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We lay the foundations of the first-order model theory of Coxeter groups. Firstly, with the exception of the $2$-spherical non-affine case (which we leave open), we characterize the superstable Coxeter groups of finite rank, which we show to be essentially the Coxeter groups of affine type. Secondly, we characterize the Coxeter groups of finite rank which are domains, a central assumption in the theory of algebraic geometry over groups, which in many respects (e.g. $\lambda$-stability) reduces the model theory of a given Coxeter system to the model theory of its associated irreducible components. In the second part of the paper we move to specific definability questions in right-angled Coxeter groups (RACGs) and $2$-spherical Coxeter groups. In this respect, firstly, we prove that RACGs of finite rank do not have proper elementary subgroups which are Coxeter groups, and prove further that reflection independent ones do not have proper elementary subgroups at all. Secondly, we prove that if the monoid $Sim(W, S)$ of $S$-self-similarities of $W$ is finitely generated, then $W$ is a prime model of its theory. Thirdly, we prove that in reflection independent RACGs of finite rank the Coxeter elements are type-determined. We then move to $2$-spherical Coxeter groups, proving that if $(W, S)$ is irreducible, $2$-spherical even and not affine, then $W$ is a prime model of its theory, and that if $W_{\Gamma}$ and $W_{\Theta}$ are as in the previous sentence, then $W_{\Gamma}$ is elementary equivalent to $W_{\Theta}$ if and only if $\Gamma \cong \Theta$, thus solving the elementary equivalence problem for most of the $2$-spherical Coxeter groups. In the last part of the paper we focus on model theoretic applications of the notion of reflection length from Coxeter group theory, proving in particular that affine Coxeter groups are not connected., Comment: 38 pages

We show that every 4-plump razor-sharp normal Tits quadrangle X is uniquely determined by a non-degenerate quadratic space whose Witt index m is at least 2. If this Witt index is finite, then X is the Tits quadrangle arising from the corresponding building of type B_m or D_m by a standard construction.

Tits polygons are generalizations of Moufang polygons in which the neighborhood of each vertex is endowed with an “opposition relation.” There is a standard construction that produces a Tits polygon from an arbitrary irreducible spherical building of rank at least 3 when paired with a suitable Tits index. In this note, we complete the proof of a characterization of the Tits quadrangles that arise in this way from the spherical building associated to an exceptional algebraic group.

A twin building consists of two buildings that are twinned by a codistance function. We prove that the local structure of a twin building uniquely determines the two buildings up to isomorphism. This has been known for twin buildings satisfying a technical condition (co)., 14 pages

A Tits polygon is a bipartite graph in which the neighborhood of each vertex is endowed with an “opposition relation” satisfying certain axioms. Moufang polygons are precisely the Tits polygons in which these opposition relations are all trivial. Every Tits polygon has a distinguished set of circuits. A Tits quadrangle is a Tits polygon in which these circuits all have length 8. There is a standard construction that produces a Tits polygon from certain pairs (∆, T), where ∆ is an irreducible spherical building and T is a Tits index of relative rank 2. We call a Tits quadrangle exceptional if it arises from such a pair (∆, T) for ∆ the spherical building associated to the group of rational points of an exceptional algebraic group. In this paper, we characterize the exceptional Tits quadrangles as extensions of orthogonal Tits quadrangles in a suitable sense.

Masures are generalizations of Bruhat–Tits buildings and the main examples are associated with almost split Kac–Moody groups G over non-Archimedean local fields. In this case, G acts strongly transitively on its corresponding masure Δ as well as on the building at infinity of Δ, which is the twin building associated with G. The aim of this article is twofold: firstly, to introduce and study the cone topology on the twin building at infinity of a masure. It turns out that this topology has various favorable properties that are required in the literature as axioms for a topological twin building. Secondly, by making use of the cone topology, we study strongly transitive actions of a group G on a masure Δ. Under some hypotheses, with respect to the masure and the group action of G, we prove that G acts strongly transitively on Δ if and only if it acts strongly transitively on the twin building at infinity ∂ Δ. Along the way a criterion for strong transitivity is given and the existence and good dynamical properties of strongly regular hyperbolic automorphisms of the masure are proven.

We introduce the notion of a Tits polygon, a generalization of the notion of a Moufang polygon, and show that Tits polygons arise in a natural way from certain configurations of parabolic subgroups in an arbitrary spherical buildings satisfying the Moufang condition. We establish numerous basic properties of Tits polygons and characterize a large class of Tits hexagons in terms of Jordan algebras. We apply this classification to give a “rank 2 2 ” presentation for the group of F F -rational points of an arbitrary exceptional simple group of F F -rank at least 4 4 and to determine defining relations for the group of F F -rational points of an an arbitrary group of F F -rank 1 1 and absolute type D 4 D_4 , E 6 E_6 , E 7 E_7 or E 8 E_8 associated to the unique vertex of the Dynkin diagram that is not orthogonal to the highest root. All of these results are over a field of arbitrary characteristic.

Veldkamp polygons are certain graphs $\Gamma=(V,E)$ such that for each $v\in V$, $\Gamma_v$ is endowed with a symmetric anti-reflexive relation $\equiv_v$. These relations are all trivial if and only if $\Gamma$ is a thick generalized polygon. A Veldkamp polygon is called flat if no two vertices have the same set of vertices that are opposite in a natural sense. We explore the connection between Veldkamp quadrangles and polar spaces. Using this connection, we give the complete classification of flat Veldkamp quadrangles in which some but not all of the relations $\equiv_v$ are trivial., Comment: 27 pages

We translate the axioms of a Weyl groupoid with (not necessarily finite) root system in terms of arrangements. The result is a correspondence between Weyl groupoids permitting a root system and Tits arrangements satisfying an integrality condition which we call the crystallographic property., Comment: 23 pages

Let q : V → K be the form of degree 4 and H the alternating bilinear form that together give rise to a Freudenthal triple system when the characteristic of K is different from 2 and 3. We give a form Θ of degree 4 on a central cover of V over an arbitrary field in arbitrary characteristic and determine the group of similitudes of Θ. This group coincides with the intersection of the group of similitudes of q and the isometry group of H when the characteristic is different from 2 and 3 and has the expected structure of a suitable Levi subgroup in general. We extend known results about the stabilizers of elements of V and orbits of the group of similitudes and show, in particular, the existence of an exotic set of orbits when char ( K ) = 2 and K is not perfect.

We introduce the notion of a Tits arrangement on a convex open cone as a special case of (infinite) simplicial arrangements. Such an object carries a simplicial structure similar to the geometric representation of Coxeter groups. The standard constructions of subarrangements and restrictions, which are known in the case of finite hyperplane arrangements, work as well in this more general setting., 45 pages, 4 figures

We define optimal EKR-sets in finite buildings. This definition is motivated by various contributions on optimal EKR-sets in finite projective spaces and polar spaces. Our main result is the classification of optimal EKR-sets of type $$\{ \ell , \ell -1\}$$ in finite building of type $$D_{\ell }$$ with $$\ell $$ even. As it is the case for most of the known optimal EKR-sets in finite buildings, our EKR-sets have a natural center. This provides some evidence that the EKR-problem for finite buildings and Tits center conjecture are closely related.

We prove a characterization of irreducible, non-spherical and non-affine Coxeter groups, motivated by applications to metric commensurators of Kac-Moody groups and twinnings at finite distance.

Quadrangular algebras arise in the theory of Tits quadrangles. They are anisotropic if and only if the corresponding Tits quadrangle is, in fact, a Moufang quadrangle. Anisotropic quadrangular algebras were classified in the course of classifying Moufang polygons. In this paper we extend the classification of anisotropic quadrangular algebras to a classification of isotropic quadrangular algebras satisfying a natural non-degeneracy condition.

An involution r in a Coxeter group W is called an intrinsic reflection of W if r ∈ S W for each Coxeter generating set S of W. In recent joint work with R.B. Howlett [13] we determined all intrinsic reflections in finitely generated Coxeter groups. In the present paper we extend this result to the infinite rank case. An important tool in [13] is the notion of the finite continuation of an involution that is only meaningful for finitely generated Coxeter groups. Here we introduce the locally finite continuation for any subset of an arbitrary group which enables us to deal with Coxeter groups of infinite rank. We apply our result to show that certain classes of Coxeter groups are reflection independent and we investigate rigidity of 2-spherical Coxeter systems of arbitrary ranks.

Let \(\Delta \) be a Moufang spherical building with field of definition F, let \(\Gamma \) be a subgroup of \(\mathrm{Aut}(\Delta )\) that acts on the set of chambers of \(\Delta \) with finite orbits and let M be the kernel of the natural homomorphism from \(\mathrm{Aut}(\Delta )\) to \(\mathrm{Aut}(F)\). We show: (1) If \(\Gamma \cap M=1\) and there exist proper residues of \(\Delta \) stabilized by \(\Gamma \), then \(\Gamma \) is a descent group of \(\Delta \). (2) If \(\Gamma \cap M\) has no finite quotients of order divisible by \(\mathrm{char}(F)\), then \(\Gamma \) is completely reducible.

This book begins with the resolution of a major open question about the local structure of Bruhat-Tits buildings. It then puts forward an algebraic solution into a geometric context by developing a general fixed point theory for groups acting on buildings of arbitrary type, giving necessary and sufficient conditions for the residues fixed by a group to form a kind of subbuilding or “form” of the original building. At the center of this theory is the notion of a Tits index, a combinatorial version of the notion of an index in the relative theory of algebraic groups. These results are combined at the end to show that every exceptional Bruhat-Tits building arises as a form of a “residually pseudo-split” building. The book concludes with a display of the Tits indices associated with each of these exceptional forms. This is the third and final volume of a trilogy that began with The Structure of Spherical Buildings and The Structure of Affine Buildings.

In this note we determine the structure of the quotient of the Bruhat–Tits tree of the locally compact group PGL2(F p ) with respect to the natural action of its S-arithmetic subgroup PGL2(O {p}), where F is a rational function field over a finite field and p is a place of F.

We prove that the unipotent horocyclic group of a Moufang twin tree of prime order is nilpotent of class at most 2.

Let $(W,S)$ be a Coxeter system and let $s \in S$. We call $s$ a right-angled generator of $(W,S)$ if $st = ts$ or $st$ has infinite order for each $t \in S$. We call $s$ an intrinsic reflection of $W$ if $s \in R^W$ for all Coxeter generating sets $R$ of $W$. We give necessary and sufficient conditions for a right-angled generator $s \in S$ of $(W,S)$ to be an intrinsic reflection of $W$.