inductively from irreducible representations of the monoid $C_2$. The proof shows also that every such representation is monomial. Since $C_n$ embeds into the algebra $K[C_n]/J(K[C_n])$, where $J(K[C_n])$ denotes the Jacobson radical of the mooned algebra $K[C_n]$, a new representation of $C_n$ as a subdirect product of the images of $C_n$ in the endomorphism algebras of the constructed simple modules follows.

math.RT↗