 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.
Contents
Definition
A module over a (not necessarily commutative) ring with unity is said to be semisimple (or completely reducible) if it is the direct sum of simple (irreducible) submodules.
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.^{[1]}
Semisimple is stronger than completely decomposable, which is a direct sum of indecomposable submodules.
Properties
 If M is semisimple and N is a submodule, then N and M/N are also semisimple.
 If each M_{i} 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.
Endomorphism rings
 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).
Semisimple rings
A ring is said to be (left)semisimple if it is semisimple as a left module over itself. Surprisingly, a leftsemisimple ring is also rightsemisimple 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 Rmodules would automatically be semisimple. Furthermore, every simple (left) Rmodule is isomorphic to a minimal left ideal of R.
Semisimple rings are both Artinian and Noetherian. From the above properties, a ring is semisimple if and only if it is Artinian and its radical is zero.
If an Artinian semisimple ring contains a field, it is called a semisimple algebra.
Examples
 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 D_{i} is a division ring and M_{n}(D) is the ring of nbyn matrices with entries in D.
 An example of a semisimple nonunital ring is , the rowfinite, columnfinite, infinite matrices over a field K.
Simple rings
Main article: simple ringOne 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>/(xyyx1) 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.
Jacobson semisimple
Main article: semiprimitive ringA ring is called Jacobson semisimple (or Jsemisimple 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 Jsemisimple 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 Jsemisimple, but not artinian semisimple.
See also
References
Notes
 ^ Nathan Jacobson, Basic Algebra II (Second Edition), p.120
Textbooks
 Jacobson, Nathan (1989), Basic algebra II (2nd ed.), W. H. Freeman, ISBN 9780716719335
 Lam, TsitYuen (2001), A First Course in Noncommutative Rings (2nd ed.), Berlin, New York: SpringerVerlag, ISBN 9780387953250, MR1838439
 R.S. Pierce. Associative Algebras. Graduate Texts in Mathematics vol 88.
Categories: Module theory
 Ring theory
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