Minimal ideal

In the branch of abstract algebra known as ring theory, a minimal right ideal of a ring ''R'' is a nonzero right ideal which contains no other nonzero right ideal. Likewise, a minimal left ideal is a nonzero left ideal of ''R'' containing no other nonzero left ideals of ''R'', and a minimal ideal of ''R'' is a nonzero ideal containing no other nonzero two-sided ideal of ''R'' .

In other words, minimal right ideals are minimal elements of the poset of nonzero right ideals of ''R'' ordered by inclusion. The reader is cautioned that outside of this context, some posets of ideals may admit the zero ideal, and so the zero ideal could potentially be a minimal element in that poset. This is the case for the poset of prime ideals of a ring, which may include the zero ideal as a minimal prime ideal.

Definition

The definition of a minimal right ideal ''N'' of a ring ''R'' is equivalent to the following conditions:
*''N'' is nonzero and if ''K'' is a right ideal of ''R'' with , then either or .
*''N'' is a simple right ''R''-module.

Minimal right ideals are the dual notion to maximal right ideals.

Properties

Many standard facts on minimal ideals can be found in standard texts such as , , , and .

* In a ring with unity, maximal right ideals always exist. In contrast, minimal right, left, or two-sided ideals in a ring with unity need not exist.
* The right socle of a ring $\mathrm(R_R)$ is an important structure defined in terms of the minimal right ideals of ''R''.
* Rings for which every right ideal contains a minimal right ideal are exactly the rings with an essential right socle.
* Any right Artinian ring or right Kasch ring has a minimal right ideal.
* Domains that are not division rings have no minimal right ideals.
* In rings with unity, minimal right ideals are necessarily principal right ideals, because for any nonzero ''x'' in a minimal right ideal ''N'', the set ''xR'' is a nonzero right ideal of ''R'' inside ''N'', and so .
* Brauer's lemma: Any minimal right ideal ''N'' in a ring ''R'' satisfies or for some idempotent element ''e'' of ''R'' .
* If ''N''₁ and ''N''₂ are nonisomorphic minimal right ideals of ''R'', then the product equals .
* If ''N''₁ and ''N''₂ are distinct minimal ideals of a ring ''R'', then
* A simple ring with a minimal right ideal is a semisimple ring.
* In a semiprime ring, there exists a minimal right ideal if and only if there exists a minimal left ideal .

Generalization

A nonzero submodule ''N'' of a right module ''M'' is called a minimal submodule if it contains no other nonzero submodules of ''M''. Equivalently, ''N'' is a nonzero submodule of ''M'' which is a simple module. This can also be extended to bimodules by calling a nonzero sub-bimodule ''N'' a minimal sub-bimodule of ''M'' if ''N'' contains no other nonzero sub-bimodules.

If the module ''M'' is taken to be the right ''R''-module ''R''_''R'', then clearly the minimal submodules are exactly the minimal right ideals of ''R''. Likewise, the minimal left ideals of ''R'' are precisely the minimal submodules of the left module _''R''''R''. In the case of two-sided ideals, we see that the minimal ideals of ''R'' are exactly the minimal sub-bimodules of the bimodule _''R''''R''_''R''.

Just as with rings, there is no guarantee that minimal submodules exist in a module. Minimal submodules can be used to define the socle of a module.

References

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*{{citation , last=Lam , first=T. Y. , title=A first course in noncommutative rings , series=Graduate Texts in Mathematics , volume=131 , edition=2 , publisher=Springer-Verlag , place=New York , year=2001 , pages=xx+385 , isbn=0-387-95183-0 , mr=1838439

External links

* http://www.encyclopediaofmath.org/index.php/Minimal_ideal

Abstract algebra
Ring theory
Ideals (ring theory)