- Semisimple module
In mathematics, especially in the area of abstract algebra known as module theory, a semisimple module or completely reducible module is a type of module that can be understood easily from its parts. A ring which is a semisimple module over itself is known as an artinian semisimple ring. Some important rings, such as group rings of finite groups over fields of characteristic zero, are semisimple rings. An artinian ring is initially understood via its largest semisimple quotient. The structure of artinian semisimple rings is well understood by the Artin–Wedderburn theorem, which exhibits these rings as finite direct products of matrix rings.
For a module M, the following are equivalent:
- M is a direct sum of irreducible modules.
- M is the sum of its irreducible submodules.
- Every submodule of M is a direct summand: for every submodule N of M, there is a complement P such that M = N ⊕ P.
For , the starting idea is to find an irreducible submodule by picking any and letting P be a maximal submodule such that . It can be shown that the complement of P is irreducible.
- If M is semisimple and N is a submodule, then N and M/N are also semisimple.
- If each Mi is a semisimple module, then so is .
- A module M is finitely generated and semisimple if and only if it is Artinian and its radical is zero.
- A semisimple module M over a ring R can also be thought of as a ring homomorphism from R into the ring of abelian group endomorphisms of M. The image of this homomorphism is a semiprimitive ring, and every semiprimitive ring is isomorphic to such an image.
- The endomorphism ring of a semisimple module is not only semiprimitive, but also von Neumann regular, (Lam 2001, p. 62).
A ring is said to be (left)-semisimple if it is semisimple as a left module over itself. Surprisingly, a left-semisimple ring is also right-semisimple and vice versa. The left/right distinction is therefore unnecessary, and one can speak of semisimple rings without ambiguity.
Semisimple rings are of particular interest to algebraists. For example, if the base ring R is semisimple, then all R-modules would automatically be semisimple. Furthermore, every simple (left) R-module is isomorphic to a minimal left ideal of R.
If an Artinian semisimple ring contains a field, it is called a semisimple algebra.
- If k is a field and G is a finite group of order n, then the group ring k[G] is semisimple if and only if the characteristic of k does not divide n. This is Maschke's theorem, an important result in group representation theory.
- By the Artin–Wedderburn theorem, a unital ring R is semisimple if and only if it is (isomorphic to) , where each Di is a division ring and Mn(D) is the ring of n-by-n matrices with entries in D.
- An example of a semisimple non-unital ring is , the row-finite, column-finite, infinite matrices over a field K.
One should beware that despite the terminology, not all simple rings are semisimple. The problem is that the ring may be "too big", that is, not (left/right) Artinian. In fact, if R is a simple ring with a minimal left/right ideal, then R is semisimple.
Classic examples of simple, but not semisimple, rings are the Weyl algebras, such as Q<x,y>/(xy-yx-1) which is a simple noncommutative domain. These and many other nice examples are discussed in more detail in several noncommutative ring theory texts, including chapter 3 of Lam's text, in which they are described as nonartinian simple rings. The module theory for the Weyl algebras is well studied and differs significantly from that of semisimple rings.
A ring is called Jacobson semisimple (or J-semisimple or semiprimitive) if the intersection of the maximal left ideals is zero, that is, if the Jacobson radical is zero. Every ring which is semisimple as a module over itself has zero Jacobson radical, but not every ring with zero Jacobson radical is semisimple as a module over itself. A J-semisimple ring is semisimple if and only if it is an artinian ring, so semisimple rings are often called artinian semisimple rings to avoid confusion.
For example the ring of integers, Z, is J-semisimple, but not artinian semisimple.
- ^ Nathan Jacobson, Basic Algebra II (Second Edition), p.120
- Jacobson, Nathan (1989), Basic algebra II (2nd ed.), W. H. Freeman, ISBN 978-0-7167-1933-5
- Lam, Tsit-Yuen (2001), A First Course in Noncommutative Rings (2nd ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-387-95325-0, MR1838439
- R.S. Pierce. Associative Algebras. Graduate Texts in Mathematics vol 88.
Wikimedia Foundation. 2010.
См. также в других словарях:
Semisimple — This article is about mathematical use. For the philosophical reduction thinking, see Reduction (philosophy). In mathematics, the term semisimple (sometimes completely reducible) is used in a number of related ways, within different subjects. The … Wikipedia
Module (mathematics) — For other uses, see Module (disambiguation). In abstract algebra, the concept of a module over a ring is a generalization of the notion of vector space, wherein the corresponding scalars are allowed to lie in an arbitrary ring. Modules also… … Wikipedia
Finitely-generated module — In mathematics, a finitely generated module is a module that has a finite generating set. A finitely generated R module also may be called a finite R module or finite over R. Related concepts include finitely cogenerated modules, finitely… … Wikipedia
Simple module — In abstract algebra, a (left or right) module S over a ring R is called simple or irreducible if it is not the zero module 0 and if its only submodules are 0 and S . Understanding the simple modules over a ring is usually helpful because these… … Wikipedia
Glossary of semisimple groups — This is a glossary for the terminology applied in the mathematical theories of semisimple Lie groups. It also covers terms related to their Lie algebras, their representation theory, and various geometric, algebraic and combinatorial structures… … Wikipedia
Projective module — In mathematics, particularly in abstract algebra and homological algebra, the concept of projective module over a ring R is a more flexible generalisation of the idea of a free module (that is, a module with basis vectors). Various equivalent… … Wikipedia
Verma module — Verma modules, named after Daya Nand Verma, are objects in the representation theory of Lie algebras, a branch of mathematics. The definition of a Verma module looks complicated, but Verma modules are very natural objects, with useful properties … Wikipedia
Serial module — Chain ring redirects here. For the bicycle part, see Chainring. In abstract algebra, a uniserial module M is a module over a ring R, whose submodules are totally ordered by inclusion. This means simply that for any two submodules N1 and N2 of M,… … Wikipedia
Indecomposable module — In abstract algebra, a module is indecomposable if it is non zero and cannot be written as a direct sum of two non zero submodules.Indecomposable is a weaker notion than simple module:simple means no proper submodule N < M,while indecomposable… … Wikipedia
Injective module — In mathematics, especially in the area of abstract algebra known as module theory, an injective module is a module Q that shares certain desirable properties with the Z module Q of all rational numbers. Specifically, if Q is a submodule of some… … Wikipedia