Langlands group

Langlands group

In representation theory, a branch of mathematics, the Langlands (dual) group "L""G" (also called L-group) is a group associated to a reductive group "G" over a field "k" that controls the representation theory of "G". It is an extension of the absolute Galois group of "k" by a complex Lie group. There is also a variation called the Weil form of the Langlands group, where the Galois group is replaced by a Weil group. The Langlands group is also often referred to as an "L-group"; here the letter "L" indicates also the connection with the theory of L-functions, particularly the "automorphic" L-functions.

The Langlands group is used heavily in the Langlands conjectures of Robert Langlands. It is used to make precise statements from ideas that automorphic forms are in a sense functorial in the group "G", when "k" is a global field. It is not exactly "G" with respect to which automorphic forms and representations are functorial, but "L""G". This makes sense of numerous phenomena, such as 'lifting' of forms from one group to another larger one, and the general fact that certain groups that become isomorphic after field extensions have related automorphic representations.

Definition for separably closed fields

From a reductive algebraic group over a separably closed field "K" we can construct its root datum ("X"*, Δ,"X"*, Δv), where "X"* is the lattice of characters of a maximal torus, "X"* the dual lattice (given by the 1-parameter subgroups), Δ the roots, and Δv the coroots. A connected reductive algebraic group over "K" is uniquely determined (up to isomorphism) by its root datum. A root datum contains slightly more information than the Dynkin diagram, because it also determines the center of the group.

For any root datum ("X"*, Δ,"X"*, Δv), we can define a dual root datum ("X"*, Δv,"X"*, Δ) by switching the characters with the 1-parameter subgroups, and switching the roots with the coroots.

If "G" is a connected reductive algebraic group over the algebraically closed field "K", then its Langlands dual group "L""G" is the complex connected reductive group whose root datum is dual to that of "G".

Examples:The Langlands dual group "L""G" has the same Dynkin diagram as "G", except that components of type "B""n" are changed to components of type "C""n" and vice versa. If "G" has trivial center then "L""G" is simply connected, and if "G" is simply connected then "L""G" has trivial center. The Langlands dual of "GL""n"("K") is "GL""n"(C).

Definition for groups over more general fields

Now suppose that "G" is a reductive group over some field "k" with separable closure "K". Over "K", "G" has a root datum, and this comes with an action of the Galois group "Gal"("K"/"k"). The identity component L"G"o of the Langlands group is the connected complex reductive group of the dual root datum; this has an induced action of the Galois group "Gal"("K"/"k"). The full Langlands group L"G" is the semidirect product:L"G" = L"G"o×"Gal"("K"/"k")of the connected component with the Galois group.

There are some variations of the definition of the Langlands group, as follows:
*Instead of using the full Galois group "Gal"("K"/"k") of the separable closure, one can just use the Galois group of a finite extension over which "G" is split (or even just quasisplit). The corresponding semidirect product then has only a finite number of components and is a complex Lie group.
*Suppos that "k" is a local, global, or finite field. Instead of using the absolute Galois group of "k", one can use the absolute Weil group, which has a natural map to the Galois group and therefore also acts on the root datum. The corresponding semidirect product is called the Weil form of the Langlands group.
*For algebraic groups "G" over finite fields, Deligne and Lusztig introduced a different dual group. As before, "G" gives a root datum with an action of the absolute Galois group of the finite field. The dual group "G"* is then the reductive algebraic group over the finite field associated to the dual root datum with the induced action of the Galois group. (This dual group is defined over a finite field, while the component of the Langlands dual group is defined over the complex numbers.)

Applications

The Langlands conjectures imply, very roughly, that if "G" is a reductive algebric group over a local or global field, then there is a correspondence between "good" representations of "G" and homomorphisms of a Galois group (or Weil group) into the Langlands dual group of "G". A more general formulation of the conjectures is Langlands functoriality, which says (roughly) that given a (well behaved) homomorphism between Langlands dual groups, there should be an induced map between "good" representations of the corresponding groups.

To make this theory explicit, there must be defined the concept of "L"-homomorphism of an "L"-group into another. That is, "L"-groups must be made into a category, so that 'functoriality' has meaning. The definition on the complex Lie groups is as expected, but "L"-homomorphisms must be 'over' the Weil group.

References

*A. Borel, [http://www.ams.org/online_bks/pspum332/pspum332-ptIII-2.pdf "Automorphic L-functions"] , in [http://www.ams.org/online_bks/pspum332/ "Automorphic forms, representations, and L-functions"] , ISBN 0-8218-1437-0
*R. Langlands, [http://www.sunsite.ubc.ca/DigitalMathArchive/Langlands/functoriality.html#weil1967 1967 letter to A. Weil] introducing the Langlands group
*I. Mirkovic and K. Vilonen, [http://arxiv.org/abs/math.RT/0401222 Geometric Langlands duality and representations of algebraic groups over commutative rings] , describes the dual group of "G" in terms of the geometry of the affine Grassmannian of "G".


Wikimedia Foundation. 2010.

Игры ⚽ Нужен реферат?

Look at other dictionaries:

  • Langlands program — The Langlands program is a web of far reaching and influential conjectures that connect number theory and the representation theory of certain groups. It was proposed by Robert Langlands beginning in 1967. Connection with number theory The… …   Wikipedia

  • Langlands — The name Langlands can refer to one of several individuals or groups:* Alan Langlands, principal and vice chancellor of the University of Dundee * Graeme Langlands, Australian rugby league former player and coach * Langlands and Bell, English… …   Wikipedia

  • Langlands & Bell — Langlands Bell, are two fine artists who work collaboratively as a duo; the two, Ben Langlands (born London 1955) and Nikki Bell (born London 1959), began collaborating in 1978, while studying Fine Art at Middlesex Polytechnic in North London,… …   Wikipedia

  • Langlands classification — In mathematics, the Langlands classification is a classification of irreducible representations of a reductive Lie group G , suggested by Robert Langlands (1973). More precisely, it classifies the irreducible admissible ( g , K ) modules,for g a… …   Wikipedia

  • Langlands–Deligne local constant — In mathematics, the Langlands–Deligne local constant (or local Artin root number un to an elementary function of s) is an elementary function associated with a representation of the Weil group of a local field. The functional equation L(ρ,s) =… …   Wikipedia

  • Langlands decomposition — In mathematics, the Langlands decomposition writes a parabolic subgroup P of a semisimple Lie group as a product P=MAN of a reductive subgroup M , an abelian subgroup A , and a nilpotent subgroup N .ee alsoLie group decompositionsReferences* A. W …   Wikipedia

  • Robert Langlands — Robert Phelan Langlands (born October 6, 1936 in New Westminster, British Columbia, Canada) was one of the most influential mathematicians of the 20th century,and remains influential in the 21st.His work in automorphic forms and representation… …   Wikipedia

  • Programme de Langlands — Pour les articles homonymes, voir Langlands. En mathématiques, le programme de Langlands est encore, au début du XXIe siècle, un domaine de recherche actif et fertile en conjectures. Ce programme souhaite relier la théorie des nombres aux… …   Wikipédia en Français

  • Alan Langlands — Sir Robert Alan Langlands FRSE FRCP (Hon.) FRCGP (Hon.) FRCS (Edin.) (Hon.) FRCPSG (Hon.) FFPH FIA is principal and vice chancellor of the University of Dundee and a former chief executive of the National Health Service executive in England.Early …   Wikipedia

  • Classical group — For the book by Weyl, see The Classical Groups. Lie groups …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”