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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Ee will prove that V and W are isomorphic. When the field is not algebraically closed, the case where the endomorphism ring is as small as possible is still of particular interest. 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.
The lemma is named after Issai Schur who used it to prove Scuur orthogonality relations and develop the basics of the representation theory of finite groups. Such modules are necessarily indecomposable, and so cannot exist over semi-simple rings such as the complex group ring of a finite echur. For other uses, see Schur’s lemma disambiguation. A simple module over k -algebra is said to be absolutely simple if its endomorphism ring is isomorphic to k.
By using this site, you agree to the Terms of Use and Privacy Policy. Retrieved from ” https: In general, Schur’s lemma cannot be reversed: 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: This page was last edited on 17 Augustat Representation theory is the study of homomorphisms from a group, Ginto the general linear group GL V of a vector space V ; i. When W has this property, we call W with the given dchur a subrepresentation of V.
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Schur’s lemma admits generalisations to Lie groups and Lie algebrasthe most common of which is due to Jacques Dixmier. In other words, the only linear transformations of M that commute with all transformations coming from R are scalar multiples of the identity. sdhur
Suppose f is a nonzero G -linear map from V to W. As a simple corollary of the second statement is that every complex irreducible representation of an Abelian group is one-dimensional. A module is said to be strongly indecomposable if its endomorphism ring is a local ring.
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Views Read Edit View history. Then Schur’s lemma says that the endomorphism ring of the module M is a division algebra over the field k. If M is finite-dimensional, this division algebra is finite-dimensional. We now describe Schur’s lemma as it is usually stated in the context of representations of Lie groups and Lie algebras. It is easy to check that this is a subspace.
A representation of G with no subrepresentations other than itself and zero is an irreducible representation. In other words, we require that f commutes with the action of G. G -linear maps are the morphisms in the category of representations of G.
From Wikipedia, the free encyclopedia. They express relations between the module-theoretic properties of M and the properties of the endomorphism ring of M.
Schur’s lemma – Wikipedia
Irreducible representations, like the prime numbers, or like the simple groups in group theory, are the building blocks of representation theory. We say W is stable under Gor stable under the schkr of G.
Even for group rings, there are examples when the characteristic of the field divides the order of the group: Schur’s lemma is frequently applied in the following particular case. If k is the field of complex numbers, the only option is that this division algebra is the complex numbers.
By assumption it is not zero, so it is surjective, in which case it is an isomorphism. 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. This is in general stronger than being irreducible over the field kand implies the module is irreducible even over the algebraic closure of k.
In mathematicsSchur’s lemma [1] is an elementary but extremely useful statement in representation theory of groups and algebras. As we are interested in homomorphisms between groups, or continuous maps between topological spaces, we are interested in certain functions between representations of G. However, even over the ring of integersthe module of rational numbers has an endomorphism ring that is a division ring, specifically the field of rational numbers.
If M and N are two simple modules over a ring Rthen any homomorphism f: The one module version of Schur’s lemma admits generalizations involving modules M that are not necessarily simple. 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.
Many of the initial questions and theorems of representation theory deal with the properties of irreducible representations. There are three parts to the result. Such a homomorphism is called a representation of G on V.