Building (mathematics)


In mathematics, a building is a combinatorial and geometric structure which simultaneously generalizes certain aspects of flag manifolds, finite projective planes, and Riemannian symmetric spaces. Buildings were initially introduced by Jacques Tits as a means to understand the structure of isotropic reductive linear algebraic groups over arbitrary fields. The more specialized theory of Bruhat–Tits buildings plays a role in the study of -adic Lie groups analogous to that of the theory of symmetric spaces in the theory of Lie groups.

Overview

The notion of a building was invented by Jacques Tits as a means of describing simple algebraic groups over an arbitrary field. Tits demonstrated how to every such group one can associate a simplicial complex with an action of, called the spherical building of. The group imposes very strong combinatorial regularity conditions on the complexes that can arise in this fashion. By treating these conditions as axioms for a class of simplicial complexes, Tits arrived at his first definition of a building. A part of the data defining a building is a Coxeter group, which determines a highly symmetrical simplicial complex, called the Coxeter complex. A building is glued together from multiple copies of, called its apartments, in a certain regular fashion. When is a finite Coxeter group, the Coxeter complex is a topological sphere, and the corresponding buildings are said to be of spherical type. When is an affine Weyl group, the Coxeter complex is a subdivision of the affine plane and one speaks of affine, or Euclidean, buildings. An affine building of type is the same as an infinite tree without terminal vertices.
Although the theory of semisimple algebraic groups provided the initial motivation for the notion of a building, not all buildings arise from a group. In particular, projective planes and generalized quadrangles form two classes of graphs studied in incidence geometry which satisfy the axioms of a building, but may not be connected with any group. This phenomenon turns out to be related to the low rank of the corresponding Coxeter system. Tits proved a remarkable theorem: all spherical buildings of rank at least three are connected with a group; moreover, if a building of rank at least two is connected with a group then the group is essentially determined by the building.
Iwahori–Matsumoto, Borel–Tits and Bruhat–Tits demonstrated that in analogy with Tits' construction of spherical buildings, affine buildings can also be constructed from certain groups, namely, reductive algebraic groups over a local non-Archimedean field. Furthermore, if the split rank of the group is at least three, it is essentially determined by its building. Tits later reworked the foundational aspects of the theory of buildings using the notion of a chamber system, encoding the building solely in terms of adjacency properties of simplices of maximal dimension; this leads to simplifications in both spherical and affine cases. He proved that, in analogy with the spherical case, every building of affine type and rank at least four arises from a group.

Definition

An -dimensional building is an abstract simplicial complex which is a union of subcomplexes called apartments such that
  • every -simplex of is within at least three -simplices if ;
  • any -simplex in an apartment lies in exactly two adjacent -simplices of and the graph of adjacent -simplices is connected;
  • any two simplices in lie in some common apartment ;
  • if two simplices both lie in apartments and, then there is a simplicial isomorphism of onto fixing the vertices of the two simplices.
An -simplex in is called a chamber.
The rank of the building is defined to be.

Elementary properties

Every apartment in a building is a Coxeter complex. In fact, for every two -simplices intersecting in an -simplex or panel, there is a unique period two simplicial automorphism of, called a reflection, carrying one -simplex onto the other and fixing their common points. These reflections generate a Coxeter group, called the Weyl group of, and the simplicial complex corresponds to the standard geometric realization of. Standard generators of the Coxeter group are given by the reflections in the walls of a fixed chamber in. Since the apartment is determined up to isomorphism by the building, the same is true of any two simplices in lying in some common apartment. When is finite, the building is said to be spherical. When it is an affine Weyl group, the building is said to be affine or Euclidean.
The chamber system is the adjacency graph formed by the chambers; each pair of adjacent chambers can in addition be labelled by one of the standard
generators of the Coxeter group.
Every building has a canonical length metric inherited from the geometric realisation obtained by identifying the vertices with an orthonormal basis of a Hilbert space. For affine buildings, this metric satisfies the CAT space| comparison inequality of Alexandrov, known in this setting as the Bruhat–Tits non-positive curvature condition for geodesic triangles: the distance from a vertex to the midpoint of the opposite side is no greater than the distance in the corresponding Euclidean triangle with the same side-lengths.

Connection with pairs

If a group acts simplicially on a building, transitively on pairs of chambers and apartments containing them, then the stabilisers of such a pair define a pair or Tits system. In fact the pair of subgroups
satisfies the axioms of a pair and the Weyl group can be identified with.
Conversely the building can be recovered from the pair, so that every pair canonically defines a building. In fact, using the terminology of pairs and calling any conjugate of a Borel subgroup and any group containing a Borel subgroup a parabolic subgroup,
  • the vertices of the building correspond to maximal parabolic subgroups;
  • vertices form a -simplex whenever the intersection of the corresponding maximal parabolic subgroups is also parabolic;
  • apartments are conjugates under of the simplicial subcomplex with vertices given by conjugates under of maximal parabolics containing.
The same building can often be described by different pairs. Moreover, not every building comes from a pair: this corresponds to the failure of classification results in low rank and dimension.
The Solomon-Tits theorem is a result which states the homotopy type of a building of a group of Lie type is the same as that of a bouquet of spheres.

Spherical and affine buildings for

The simplicial structure of the affine and spherical buildings associated to, as well as their interconnections, are easy to explain directly using only concepts from elementary algebra and geometry. In this case there are three different buildings, two spherical and one affine. Each is a union of apartments, themselves simplicial complexes. For the affine building, an apartment is a simplicial complex tessellating Euclidean space by -dimensional simplices; while for a spherical building it is the finite simplicial complex formed by all simplices with a given common vertex in the analogous tessellation in.
Each building is a simplicial complex which has to satisfy the following axioms:
  • is a union of apartments.
  • Any two simplices in are contained in a common apartment.
  • If a simplex is contained in two apartments, there is a simplicial isomorphism of one onto the other fixing all common points.

    Spherical building

Let be a field and let be the simplicial complex with vertices the non-trivial vector subspaces of. Two subspaces and are connected if one of them is a subset of the other. The -simplices of are formed by sets of mutually connected subspaces. Maximal connectivity is obtained by taking proper non-trivial subspaces and the corresponding -simplex corresponds to a complete flag
Lower dimensional simplices correspond to partial flags with fewer intermediary subspaces.
To define the apartments in, it is convenient to define a frame in as a basis determined up to scalar multiplication of each of its vectors ; in other words a frame is a set of one-dimensional subspaces such that any of them generate a -dimensional subspace. Now an ordered frame defines a complete flag via
Since reorderings of the various also give a frame, it is straightforward to see that the subspaces, obtained as sums of the, form a simplicial complex of the type required for an apartment of a spherical building. The axioms for a building can easily be verified using the classical Schreier refinement argument used to prove the uniqueness of the Jordan–Hölder decomposition.

Affine building

Let be a field lying between and its -adic completion with respect to the usual non-Archimedean -adic norm on for some prime. Let be the subring of defined by
When, is the localization of at and, when,, the -adic integers, i.e. the closure of in.
The vertices of the building are the -lattices in, i.e. -submodules of the form
where is a basis of over. Two lattices are said to be equivalent if one is a scalar multiple of the other by an element of the multiplicative group of . Two lattices and are said to be adjacent if some lattice equivalent to lies between and its sublattice : this relation is symmetric. The -simplices of are equivalence classes of mutually adjacent lattices, The -simplices correspond, after relabelling, to chains
where each successive quotient has order. Apartments are defined by fixing a basis of and taking all lattices with basis where lies in and is uniquely determined up to addition of the same integer to each entry.
By definition each apartment has the required form and their union is the whole of. The second axiom follows by a variant of the Schreier refinement argument. The last axiom follows by a simple counting argument based on the orders of finite Abelian groups of the form
A standard compactness argument shows that is in fact independent of the choice of. In particular taking, it follows that is countable. On the other hand, taking, the definition shows that admits a natural simplicial action on the building.
The building comes equipped with a labelling of its vertices with values in. Indeed, fixing a reference lattice, the label of is given by
for sufficiently large. The vertices of any -simplex in has distinct labels, running through the whole of. Any simplicial automorphism of defines a permutation of such that. In particular for in,
Thus preserves labels if lies in.