
Radical of a ring

In ring theory, a branch of mathematics, a radical of a ring is an ideal of "not-good" elements of the ring.
The first example of a radical was the nilradical introduced by , based on a suggestion of . In the next few years several other radicals were discovered, of which the most important example is the Jacobson radical. The general theory of radicals was defined independently by and .
Definitions.
In the theory of radicals, rings are usually assumed to be associative, but need not be commutative and need not have an identity element. In particular, every ideal in a ring is also a ring.
A radical class (also called radical property or just radical) is a class σ of rings possibly without identities, such that:
The study of such radicals is called torsion theory.
For any class δ of rings, there is a smallest radical class "L"δ containing it, called the lower radical of δ. The operator "L" is called the lower radical operator.
A class of rings is called regular if every non-zero ideal of a ring in the class has a non-zero image in the class. For every regular class δ of rings, there is a largest radical class "U"δ, called the upper radical of δ, having zero intersection with δ. The operator "U" is called the upper radical operator.
A class of rings is called hereditary if every ideal of a ring in the class also belongs to the class.
Examples.
The Jacobson radical.
Let "R" be any ring, not necessarily commutative. The Jacobson radical of R is the intersection of the annihilators of all simple right "R"-modules.
There are several equivalent characterizations of the Jacobson radical, such as:
As with the nilradical, we can extend this definition to arbitrary two-sided ideals "I" by defining J("I") to be the preimage of J("R/I") under the projection map "R"→"R/I".
If "R" is commutative, the Jacobson radical always contains the nilradical. If the ring "R" is a finitely generated Z-algebra, then the nilradical is equal to the Jacobson radical, and more generally: the radical of any ideal "I" will always be equal to the intersection of all the maximal ideals of "R" that contain "I". This says that "R" is a Jacobson ring.
The Baer radical.
The Baer radical of a ring is the intersection of the prime ideals of the ring "R". Equivalently it is the smallest semiprime ideal in "R". The Baer radical is the lower radical of the class of nilpotent rings. Also called the "lower nilradical" (and denoted Nil∗"R"), the "prime radical", and the "Baer-McCoy radical". Every element of the Baer radical is nilpotent, so it is a nil ideal.
For commutative rings, this is just the nilradical and closely follows the definition of the radical of an ideal.
The upper nil radical or Köthe radical.
The sum of the nil ideals of a ring "R" is the upper nilradical Nil*"R" or Köthe radical and is the unique largest nil ideal of "R". Köthe's conjecture asks whether any left nil ideal is in the nilradical.
Singular radical.
An element of a (possibly non-commutative ring) is called left singular if it annihilates an essential left ideal, that is, "r" is left singular if "Ir" = 0 for some essential left ideal "I". The set of left singular elements of a ring "R" is a two-sided ideal, called the left singular ideal, and is denoted formula_1. The ideal "N" of "R" such that formula_2 is denoted by formula_3 and is called the singular radical or the Goldie torsion of "R". The singular radical contains the prime radical (the nilradical in the case of commutative rings) but may properly contain it, even in the commutative case. However, the singular radical of a Noetherian ring is always nilpotent.
The Levitzki radical.
The Levitzki radical is defined as the largest locally nilpotent ideal, analogous to the Hirsch–Plotkin radical in the theory of groups. If the ring is noetherian, then the Levitzki radical is itself a nilpotent ideal, and so is the unique largest left, right, or two-sided nilpotent ideal.
The Brown–McCoy radical.
The Brown–McCoy radical (called the strong radical in the theory of Banach algebra) can be defined in any of the following ways: 
The Brown–McCoy radical is studied in much greater generality than associative rings with 1.
The von Neumann regular radical.
A von Neumann regular ring is a ring "A" (possibly non-commutative without identity) such that for every "a" there is some "b" with "a" = "aba". The von Neumann regular rings form a radical class. It contains every matrix ring over a division algebra, but contains no nil rings.
The Artinian radical.
The Artinian radical is usually defined for two-sided Noetherian rings as the sum of all right ideals that are Artinian modules. The definition is left-right symmetric, and indeed produces a two-sided ideal of the ring. This radical is important in the study of Noetherian rings, as outlined by .
See also.
Related uses of "radical" that are not radicals of rings: