Request PDF on ResearchGate | Le lemme de Schur pour les représentations orthogonales | Let σ be an orthogonal representation of a group G on a real. Statement no. Condition, Conclusion in abstract formulation for vector spaces: \ rho_1: G \to GL(V_1), \rho_2: G \ are linear representations of G. Ensuite nous démontrons un lemme (le théorème II) qui est fondamental pour pour la convexité S en généralisant et précisant quelques résultats de Schur.

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The lemma is lemne after Issai Schur who used it to prove Schur orthogonality schu and develop the basics of the representation theory of finite groups. A simple module over k -algebra is said to be absolutely simple if its endomorphism ring is isomorphic to k. If M and N are two simple modules over a ring Rthen any homomorphism f: A representation of G with no subrepresentations other than itself and zero is an irreducible representation.

The group version is a special case of the module version, since any representation of a group G can equivalently be viewed as a module over the group ring of G.

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As we are interested in homomorphisms between groups, or continuous maps between topological spaces, we are interested in certain functions between representations of G. By using this site, you agree to the Terms of Use and Privacy Policy. Representation theory is the study of homomorphisms from a group, Ginto the general linear group GL V of a vector space V ; i.

As a simple corollary of the second statement is that every complex irreducible representation of an Abelian group is one-dimensional. They express relations achur the module-theoretic properties of M and the properties of the endomorphism ring of M. Retrieved from ” https: Suppose f is a nonzero G -linear map from V to W.

### Schur’s lemma – Wikidata

Many of the initial questions and theorems of representation theory deal with the properties of irreducible representations.

A representation on V is a special case of a group action on Vbut rather than permit any arbitrary permutations of the underlying set of Vwe restrict ourselves to invertible linear transformations. We now describe Schur’s lemma as it is usually stated in the context of representations of Lie groups and Lie algebras. When the field is not algebraically closed, the case where the endomorphism ring is as small as possible is scnur of particular interest. We say W is stable under Gor stable under the action of G.

If M is finite-dimensional, this division algebra is finite-dimensional.

This holds more generally for any algebra R over an uncountable algebraically closed field k and for any simple module M that is at most countably-dimensional: For other uses, see Schur’s lemma disambiguation.

In mathematicsSchur’s lemma [1] is an elementary dr extremely useful statement in representation theory of groups and algebras. Views Read Edit View history. This page was last edited on 17 Augustat G -linear maps are the morphisms in the category of representations of G.

The one module version of Schur’s lemma admits generalizations involving modules M that are not necessarily simple.

From Wikipedia, the free encyclopedia. Such a homomorphism is called a representation of G on V. This is in general stronger than lemem irreducible over the field kand implies the module is irreducible even over the algebraic closure of k. A module is said to be strongly indecomposable if its endomorphism ring is a local ring.

If k is the field of complex numbers, the only option is that this division algebra is the complex numbers. Irreducible representations, like the prime numbers, or like the simple groups in group theory, are the le,me blocks of representation theory.

Such modules are necessarily indecomposable, and so cannot exist over semi-simple rings such as the complex group ring of a finite group. Thus the endomorphism ring of the module M is “as small as possible”.

Schur’s Lemma is a theorem that describes what G -linear maps can exist between two irreducible representations of G. In general, Schur’s lemma cannot be reversed: In other words, the only linear transformations of M that commute with all transformations coming from R are scalar multiples of the identity.

We will prove that V and W are isomorphic. Schur’s lemma admits generalisations to Lie groups and Lie algebrasthe most common of which is due to Jacques Dixmier. In other words, we require that f commutes with the action of G.

## Le lemme de Schur pour les représentations orthogonales.

It is easy to check that this is a subspace. Schur’s lemma is frequently applied in the following particular case.

However, even over the ring of integersthe module of rational numbers has an endomorphism ring that is leemme division ring, specifically the field of rational numbers.

Even for group rings, there are examples when the characteristic of the field divides the order of the group: Then Schur’s lemma says that the endomorphism ring of the module M is a division algebra over the field k.

### Schur’s lemma – Wikipedia

When W has this property, we call W with the given representation a subrepresentation of V. There are three parts to the result. By assumption it is not zero, so it is surjective, in which case it is an isomorphism.