In
mathematics, an arithmetic group is a group obtained as the integer points of an
algebraic group
In mathematics, an algebraic group is an algebraic variety endowed with a group structure which is compatible with its structure as an algebraic variety. Thus the study of algebraic groups belongs both to algebraic geometry and group theory.
...
, for example
They arise naturally in the study of arithmetic properties of
quadratic form
In mathematics, a quadratic form is a polynomial with terms all of degree two (" form" is another name for a homogeneous polynomial). For example,
:4x^2 + 2xy - 3y^2
is a quadratic form in the variables and . The coefficients usually belong t ...
s and other classical topics in
number theory. They also give rise to very interesting examples of
Riemannian manifolds and hence are objects of interest in
differential geometry and
topology. Finally, these two topics join in the theory of
automorphic form
In harmonic analysis and number theory, an automorphic form is a well-behaved function from a topological group ''G'' to the complex numbers (or complex vector space) which is invariant under the action of a discrete subgroup \Gamma \subset G of ...
s which is fundamental in modern number theory.
History
One of the origins of the mathematical theory of arithmetic groups is algebraic number theory. The classical reduction theory of quadratic and Hermitian forms by
Charles Hermite
Charles Hermite () FRS FRSE MIAS (24 December 1822 – 14 January 1901) was a French mathematician who did research concerning number theory, quadratic forms, invariant theory, orthogonal polynomials, elliptic functions, and algebra.
Hermite p ...
,
Hermann Minkowski and others can be seen as computing
fundamental domains for the action of certain arithmetic groups on the relevant
symmetric spaces. The topic was related to Minkowski's
geometry of numbers and the early development of the study of arithmetic invariant of number fields such as the
discriminant. Arithmetic groups can be thought of as a vast generalisation of the
unit groups of number fields to a noncommutative setting.
The same groups also appeared in analytic number theory as the study of classical modular forms and their generalisations developed. Of course the two topics were related, as can be seen for example in Langlands' computation of the volume of certain fundamental domains using analytic methods. This classical theory culminated with the work of Siegel, who showed the finiteness of the volume of a fundamental domain in many cases.
For the modern theory to begin foundational work was needed, and was provided by the work of
Armand Borel
Armand Borel (21 May 1923 – 11 August 2003) was a Swiss mathematician, born in La Chaux-de-Fonds, and was a permanent professor at the Institute for Advanced Study in Princeton, New Jersey, United States from 1957 to 1993. He worked in ...
,
André Weil
André Weil (; ; 6 May 1906 – 6 August 1998) was a French mathematician, known for his foundational work in number theory and algebraic geometry. He was a founding member and the ''de facto'' early leader of the mathematical Bourbaki group. Th ...
,
Jacques Tits and others on algebraic groups. Shortly afterwards the finiteness of covolume was proven in full generality by Borel and Harish-Chandra. Meanwhile, there was progress on the general theory of lattices in Lie groups by
Atle Selberg
Atle Selberg (14 June 1917 – 6 August 2007) was a Norwegian mathematician known for his work in analytic number theory and the theory of automorphic forms, and in particular for bringing them into relation with spectral theory. He was awarded ...
,
Grigori Margulis,
David Kazhdan
David Kazhdan ( he, דוד קשדן), born Dmitry Aleksandrovich Kazhdan (russian: Дми́трий Александро́вич Кажда́н), is a Soviet and Israeli mathematician known for work in representation theory. Kazhdan is a 1990 Ma ...
,
M. S. Raghunathan and others. The state of the art after this period was essentially fixed in Raghunathan's treatise, published in 1972.
In the seventies Margulis revolutionised the topic by proving that in "most" cases the arithmetic constructions account for all lattices in a given Lie group. Some limited results in this direction had been obtained earlier by Selberg, but Margulis' methods (the use of
ergodic-theoretical tools for actions on homogeneous spaces) were completely new in this context and were to be extremely influential on later developments, effectively renewing the old subject of geometry of numbers and allowing Margulis himself to prove the
Oppenheim conjecture
In Diophantine approximation, the Oppenheim conjecture concerns representations of numbers by real quadratic forms in several variables. It was formulated in 1929 by Alexander Oppenheim and later the conjectured property was further strengthened ...
; stronger results (
Ratner's theorems) were later obtained by
Marina Ratner.
In another direction the classical topic of modular forms has blossomed into the modern theory of automorphic forms. The driving force behind this effort is mainly the
Langlands program initiated by
Robert Langlands. One of the main tool used there is the
trace formula originating in Selberg's work and developed in the most general setting by
James Arthur.
Finally arithmetic groups are often used to construct interesting examples of
locally symmetric
In mathematics, a symmetric space is a Riemannian manifold (or more generally, a pseudo-Riemannian manifold) whose group of symmetries contains an inversion symmetry about every point. This can be studied with the tools of Riemannian geometry, l ...
Riemannian manifolds. A particularly active research topic has been
arithmetic hyperbolic 3-manifolds, which as
William Thurston wrote, "...often seem to have special beauty."
Definition and construction
Arithmetic groups
If
is an algebraic subgroup of
for some
then we can define an arithmetic subgroup of
as the group of integer points
In general it is not so obvious how to make precise sense of the notion of "integer points" of a
-group, and the subgroup defined above can change when we take different embeddings
Thus a better notion is to take for definition of an arithmetic subgroup of
any group
which is
commensurable (this means that both
and
are finite sets) with a group
defined as above (with respect to any embedding into
). With this definition, to the algebraic group
is associated a collection of "discrete" subgroups all commensurable to each other.
Using number fields
A natural generalisation of the construction above is as follows: let
be a
number field with ring of integers
and
an algebraic group over
. If we are given an embedding
defined over
then the subgroup
can legitimately be called an arithmetic group.
On the other hand, the class of groups thus obtained is not larger than the class of arithmetic groups as defined above. Indeed, if we consider the algebraic group
over
obtained by
restricting scalars from
to
and the
-embedding
induced by
(where