Searcharxiv⌕ Search

arXiv subjects

Tom De Medts

Publications and source records attributed to Tom De Medts.

At least 19 recordsLinked to original sources
arXiv2026

From cubic norm pairs to $G_2$- and $F_4$-graded groups and Lie algebras

We construct Lie algebras arising from cubic norm pairs over arbitrary commutative base rings. Such Lie algebras admit a grading by a root system of type $G_2$, and when the cubic norm pair is a cubic Jordan matrix algebra, the $G_2$-grading can be further refined to an $F_4$-grading. We then use these Lie algebras and their gradings to construct corresponding root graded groups. Along the way, we produce many results providing detailed information about the structure of these Lie algebras and groups.

math.RA↗
arXiv2026

Constructing Chayet-Garibaldi algebras from affine vertex algebras (including the 3876-dimensional algebra for $E_8$)

In 2021, Maurice Chayet and Skip Garibaldi provided an explicit construction of a commutative non-associative algebra on the second smallest representation of $E_8$ (of dimension $3875$) adjoined with a unit. In fact, they define such an algebra $A(\mathfrak{g})$ for each simple Lie algebra $\mathfrak{g}$, in terms of explicit but ad-hoc formulas. We discovered that their algebras $A(\mathfrak{g})$ have a natural interpretation in terms of affine vertex algebras, and their ad-hoc formulas take an extremely simple form in this new interpretation. It is our hope that this point of view will lead to a better understanding of this interesting class of algebras.

math.RA↗
arXiv2025

Lattice envelopes of right-angled Artin groups

Let $Γ$ be a finite simplicial graph with at least two vertices, and let $G(Γ)$ be the associated right-angled Artin group. We describe a locally compact group $\mathcal U$ containing $G(Γ)$ as a cocompact lattice. If $Γ$ is not a join (i.e. the complement graph is connected), the group $\mathcal U$ is non-discrete, almost simple, but not virtually simple: it has a smallest normal subgroup $\mathcal U^+$ which is an open simple subgroup, and the quotient $\mathcal U/\mathcal U^+$ is isomorphic to the right-angled Coxeter group $W(Γ)$. Under suitable assumptions on $Γ$, we rely on work by Bader-Furman-Sauer and Huang-Kleiner to show that $\mathcal U \rtimes \mathrm{Aut}(Γ)$ is the universal lattice envelope of $G(Γ)$: for every lattice envelope $H$ of $G(Γ)$, there is a continuous proper homomorphism $H \to \mathcal U \rtimes \mathrm{Aut}(Γ)$. In particular, no lattice envelope of $G(Γ)$ is virtually simple. We also show that no locally compact group quasi-isometric to $G(Γ)$ is virtually simple. This contrasts with the case of free groups. The group $\mathcal U$ is a universal automorphism group of the Davis building of $G(Γ)$, with prescribed local actions. As an application, we describe the algebraic structure of the full automorphism group of the Cayley graph of $G(Γ)$ with respect to its standard generating set.

math.GR↗
arXiv2025

$5 \times 5$-graded Lie algebras, cubic norm structures and quadrangular algebras

We study simple Lie algebras generated by extremal elements, over arbitrary fields of arbitrary characteristic. We show: (1) If the extremal geometry contains lines, then the Lie algebra admits a $5 \times 5$-grading that can be parametrized by a cubic norm structure; (2) If there exists a field extension of degree at most $2$ such that the extremal geometry over that field extension contains lines, and in addition, there exist symplectic pairs of extremal elements, then the Lie algebra admits a $5 \times 5$-grading that can be parametrized by a quadrangular algebra. One of our key tools is a new definition of exponential maps that makes sense even over fields of characteristic $2$ and $3$, which ought to be interesting in its own right.

math.RA↗
arXiv2024

Operator Kantor Pairs

Kantor pairs, (quadratic) Jordan pairs, and similar structures have been instrumental in the study of $\mathbb{Z}$-graded Lie algebras and algebraic groups. We introduce the notion of an operator Kantor pair, a generalization of Kantor pairs to arbitrary (commutative, unital) rings, similar in spirit as to how quadratic Jordan pairs and algebras generalize linear Jordan pairs and algebras. Such an operator Kantor pair is formed by a pair of $Φ$-groups $(G^+,G^-)$ of a specific kind, equipped with certain homogeneous operators. For each such a pair $(G^+,G^-)$, we construct a $5$-graded Lie algebra $L$ together with actions of $G^\pm$ on $L$ as automorphisms. Moreover, we can associate a group $G(G^+,G^-) \subset \operatorname{Aut}(L)$ to this pair generalizing the projective elementary group of Jordan pairs. If the non-$0$-graded part of $L$ is projective, we can uniquely recover $G^+,G^-$ from $G(G^+,G^-)$ and the grading on $L$ alone. We establish, over rings $Φ$ with $1/30 \in Φ$, a one to one correspondence between Kantor pairs and operator Kantor pairs. Finally, we construct operator Kantor pairs for the different families of central simple structurable algebras.

math.RA↗
arXiv2023

Matsuo algebras in characteristic 2

We extend the theory of Matsuo algebras, which are certain non-associative algebras related to 3-transposition groups, to characteristic 2. Instead of idempotent elements associated to points in the corresponding Fischer space, our algebras are now generated by nilpotent elements associated to lines. For many 3-transposition groups, this still gives rise to a $\mathbb{Z}/2\mathbb{Z}$-graded fusion law, and we provide a complete classification of when this occurs. In one particular small case, arising from the 3-transposition group $\operatorname{Sym}(4)$, the fusion law is even stronger, and the resulting Miyamoto group is an algebraic group $\mathbb{G}_a^2 \rtimes \mathbb{G}_m$.

math.GR↗
arXiv2022

City products of right-angled buildings and their universal groups

We introduce the notion of city products of right-angled buildings that produces a new right-angled building out of smaller ones. More precisely, if $M$ is a right-angled Coxeter diagram of rank $n$ and $Δ_1,\dots,Δ_n$ are right-angled buildings, then we construct a new right-angled building $Δ:= \mathrm{cityproduct}_M(Δ_1,\dots,Δ_n)$. We can recover the buildings $Δ_1,\dots,Δ_n$ as residues of $Δ$, but we can also construct a skeletal building of type $M$ from $Δ$ that captures the large-scale geometry of $Δ$. We then proceed to study universal groups for city products of right-angled buildings, and we show that the universal group of $Δ$ can be expressed in terms of the universal groups for the buildings $Δ_1,\dots,Δ_n$ and the structure of $M$. As an application, we show the existence of many examples of pairs of different buildings of the same type that admit (topologically) isomorphic universal groups, thereby vastly generalizing a recent example by Lara Beßmann.

math.GR↗
arXiv2022

Restricted universal groups for right-angled buildings

In 2000, Marc Burger and Shahar Mozes introduced universal groups acting on trees. Such groups provide interesting examples of totally disconnected locally compact groups. Intuitively, these are the largest groups for which all local actions satisfy a prescribed behavior. Since then, their study has evolved in various directions. In particular, Adrien Le Boudec has studied restricted universal groups, where the prescribed behavior is allowed to be violated in a finite number of vertices. On the other hand, we have been studying universal groups acting on right-angled buildings, a class of geometric objects with a much more general structure than trees. The aim of the current paper is to combine both ideas: we will study restricted universal groups acting on right-angled buildings. We show several permutational and topological properties of those groups, with as main result a precise criterion for when these groups are simple.

math.GR↗
arXiv2021

Topological and algebraic properties of universal groups for right-angled buildings

We study universal groups for right-angled buildings. Inspired by Simon Smith's work on universal groups for trees, we explicitly allow local groups that are not necessarily finite nor transitive. We discuss various topological and algebraic properties in this extended setting. In particular, we characterise when these groups are locally compact, when they are abstractly simple, when they act primitively on residues of the building, and we discuss some necessary and sufficient conditions for the groups to be compactly generated. We point out that there are unexpected aspects related to the geometry and the diagram of these buildings that influence the topological and algebraic properties of the corresponding universal groups.

math.GR↗
arXiv2020

Decomposition algebras and axial algebras

We introduce decomposition algebras as a natural generalization of axial algebras, Majorana algebras and the Griess algebra. They remedy three limitations of axial algebras: (1) They separate fusion laws from specific values in a field, thereby allowing repetition of eigenvalues; (2) They allow for decompositions that do not arise from multiplication by idempotents; (3) They admit a natural notion of homomorphisms, making them into a nice category. We exploit these facts to strengthen the connection between axial algebras and groups. In particular, we provide a definition of a universal Miyamoto group which makes this connection functorial under some mild assumptions. We illustrate our theory by explaining how representation theory and association schemes can help to build a decomposition algebra for a given (permutation) group. This construction leads to a large number of examples. We also take the opportunity to fix some terminology in this rapidly expanding subject.

math.RA↗
arXiv2020

Inner ideals and structurable algebras: Moufang sets, triangles and hexagons

We construct Moufang sets, Moufang triangles and Moufang hexagons using inner ideals of Lie algebras obtained from structurable algebras via the Tits--Kantor--Koecher construction. The three different types of structurable algebras we use are, respectively: (1) structurable division algebras, (2) algebras $D \oplus D$ for some alternative division algebra $D$, equipped with the exchange involution, (3) matrix structurable algebras $M(J,1)$ for some cubic Jordan division algebra $J$. In each case, we also determine the root groups directly in terms of the structurable algebra.

math.RA↗
arXiv2020

Non-associative Frobenius algebras for simply laced Chevalley groups

We provide an explicit construction for a class of commutative, non-associative algebras for each of the simple Chevalley groups of simply laced type. Moreover, we equip these algebras with an associating bilinear form, which turns them into Frobenius algebras. This class includes a 3876-dimensional algebra on which the Chevalley group of type E8 acts by automorphisms. We also prove that these algebras admit the structure of (axial) decomposition algebras.

math.RA↗
arXiv2019

Suzuki-Ree groups as algebraic groups over $\mathbb{F}_{\sqrt{\smash[b]p}}$

Among the infinite classes of finite simple groups, the most exotic classes are probably the Suzuki groups and the Ree groups. They are "twisted versions" of groups of Lie type, but they cannot be directly obtained as groups of rational points of a suitable linear algebraic group. We provide a framework in which these groups do arise as groups of rational points of algebraic groups over a "twisted field"; in the finite case, such a twisted field can be interpreted as a "field with $\sqrt{\smash[b]p}$ elements". Our framework at once allows for other, perhaps less known, exotic families of groups. Most notably, there is a class of "mixed groups", introduced by J. Tits but also apparent in the work of Steinberg, and we show that they can be obtained as groups of rational points of algebraic groups over a "mixed field". We show that a base change from $\mathbb{F}_{\sqrt{\smash[b]p}}$ to $\mathbb{F}_p$ transforms twisted groups into mixed groups, and we formulate a notion of "twisted descent" that allows to detect which mixed groups arise in this fashion.

math.GR↗
arXiv2019

Open subgroups of the automorphism group of a right-angled building

We study the group of type-preserving automorphisms of a right-angled building, in particular when the building is locally finite. Our aim is to characterize the proper open subgroups as the finite index closed subgroups of the stabilizers of proper residues. One of the main tools is the new notion of firm elements in a right-angled Coxeter group, which are those elements for which the final letter in each reduced representation is the same. We also introduce the related notions of firmness for arbitrary elements of such a Coxeter group and $n$-flexibility of chambers in a right-angled building. These notions and their properties are used to determine the set of chambers fixed by the fixator of a ball. Our main result is obtained by combining these facts with ideas by Pierre-Emmanuel Caprace and Timothée Marquis in the context of Kac-Moody groups over finite fields, where we had to replace the notion of root groups by a new notion of root wing groups.

math.GR↗
arXiv2018

Modules over axial algebras

We introduce axial representations and modules over axial algebras as new tools to study axial algebras. All known interesting examples of axial algebras fall into this setting, in particular the Griess algebra whose automorphism group is the Monster group. Our results become especially interesting for Matsuo algebras. We vitalize the connection between Matsuo algebras and 3-transposition groups by relating modules over Matsuo algebras with representations of 3-transposition groups. As a by-product, we define, given a Fischer space, a group that can fulfill the role of a universal 3-transposition group.

math.RA↗
arXiv2018

Universal groups for right-angled buildings

In 2000, M. Burger and S. Mozes introduced universal groups acting on trees with a prescribed local action. We generalize this concept to groups acting on right-angled buildings. When the right-angled building is thick and irreducible of rank at least 2 and each of the local permutation groups is transitive and generated by its point stabilizers, we show that the corresponding universal group is a simple group. When the building is locally finite, these universal groups are compactly generated totally disconnected locally compact groups, and we describe the structure of the maximal compact open subgroups of the universal groups as a limit of generalized wreath products.

math.GR↗
arXiv2017

Structurable algebras of skew-dimension one and hermitian cubic norm structures

We study structurable algebras of skew-dimension one. We present two different equivalent constructions for such algebras: one in terms of non-linear isotopies of cubic norm structures, and one in terms of hermitian cubic norm structures. After this work was essentially finished, we became aware of the fact that both descriptions already occur in (somewhat hidden places in) the literature. Nevertheless, we prove some facts that had not been noticed before: (1) We show that every form of a matrix structurable algebra can be described by our constructions; (2) We give explicit formulas for the norm $ν$; (3) We make a precise connection with the Cayley-Dickson process for structurable algebras.

math.RA↗