Mumford–Tate group

Mumford–Tate group

In algebraic geometry, the Mumford–Tate group MT(F) constructed from a Hodge structure F is a certain algebraic group G, named for David Mumford and John Tate. When F is given by a rational representation of an algebraic torus, the definition of G is as the Zariski closure of the image in the representation of the circle group, over the rational numbers. It has been said that “Mumford–Tate groups have emerged as the principal symmetry groups in Hodge theory”.[1] They are applied to problems in number theory and physics, as well as geometry.



The algebraic torus T used to describe Hodge structures has a concrete matrix representation, as the 2×2 invertible matrices of the shape that is given by the action of a+bi on the basis {1,i} of the complex numbers C over R:

\begin{bmatrix} a & b \\ -b & a \end{bmatrix}.

The circle group inside this group of matrices is the unitary group U(1).

Hodge structures arising in geometry, for example on the cohomology groups of Kähler manifolds, have a lattice consisting of the integral cohomology classes. Not quite so much is needed for the definition of the Mumford–Tate group, but it does assume that the vector space V underlying the Hodge structure has a given rational structure, i.e. is given over the rational numbers Q. For the purposes of the theory the complex vector space VC, obtained by extending the scalars of V from Q to C, is used.

The weight k of the Hodge structure describes the action of the diagonal matrices of T, and V is supposed therefore to be homogeneous of weight k, under that action. Under the action of the full group VC breaks up into subspaces Vpq, complex conjugate in pairs under switching p and q. Thinking of the matrix in terms of the complex number λ it represents, Vpq has the action of λ by the pth power and of the complex conjugate of λ by the qth power. Here necessarily

p + q = k.

In more abstract terms, the torus T underlying the matrix group is the Weil restriction of the multiplicative group GL(1), from the complex field to the real field, an algebraic torus whose character group consists of the two homomorphisms to GL(1), interchanged by complex conjugation.

Once formulated in this fashion, the rational representation ρ of T on V setting up the Hodge structure F determines the image ρ(U(1)) in GL(VC); and MT(F) is by definition the Zariski closure, for the Q-Zariski topology on GL(V), of this image.[2]

Mumford–Tate conjecture

The original context for the formulation of the group in question was the question of the Galois representation on the Tate module of an abelian variety A. Conjecturally, the image of such a Galois representation, which is an l-adic Lie group for a given prime number l, is determined by the corresponding Mumford–Tate group G (coming from the Hodge structure on H1(A)), to the extent that knowledge of G determines the Lie algebra of the Galois image. This conjecture is known only in particular cases.[3] Through generalisations of this conjecture, the Mumford–Tate group has been connected to the motivic Galois group, and, for example, the general issue of extending the Sato–Tate conjecture (now a theorem).

Period conjecture

A related conjecture on abelian varieties states that the period matrix of A over number field has transcendence degree, in the sense of the field generated by its entries, predicted by the dimension of its Mumford–Tate group, as in the previous section. Work of Pierre Deligne has shown that the dimension bounds the transcendence degree; so that the Mumford–Tate group catches sufficiently many algebraic relations between the periods. This is a special case of the full Grothendieck period conjecture.[4][5]


External links

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • David Mumford — David Bryant Mumford (né le 11 juin 1937) est un mathématicien américain connu pour son travail en géométrie algébrique puis pour sa recherche en théorie de la vision. Il est actuellement professeur dans la division de mathématiques… …   Wikipédia en Français

  • David Mumford — in 1975 Born 11 June 1937 (1937 06 11) (age 74) …   Wikipedia

  • Motive (algebraic geometry) — For other uses, see Motive (disambiguation). In algebraic geometry, a motive (or sometimes motif, following French usage) denotes some essential part of an algebraic variety . To date, pure motives have been defined, while conjectural mixed… …   Wikipedia

  • Tannakian category — In mathematics, a tannakian category is a particular kind of monoidal category C , equipped with some extra structure relative to a given field K . The role of such categories C is to approximate, in some sense, the category of linear… …   Wikipedia

  • Arithmetic of abelian varieties — In mathematics, the arithmetic of abelian varieties is the study of the number theory of an abelian variety, or family of those. It goes back to the studies of Fermat on what are now recognised as elliptic curves; and has become a very… …   Wikipedia

  • architecture — /ahr ki tek cheuhr/, n. 1. the profession of designing buildings, open areas, communities, and other artificial constructions and environments, usually with some regard to aesthetic effect. Architecture often includes design or selection of… …   Universalium

  • Grammy Awards 2011 — Grammy Award Am 13. Februar 2011 wurden im Staples Center von Los Angeles die Grammy Awards 2011 verliehen. Es war die 53. Verleihung des Grammys, des wichtigsten US amerikanischen Musikpreises. Gewürdigt wurden Anfang 2011 die musikalischen… …   Deutsch Wikipedia

  • 53e cérémonie des Grammy Awards — Grammy Award Organisés par la National Academy of Recording Arts and Sciences Cérémonie Date 13  …   Wikipédia en Français

  • Glossary of arithmetic and Diophantine geometry — This is a glossary of arithmetic and Diophantine geometry in mathematics, areas growing out of the traditional study of Diophantine equations to encompass large parts of number theory and algebraic geometry. Much of the theory is in the form of… …   Wikipedia

  • Étale cohomology — In mathematics, the étale cohomology groups of an algebraic variety or scheme are algebraic analogues of the usual cohomology groups with finite coefficients of a topological space, introduced by Grothendieck in order to prove the Weil… …   Wikipedia