Jacobson radical

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In ring theory, a branch of abstract algebra, the Jacobson radical of a ring R is an ideal of R which contains those elements of R which in a sense are "close to zero".

1 Definition
2 Examples
3 Properties
4 See also
5 References

[edit] Definition

The Jacobson radical is denoted by J(R) and can be defined in the following equivalent ways:

the intersection of all maximal left ideals.
the intersection of all maximal right ideals.
the intersection of all annihilators of simple left R-modules
the intersection of all annihilators of simple right R-modules
the intersection of all left primitive ideals.
the intersection of all right primitive ideals.
{ x ∈ R : for every r ∈ R there exists u ∈ R with u (1-rx) = 1 }
{ x ∈ R : for every r ∈ R there exists u ∈ R with (1-xr) u = 1 }
if R is commutative, the intersection of all maximal ideals in R.
the largest ideal I such that for all x ∈ I, 1-x is invertible in R

Note that the last property does not mean that every element x of R such that 1-x is invertible must be an element of J(R). Also, if R is not commutative, then J(R) is not necessarily equal to the intersection of all two-sided maximal ideals in R.

The Jacobson radical is named for Nathan Jacobson, who first studied the Jacobson radical.

[edit] Examples

The Jacobson radical of any field is {0}. The Jacobson radical of the integers is {0}.
The Jacobson radical of the ring Z/8Z (see modular arithmetic) is 2Z/8Z.
If K is a field and R is the ring of all upper triangular n-by-n matrices with entries in K, then J(R) consists of all upper triangular matrices with zeros on the main diagonal.
If K is a field and R = K[[X₁,...,X_n]] is a ring of formal power series, then J(R) consists of those power series whose constant term is zero. More generally: the Jacobson radical of every local ring consists precisely of the ring's non-units.
Start with a finite quiver Γ and a field K and consider the quiver algebra KΓ (as described in the quiver article). The Jacobson radical of this ring is generated by all the paths in Γ of length ≥ 1.
The Jacobson radical of a C*-algebra is {0}. This follows from the Gelfand–Naimark theorem and the fact for a C*-algebra, a topologically irreducible *-representation on a Hilbert space is algebraically irreducible, so that its kernel is a primitive ideal in the purely algebraic sense (see spectrum of a C*-algebra).

[edit] Properties

Unless R is the trivial ring {0}, the Jacobson radical is always an ideal in R distinct from R.

If R is commutative and finitely generated, then J(R) is equal to the nilradical of R.

The Jacobson radical of the ring R/J(R) is zero. Rings with zero Jacobson radical are called semiprimitive rings.

If f : R → S is a surjective ring homomorphism, then f(J(R)) ⊆ J(S).

If M is a finitely generated left R-module with J(R)M = M, then M = 0 (Nakayama lemma).

J(R) contains every nil ideal of R. If R is left or right artinian, then J(R) is a nilpotent ideal. Note however that in general the Jacobson radical need not contain every nilpotent element of the ring.

R is a semisimple ring if and only if it is Artinian and its Jacobson radical is zero.

[edit] See also

[edit] References

M.F. Atiyah, I.G. Macdonald. Introduction to Commutative Algebra.
N. Bourbaki. Éléments de Mathématique.
R.S. Pierce. Associative Algebras. Graduate Texts in Mathematics vol 88.
T.Y. Lam. A First Course in Non-commutative Rings. Graduate Texts in Mathematics vol 131.

This article incorporates material from Jacobson radical on PlanetMath, which is licensed under the GFDL.

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