Ring theory

In mathematics, ring theory is the study of ring (mathematics), algebraic structures in which addition and multiplication are defined and have similar properties to those familiar from the integers.

Please refer to the glossary of ring theory for the definitions of terms used throughout ring theory.

=History=

The study of rings originated from the theory of polynomial rings and the theory of algebraic integers. Furthermore, the appearance of hypercomplex numbers in the mid-nineteenth century undercut the pre-eminence of field (mathematics) in mathematical analysis.

Richard Dedekind introduced the concept of a ring.

The term ring (Zahlring) was coined by David Hilbert in the article Die Theorie der algebraischen Zahlkörper, Jahresbericht der Deutschen Mathematiker Vereinigung, Vol. 4, 1897.

The first axiomatic definition of a ring was given by Adolf Fraenkel in an essay in Journal für die reine und angewandte Mathematik (A. L. Crelle), vol. 145, 1914.

In 1921, Emmy Noether gave the first axiomatic foundation of the theory of commutative rings in her monumental paper Ideal Theory in Rings .

=Elementary introduction=

== Definition ==

Formally, a ring is an abelian group ( R , +), together with a second binary operation * such that for all a , b and c in R , : a * (b*c) = (a*b) * c : a * (b+c) = (a*b) + (a*c) : (a+b) * c = (a*c) + (b*c)

and such that there exists a multiplicative identity , or unity , that is, an element 1 so that for all a in R , : a*1 = 1*a = a

It is simple to show that any ring in which 1 = 0 must have just one element; any such ring is called a zero ring.

Rings that sit inside other rings are called subrings. Maps between rings which respect the ring operations are called ring homomorphisms. Rings, together with ring homomorphisms, form a category theory. Closely related is the notion of ideal (ring theory), certain subsets of rings which arise as kernel of a homomorphism of homomorphisms and can serve to define factor rings. Basic facts about ideals, homomorphisms and factor rings are recorded in the isomorphism theorems and in the Chinese remainder theorem.

A ring is called commutative if its multiplication is .

Non-commutative rings resemble rings of matrix (mathematics) in many respects. Following the model of algebraic geometry, attempts have been made recently at defining non-commutative geometry based on non-commutative rings. Non-commutative rings and associative algebras (rings that are also vector spaces) are often studied via their Category theory of modules. A module (mathematics) over a ring is an abelian group (mathematics) that the ring acts on as a ring of Endomorphisms, very much akin to the way field (mathematics)s (integral domains in which every non-zero element is invertible) act on vector spaces. Examples of non-commutative rings are given by rings of square matrix (mathematics) or more generally by rings of endomorphisms of abelian groups or modules, and by monoid rings.

=Some useful theorems=

*Artin-Wedderburn theorem

=Generalizations=

Any ring can be seen as a preadditive category with a single object. It is therefore natural to consider arbitrary preadditive categories to be generalizations of rings. And indeed, many definitions and theorems originally given for rings can be translated to this more general context. Additive functors between preadditive categories generalize the concept of ring homomorphism, and ideals in additive categories can be defined as sets of morphisms closed under addition and under composition with arbitrary morphisms.

=External links=

*[http://www-gap.dcs.st-and.ac.uk/~history/HistTopics/Ring_theory.html History of ring theory at the MacTutor Archive]