In
abstract algebra
In mathematics, more specifically algebra, abstract algebra or modern algebra is the study of algebraic structures. Algebraic structures include groups, rings, fields, modules, vector spaces, lattices, and algebras over a field. The ter ...
, a branch of
mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, a simple ring is a
non-zero ring that has no two-sided
ideal
Ideal may refer to:
Philosophy
* Ideal (ethics), values that one actively pursues as goals
* Platonic ideal, a philosophical idea of trueness of form, associated with Plato
Mathematics
* Ideal (ring theory), special subsets of a ring considered ...
besides the
zero ideal and itself. In particular, a
commutative ring
In mathematics, a commutative ring is a ring in which the multiplication operation is commutative. The study of commutative rings is called commutative algebra. Complementarily, noncommutative algebra is the study of ring properties that are not ...
is a simple ring if and only if it is a
field.
The
center of a simple ring is necessarily a field. It follows that a simple ring is an
associative algebra over this field. So, simple algebra and ''simple ring'' are synonyms.
Several references (e.g., Lang (2002) or Bourbaki (2012)) require in addition that a simple ring be left or right
Artinian (or equivalently
semi-simple). Under such terminology a non-zero ring with no non-trivial two-sided ideals is called quasi-simple.
Rings which are simple as rings but are not a
simple module over themselves do exist: a full
matrix ring
In abstract algebra, a matrix ring is a set of matrices with entries in a ring ''R'' that form a ring under matrix addition and matrix multiplication . The set of all matrices with entries in ''R'' is a matrix ring denoted M''n''(''R'')Lang, ...
over a field does not have any nontrivial ideals (since any ideal of
is of the form
with
an ideal of
), but has nontrivial left ideals (for example, the sets of matrices which have some fixed zero columns).
According to the
Artin–Wedderburn theorem, every simple ring that is left or right
Artinian is a
matrix ring
In abstract algebra, a matrix ring is a set of matrices with entries in a ring ''R'' that form a ring under matrix addition and matrix multiplication . The set of all matrices with entries in ''R'' is a matrix ring denoted M''n''(''R'')Lang, ...
over a
division ring. In particular, the only simple rings that are a
finite-dimensional vector space over the
real number
In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...
s are rings of matrices over either the real numbers, the
complex number
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the fo ...
s, or the
quaternion
In mathematics, the quaternion number system extends the complex numbers. Quaternions were first described by the Irish mathematician William Rowan Hamilton in 1843 and applied to mechanics in three-dimensional space. Hamilton defined a quater ...
s.
An example of a simple ring that is not a matrix ring over a division ring is the
Weyl algebra.
Characterization
A
ring is a simple algebra if it contains no non-trivial two-sided
ideal
Ideal may refer to:
Philosophy
* Ideal (ethics), values that one actively pursues as goals
* Platonic ideal, a philosophical idea of trueness of form, associated with Plato
Mathematics
* Ideal (ring theory), special subsets of a ring considered ...
s.
An immediate example of simple algebras are
division algebras, where every nonzero element has a multiplicative inverse, for instance, the real algebra of
quaternions
In mathematics, the quaternion number system extends the complex numbers. Quaternions were first described by the Irish mathematician William Rowan Hamilton in 1843 and applied to mechanics in three-dimensional space. Hamilton defined a quater ...
. Also, one can show that the algebra of
matrices with entries in a
division ring is simple. In fact, this characterizes all finite-dimensional simple algebras up to
isomorphism
In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word i ...
, i.e. any simple algebra that is finite dimensional over its center is isomorphic to a
matrix algebra
In abstract algebra, a matrix ring is a set of matrices with entries in a ring ''R'' that form a ring under matrix addition and matrix multiplication . The set of all matrices with entries in ''R'' is a matrix ring denoted M''n''(''R'')Lang, '' ...
over some division ring. This was proved in 1907 by
Joseph Wedderburn in his doctoral thesis, ''On hypercomplex numbers'', which appeared in the
Proceedings of the London Mathematical Society. Wedderburn's thesis classified simple and
semisimple algebras. Simple algebras are building blocks of semi-simple algebras: any finite-dimensional semi-simple algebra is a Cartesian product, in the sense of algebras, of simple algebras.
Wedderburn's result was later generalized to
semisimple rings in the
Artin–Wedderburn theorem.
Examples
* A
central simple algebra
In ring theory and related areas of mathematics a central simple algebra (CSA) over a field ''K'' is a finite-dimensional associative ''K''-algebra ''A'' which is simple, and for which the center is exactly ''K''. (Note that ''not'' every simpl ...
(sometimes called Brauer algebra) is a simple finite-dimensional algebra over a
field whose
center is
.
Let
be the field of real numbers,
be the field of complex numbers, and
the
quaternion
In mathematics, the quaternion number system extends the complex numbers. Quaternions were first described by the Irish mathematician William Rowan Hamilton in 1843 and applied to mechanics in three-dimensional space. Hamilton defined a quater ...
s.
* Every finite-dimensional
simple algebra over
is isomorphic to a matrix ring over
,
, or
. Every
central simple algebra
In ring theory and related areas of mathematics a central simple algebra (CSA) over a field ''K'' is a finite-dimensional associative ''K''-algebra ''A'' which is simple, and for which the center is exactly ''K''. (Note that ''not'' every simpl ...
over
is isomorphic to a matrix ring over
or
. These results follow from the
Frobenius theorem.
* Every finite-dimensional simple algebra over
is a central simple algebra, and is isomorphic to a matrix ring over
.
* Every finite-dimensional central simple algebra over a
finite field
In mathematics, a finite field or Galois field (so-named in honor of Évariste Galois) is a field that contains a finite number of elements. As with any field, a finite field is a set on which the operations of multiplication, addition, subtr ...
is isomorphic to a matrix ring over that field.
* For a
commutative ring
In mathematics, a commutative ring is a ring in which the multiplication operation is commutative. The study of commutative rings is called commutative algebra. Complementarily, noncommutative algebra is the study of ring properties that are not ...
, the four following properties are equivalent: being a
semisimple ring; being
Artinian and
reduced; being a
reduced Noetherian ring of
Krull dimension 0; and being isomorphic to a finite direct product of fields.
Wedderburn's theorem
Wedderburn's theorem characterizes simple rings with a unit and a minimal left ideal. (The left Artinian condition is a generalization of the second assumption.) Namely it says that every such ring is, up to
isomorphism
In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word i ...
, a ring of
matrices over a division ring.
Let
be a division ring and
be the ring of matrices with entries in
. It is not hard to show that every left ideal in
takes the following form:
:
,
for some fixed subset
. So a minimal ideal in
is of the form
:
,
for a given
. In other words, if
is a minimal left ideal, then
, where
is the
idempotent matrix with 1 in the
entry and zero elsewhere. Also,
is isomorphic to
. The left ideal ''
'' can be viewed as a right module over
, and the ring
is clearly isomorphic to the algebra of
homomorphisms
In algebra, a homomorphism is a structure-preserving map between two algebraic structures of the same type (such as two groups, two rings, or two vector spaces). The word ''homomorphism'' comes from the Ancient Greek language: () meaning "same ...
on this module.
The above example suggests the following lemma:
Lemma. is a ring with identity and an idempotent element '''', where . Let '''' be the left ideal , considered as a right module over . Then '''' is isomorphic to the algebra of homomorphisms on '''', denoted by .
Proof: We define the "left regular representation" by for . Then is injective because if , then , which implies that .
For surjectivity, let . Since , the unit can be expressed as . So
:.
Since the expression does not depend on , is surjective. This proves the lemma.
Wedderburn's theorem follows readily from the lemma.
Theorem (Wedderburn). If '''' is a simple ring with unit and a minimal left ideal '''', then '''' is isomorphic to the ring of matrices over a division ring.
One simply has to verify the assumptions of the lemma hold, i.e. find an idempotent ''
'' such that
, and then show that
is a division ring. The assumption
follows from
being simple.
See also
*
Simple (algebra)
*
Simple universal algebra
References
*
A. A. Albert, ''Structure of algebras'', Colloquium publications 24,
American Mathematical Society
The American Mathematical Society (AMS) is an association of professional mathematicians dedicated to the interests of mathematical research and scholarship, and serves the national and international community through its publications, meeting ...
, 2003, . P.37.
*
*
*
*
* {{Citation , last1=Jacobson , first1=Nathan , author1-link=Nathan Jacobson , title=Basic algebra II , publisher=W. H. Freeman , edition=2nd , isbn=978-0-7167-1933-5 , year=1989
Ring theory