# Lattice (discrete subgroup)

﻿
Lattice (discrete subgroup)

In Lie theory and related areas of mathematics, a lattice in a locally compact topological group is a discrete subgroup with the property that the quotient space has finite invariant measure. In the special case of subgroups of R"n", this amounts to the usual geometric notion of a lattice, and both the algebraic structure of lattices and the geometry of the totality of all lattices are relatively well understood. Deep results of Borel, Harish-Chandra, Mostow, Tamagawa, M.S. Raghunathan, Margulis, Zimmer obtained from the 1950s through the 1970s provided examples and generalized much of the theory to the setting of nilpotent Lie groups and semisimple algebraic groups over a local field. In the 1990s, Bass and Lubotzky initiated the study of "tree lattices", which remains an active research area.

Definition

Let "G" be a locally compact topological group with the Haar measure "&mu;". A discrete subgroup "&Gamma;" is called a lattice in "G" if the quotient space "G"/"&Gamma;" has finite invariant measure, that is, if "G" is a unimodular group and the volume "&mu;"("G"/"&Gamma;") is finite. The lattice is uniform (or "cocompact") if the quotient space is compact, and nonuniform otherwise.

Arithmetic lattices

An archetypical example of a nonuniform lattice is given by the group "SL"(2,Z), which is a lattice in the special linear group "SL"(2,R), and by the closely related modular group. This construction admits a far-reaching generalization to a class of lattices in all semisimple algebraic groups over a local field "F" called "arithmetic lattices". For example, let "F" = R be the field of real numbers. Roughly speaking, the Lie group "G"(R) is formed by all matrices with entries in R satisfying certain algebraic conditions, and by restricting the entries to the integers Z, one obtains a lattice "G"(Z). Conversely, Grigory Margulis proved that under certain assumptions on "G", any lattice in it essentially arises in this way. This remarkable statement is known as "Arithmeticity of lattices" or "Margulis Arithmeticity Theorem".

"S"-arithmetic lattices

Arithmetic lattices admit an important generalization, known as the "S"-"arithmetic lattices". The first example is given by the diagonally embedded subgroup

: $SLleft\left(2,mathbb\left\{Z\right\}left \left[frac\left\{1\right\}\left\{p\right\} ight\right] ight\right) subset SL\left(2,mathbb\left\{R\right\}\right) imes SL\left(2,mathbb\left\{Q\right\}_p\right), S=\left\{p, infty\right\}.$

This is a lattice in the product of algebraic groups over "different" local fields, both real and p-adic. It is formed by the unimodular matrices of order 2 with entries in the localization of the ring of integers at the prime "p". The set "S" is a finite set of places of Q which includes all archimedean places and the locally compact group is the direct product of the groups of points of a fixed linear algebraic group "G" defined over Q (or a more general global field) over the completions of Q at the places from "S". To form the discrete subgroup, instead of matrices with integer entries, one considers matrices with entries in the localization over the primes (nonarchimedean places) in "S". Under fairly general assumptions, this construction indeed produces a lattice. The class of "S"-arithmetic lattices is much wider than the class of arithmetic lattices, but they share many common features.

A lattice of fundamental importance for the theory of automorphic forms is given by the group "G"("K") of "K"-points of a semisimple (or reductive) linear algebraic group "G" defined over a global field "K". This group diagonally embeds into the adelic algebraic group "G"("A"), where "A" is the ring of adeles of "K", and is a lattice there. Unlike arithmetic lattices, "G"("K") is not finitely generated.

Rigidity

Another group of phenomena concerning lattices in semisimple algebraic groups is collectively known as "rigidity". Mostow rigidity theorem showed that the algebraic structure of a lattice in simple Lie group "G" of split rank at least two determines "G". Thus any isomorphism of lattices in two such groups is essentially induced by an isomorphism between the groups themselves. "Superrigidity" provides a generalization dealing with homomorphisms from a lattice in an algebraic group "G" into another algebraic group "H".

Tree lattices

Let "X" be a locally finite tree. Then the automorphism group "G" of "X" is a locally compact topological group, in which the basis of the topology is given by the stabilizers of finite sets of vertices. Vertex stabilizers "G""x" are thus compact open subgroups, and a subgroup "&Gamma;" of "G" is discrete if "&Gamma;""x" is finite for some (and hence, for any) vertex "x". The subgroup "&Gamma;" is an "X"-lattice if the suitably defined volume of is finite, and a uniform "X"-lattice if this quotient is a finite graph. In case is finite, this is equivalent to "&Gamma;" being a lattice (respectively, a uniform lattice) in "G".

* Kazhdan's property (T)
* Graph of groups

References

* Hyman Bass and Alexander Lubotzky, "Tree lattices". With appendices by H. Bass, L. Carbone, A. Lubotzky, G. Rosenberg, and J. Tits. Progress in Mathematics, vol 176, Birkhäuser Verlag, Boston, 2001 ISBN 0-8176-4120-3
* Grigory Margulis, "Discrete subgroups of semisimple Lie groups", Ergebnisse der Mathematik und ihrer Grenzgebiete (3) [Results in Mathematics and Related Areas (3)] , 17. Springer-Verlag, Berlin, 1991. x+388 pp. ISBN 3-540-12179-X MathSciNet|id=1090825
* [http://people.uleth.ca/~dave.morris/LectureNotes.shtml#ArithmeticGroups Dave Witte Morris: Introduction to Arithmetic Groups] , draft of a book
*citation|id=MR|1278263
last=Platonov|first= Vladimir|author-link=Vladimir Platonov|last2= Rapinchuk|first2= Andrei|title=Algebraic groups and number theory. (Translated from the 1991 Russian original by Rachel Rowen.) |series=Pure and Applied Mathematics|volume= 139|publisher= Academic Press, Inc.|publication-place= Boston, MA|year= 1994|ISBN= 0-12-558180-7

* M.S.Raghunathan, "Discrete subgroups of Lie groups". Ergebnisse der Mathematik und ihrer Grenzgebiete, Band 68. Springer-Verlag, New York-Heidelberg, 1972 MathSciNet|id=0507234

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Lattice — may refer to: In art and design: Latticework an ornamental criss crossed framework, an arrangement of crossing laths or other thin strips of material Lattice (pastry) In engineering: A lattice shape truss structure In mathematics: Lattice (order) …   Wikipedia

• Lattice (group) — A lattice in the Euclidean plane. In mathematics, especially in geometry and group theory, a lattice in Rn is a discrete subgroup of Rn which spans the real vector space Rn. Every lattice in Rn …   Wikipedia

• Lattice (mathematics) — In mathematics, the term lattice can mean:* A partially ordered set (poset) in which any two elements have a supremum and an infimum mdash;see lattice (order). The Hasse diagrams of these posets look (in some simple cases) like the lattices of… …   Wikipedia

• Discrete group — Concepts in group theory category of groups subgroups, normal subgroups group homomorphisms, kernel, image, quotient direct product, direct sum semidirect product, wreath product …   Wikipedia

• Lattice group — In mathematics, the term lattice group is used for two distinct notions:* a lattice, a discrete subgroup of R n and its generalizations.* a lattice ordered group, a group that with a partial ordering that is a lattice order …   Wikipedia

• Discrete series representation — In mathematics, a discrete series representation is an irreducible unitary representation of a locally compact topological group G that is a subrepresentation of the left regular representation of G on L²(G). In the Plancherel measure, such… …   Wikipedia

• Subgroup — This article is about the mathematical concept For the galaxy related concept, see Galaxy subgroup. Concepts in group theory category of groups subgroups, normal subgroups group homomorphisms, kernel, image, quotient direct product, direct sum …   Wikipedia

• E₈ lattice — In mathematics, the E8 lattice is a special lattice in R8. It can be characterized as the unique positive definite, even, unimodular lattice of rank 8. The name derives from the fact that it is the root lattice of the E8 root system. The normIn… …   Wikipedia

• Normal subgroup — Concepts in group theory category of groups subgroups, normal subgroups group homomorphisms, kernel, image, quotient direct product, direct sum semidirect product, wreath product …   Wikipedia

• Reciprocal lattice — In physics, the reciprocal lattice of a lattice (usually a Bravais lattice) is the lattice in which the Fourier transform of the spatial function of the original lattice (or direct lattice) is represented. This space is also known as momentum… …   Wikipedia