Algebraic structure → Ring theoryRing theory |
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In mathematics, an **associative algebra***A* is an algebraic structure with compatible operations of addition, multiplication (assumed to be associative), and a scalar multiplication by elements in some field. The addition and multiplication operations together give *A* the structure of a ring; the addition and scalar multiplication operations together give *A* the structure of a vector space over *K*. In this article we will also use the term *K*-algebra** to mean an associative algebra over the field K. A standard first example of a K-algebra is a ring of square matrices over a field K, with the usual matrix multiplication.**

**Definition****As a monoid object in the category of modules****From ring homomorphisms****Algebra homomorphisms****Examples****Algebra****Representation theory****Analysis****Geometry and combinatorics****Constructions****Separable algebra****Finite-dimensional algebra****Commutative case****Noncommutative case****Lattices and orders****Related concepts****Coalgebras****Representations****Motivation for a Hopf algebra****Motivation for a Lie algebra****Non-unital algebras****See also****Notes****References**

**A commutative algebra is an associative algebra that has a commutative multiplication, or, equivalently, an associative algebra that is also a commutative ring.**

**In this article associative algebras are assumed to have a multiplicative identity, denoted 1; they are sometimes called unital associative algebras for clarification. In some areas of mathematics this assumption is not made, and we will call such structures non-unital associative algebras. We will also assume that all rings are unital, and all ring homomorphisms are unital.**

**Many authors consider the more general concept of an associative algebra over a commutative ring R, instead of a field: An R-algebra is an R-module with an associative R-bilinear binary operation, which also contains a multiplicative identity. For examples of this concept, if S is any ring with center C, then S is an associative C-algebra.**

Algebraic structures |
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**Let R be a commutative ring (so R could be a field). An associative R-algebra (or more simply, an R-algebra) is a ring that is also an R-module in such a way that the ring addition and the module addition are the same operation, and scalar multiplication satisfies**

**for all r ∈ R and x, y ∈ A. (This implies that A is assumed to be unital, as rings are supposed to have a multiplicative identity).**

**Equivalently, an associative algebra A is a ring together with a ring homomorphism from R to the center of A. If f is such a homomorphism, the scalar multiplication is (here the multiplication is the ring multiplication); if the scalar multiplication is given, the ring homomorphism is given by (See also § From ring homomorphisms below).**

**Every ring is an associative -algebra, where denotes the ring of the integers.**

**A commutative algebra is an associative algebra that is also a commutative ring.**

**The definition is equivalent to saying that a unital associative R-algebra is a monoid object in R-Mod (the monoidal category of R-modules). By definition, a ring is a monoid object in the category of abelian groups; thus, the notion of an associative algebra is obtained by replacing the category of abelian groups with the category of modules.**

**Pushing this idea further, some authors have introduced a "generalized ring" as a monoid object in some other category that behaves like the category of modules. Indeed, this reinterpretation allows one to avoid making an explicit reference to elements of an algebra A. For example, the associativity can be expressed as follows. By the universal property of a tensor product of modules, the multiplication (the R-bilinear map) corresponds to a unique R-linear map**

**.**

**The associativity then refers to the identity:**

**An associative algebra amounts to a ring homomorphism whose image lies in the center. Indeed, starting with a ring A and a ring homomorphism whose image lies in the center of A, we can make A an R-algebra by defining**

**for all r ∈ R and x ∈ A. If A is an R-algebra, taking x = 1, the same formula in turn defines a ring homomorphism whose image lies in the center.**

**If a ring is commutative then it equals its center, so that a commutative R-algebra can be defined simply as a commutative ring A together with a commutative ring homomorphism .**

**The ring homomorphism η appearing in the above is often called a structure map. In the commutative case, one can consider the category whose objects are ring homomorphisms R → A; i.e., commutative R-algebras and whose morphisms are ring homomorphisms A → A' that are under R; i.e., R → A → A' is R → A' (i.e., the coslice category of the category of commutative rings under R.) The prime spectrum functor Spec then determines an anti-equivalence of this category to the category of affine schemes over Spec R.**

**How to weaken the commutativity assumption is a subject matter of noncommutative algebraic geometry and, more recently, of derived algebraic geometry. See also: generic matrix ring.**

**A homomorphism between two R-algebras is an R-linear ring homomorphism. Explicitly, is an associative algebra homomorphism if**

**The class of all R-algebras together with algebra homomorphisms between them form a category, sometimes denoted R-Alg.**

**The subcategory of commutative R-algebras can be characterized as the coslice category R/CRing where CRing is the category of commutative rings.**

**The most basic example is a ring itself; it is an algebra over its center or any subring lying in the center. In particular, any commutative ring is an algebra over any of its subrings. Other examples abound both from algebra and other fields of mathematics.**

**Any ring***A*can be considered as a**Z**-algebra. The unique ring homomorphism from**Z**to*A*is determined by the fact that it must send 1 to the identity in*A*. Therefore, rings and**Z**-algebras are equivalent concepts, in the same way that abelian groups and**Z**-modules are equivalent.**Any ring of characteristic***n*is a (**Z**/*n***Z**)-algebra in the same way.**Given an***R*-module*M*, the endomorphism ring of*M*, denoted End_{R}(*M*) is an*R*-algebra by defining (*r*·φ)(*x*) =*r*·φ(*x*).**Any ring of matrices with coefficients in a commutative ring***R*forms an*R*-algebra under matrix addition and multiplication. This coincides with the previous example when*M*is a finitely-generated, free*R*-module.- In particular, the square
*n*-by-*n*matrices with entries from the field*K*form an associative algebra over*K*.

- In particular, the square
**The complex numbers form a 2-dimensional commutative algebra over the real numbers.****The quaternions form a 4-dimensional associative algebra over the reals (but not an algebra over the complex numbers, since the complex numbers are not in the center of the quaternions).****The polynomials with real coefficients form a commutative algebra over the reals.****Every polynomial ring***R*[*x*_{1}, ...,*x*] is a commutative_{n}*R*-algebra. In fact, this is the free commutative*R*-algebra on the set {*x*_{1}, ...,*x*}._{n}**The free***R*-algebra on a set*E*is an algebra of "polynomials" with coefficients in*R*and noncommuting indeterminates taken from the set*E*.**The tensor algebra of an***R*-module is naturally an associative*R*-algebra. The same is true for quotients such as the exterior and symmetric algebras. Categorically speaking, the functor that maps an*R*-module to its tensor algebra is left adjoint to the functor that sends an*R*-algebra to its underlying*R*-module (forgetting the multiplicative structure).**The following ring is used in the theory of λ-rings. Given a commutative ring***A*, let the set of formal power series with constant term 1. It is an abelian group with the group operation that is the multiplication of power series. It is then a ring with the multiplication, denoted by , such that determined by this condition and the ring axioms. The additive identity is 1 and the multiplicative identity is . Then has a canonical structure of a -algebra given by the ring homomorphism

**On the other hand, if***A*is a λ-ring, then there is a ring homomorphism**giving a structure of an***A*-algebra.

**The universal enveloping algebra of a Lie algebra is an associative algebra that can be used to study the given Lie algebra.****If***G*is a group and*R*is a commutative ring, the set of all functions from*G*to*R*with finite support form an*R*-algebra with the convolution as multiplication. It is called the group algebra of*G*. The construction is the starting point for the application to the study of (discrete) groups.**If***G*is an algebraic group (e.g., semisimple complex Lie group), then the coordinate ring of*G*is the Hopf algebra*A*corresponding to*G*. Many structures of*G*translate to those of*A*.**A quiver algebra (or a path algebra) of a directed graph is the free associative algebra over a field generated by the paths in the graph.**

**Given any Banach space***X*, the continuous linear operators*A*:*X*→*X*form an associative algebra (using composition of operators as multiplication); this is a Banach algebra.**Given any topological space***X*, the continuous real- or complex-valued functions on*X*form a real or complex associative algebra; here the functions are added and multiplied pointwise.**The set of semimartingales defined on the filtered probability space (Ω,***F*, (*F*)_{t}_{t ≥ 0}, P) forms a ring under stochastic integration.**The Weyl algebra****An Azumaya algebra**

**The Clifford algebras, which are useful in geometry and physics.****Incidence algebras of locally finite partially ordered sets are associative algebras considered in combinatorics.**

**Subalgebras****A subalgebra of an***R*-algebra*A*is a subset of*A*which is both a subring and a submodule of*A*. That is, it must be closed under addition, ring multiplication, scalar multiplication, and it must contain the identity element of*A*.**Quotient algebras****Let***A*be an*R*-algebra. Any ring-theoretic ideal*I*in*A*is automatically an*R*-module since*r*·*x*= (*r*1_{A})*x*. This gives the quotient ring*A*/*I*the structure of an*R*-module and, in fact, an*R*-algebra. It follows that any ring homomorphic image of*A*is also an*R*-algebra.**Direct products****The direct product of a family of***R*-algebras is the ring-theoretic direct product. This becomes an*R*-algebra with the obvious scalar multiplication.**Free products****One can form a free product of***R*-algebras in a manner similar to the free product of groups. The free product is the coproduct in the category of*R*-algebras.**Tensor products****The tensor product of two***R*-algebras is also an*R*-algebra in a natural way. See tensor product of algebras for more details. Given a commutative ring*R*and any ring*A*the tensor product*R*⊗_{Z}*A*can be given the structure of an*R*-algebra by defining*r*· (*s*⊗*a*) = (*rs*⊗*a*). The functor which sends*A*to*R*⊗_{Z}*A*is left adjoint to the functor which sends an*R*-algebra to its underlying ring (forgetting the module structure). See also: Change of rings.

**Let A be an algebra over a commutative ring R. Then the algebra A is a right^{ [1] } module over with the action . Then, by definition, A is said to separable if the multiplication map splits as an -linear map,^{ [2] } where is an -module by . Equivalently,^{ [3] } is separable if it is a projective module over ; thus, the -projective dimension of A, sometimes called the bidimension of A, measures the failure of separability.**

**Let A be a finite-dimensional algebra over a field k. Then A is an Artinian ring.**

**As A is Artinian, if it is commutative, then it is a finite product of Artinian local rings whose residue fields are algebras over the base field k. Now, a reduced Artinian local ring is a field and thus the following are equivalent^{ [4] }**

**is separable.****is reduced, where is some algebraic closure of***k*.**for some***n*.**is the number of -algebra homomorphisms .**

**Since a simple Artinian ring is a (full) matrix ring over a division ring, if A is a simple algebra, then A is a (full) matrix algebra over a division algebra D over k; i.e., . More generally, if A is a semisimple algebra, then it is a finite product of matrix algebras (over various division k-algebras), the fact known as the Artin–Wedderburn theorem.**

**The fact that A is Artinian simplifies the notion of a Jacobson radical; for an Artinian ring, the Jacobson radical of A is the intersection of all (two-sided) maximal ideals (in contrast, in general, a Jacobson radical is the intersection of all left maximal ideals or the intersection of all right maximal ideals.)**

**The Wedderburn principal theorem states:^{ [5] } for a finite-dimensional algebra A with a nilpotent ideal I, if the projective dimension of as an -module is at most one, then the natural surjection splits; i.e., contains a subalgebra such that is an isomorphism. Taking I to be the Jacobson radical, the theorem says in particular that the Jacobson radical is complemented by a semisimple algebra. The theorem is an analog of Levi's theorem for Lie algebras.**

**Let R be a Noetherian integral domain with field of fractions K (for example, they can be ). A lattice L in a finite-dimensional K-vector space V is a finitely generated R-submodule of V that spans V; in other words, .**

**Let be a finite-dimensional K-algebra. An order in is an R-subalgebra that is a lattice. In general, there are a lot fewer orders than lattices; e.g., is a lattice in but not an order (since it is not an algebra).^{ [6] }**

**A maximal order is an order that is maximal among all the orders.**

**An associative algebra over K is given by a K-vector space A endowed with a bilinear map A × A → A having two inputs (multiplicator and multiplicand) and one output (product), as well as a morphism K → A identifying the scalar multiples of the multiplicative identity. If the bilinear map A × A → A is reinterpreted as a linear map (i. e., morphism in the category of K-vector spaces) A ⊗ A → A (by the universal property of the tensor product), then we can view an associative algebra over K as a K-vector space A endowed with two morphisms (one of the form A ⊗ A → A and one of the form K → A) satisfying certain conditions that boil down to the algebra axioms. These two morphisms can be dualized using categorial duality by reversing all arrows in the commutative diagrams that describe the algebra axioms; this defines the structure of a coalgebra.**

**There is also an abstract notion of F-coalgebra, where F is a functor. This is vaguely related to the notion of coalgebra discussed above.**

**A representation of an algebra A is an algebra homomorphism ρ : A → End(V) from A to the endomorphism algebra of some vector space (or module) V. The property of ρ being an algebra homomorphism means that ρ preserves the multiplicative operation (that is, ρ(xy) = ρ(x)ρ(y) for all x and y in A), and that ρ sends the unit of A to the unit of End(V) (that is, to the identity endomorphism of V).**

**If A and B are two algebras, and ρ : A → End(V) and τ : B → End(W) are two representations, then there is a (canonical) representation AB → End(VW) of the tensor product algebra A B on the vector space V W. However, there is no natural way of defining a tensor product of two representations of a single associative algebra in such a way that the result is still a representation of that same algebra (not of its tensor product with itself), without somehow imposing additional conditions. Here, by tensor product of representations , the usual meaning is intended: the result should be a linear representation of the same algebra on the product vector space. Imposing such additional structure typically leads to the idea of a Hopf algebra or a Lie algebra, as demonstrated below.**

**Consider, for example, two representations and . One might try to form a tensor product representation according to how it acts on the product vector space, so that**

**However, such a map would not be linear, since one would have**

**for k ∈ K. One can rescue this attempt and restore linearity by imposing additional structure, by defining an algebra homomorphism Δ: A → A⊗A, and defining the tensor product representation as**

**Such a homomorphism Δ is called a comultiplication if it satisfies certain axioms. The resulting structure is called a bialgebra. To be consistent with the definitions of the associative algebra, the coalgebra must be co-associative, and, if the algebra is unital, then the co-algebra must be co-unital as well. A Hopf algebra is a bialgebra with an additional piece of structure (the so-called antipode), which allows not only to define the tensor product of two representations, but also the Hom module of two representations (again, similarly to how it is done in the representation theory of groups).**

**One can try to be more clever in defining a tensor product. Consider, for example,**

**so that the action on the tensor product space is given by**

**.**

**This map is clearly linear in x, and so it does not have the problem of the earlier definition. However, it fails to preserve multiplication:**

**.**

**But, in general, this does not equal**

**.**

**This shows that this definition of a tensor product is too naive; the obvious fix is to define it such that it is antisymmetric, so that the middle two terms cancel. This leads to the concept of a Lie algebra.**

**Some authors use the term "associative algebra" to refer to structures which do not necessarily have a multiplicative identity, and hence consider homomorphisms which are not necessarily unital.**

**One example of a non-unital associative algebra is given by the set of all functions f: R → R whose limit as x nears infinity is zero.**

**Another example is the vector space of continuous periodic functions, together with the convolution product.**

**Abstract algebra****Algebraic structure****Algebra over a field****Sheaf of algebras, a sort of an algebra over a ringed space**

**↑ Editorial note: as it turns, is a full matrix ring in interesting cases and it is more conventional to let matrices act from the right.****↑ Cohn 2003 , § 4.7.****↑ To see the equivalence, note a section of can be used to construct a section of a surjection.****↑ Waterhouse 1979 , § 6.2.****↑ Cohn 2003 , Theorem 4.7.5.****↑ Artin 1999 , Ch. IV, § 1.**

**In mathematics, the tensor product of two vector spaces V and W is a vector space which can be thought of as the space of all tensors that can be built from vectors from its constituent spaces using an additional operation which can be considered as a generalization and abstraction of the outer product. Because of the connection with tensors, which are the elements of a tensor product, tensor products find uses in many areas of application including in physics and engineering, though the full theoretical mechanics of them described below may not be commonly cited there. For example, in general relativity, the gravitational field is described through the metric tensor, which is a field of tensors, one at each point in the space-time manifold, and each of which lives in the tensor self-product of tangent spaces at its point of residence on the manifold.**

**In abstract algebra, the endomorphisms of an abelian group X form a ring. This ring is called the endomorphism ringX, denoted by End(X); the set of all homomorphisms of X into itself. Addition of endomorphisms arises naturally in a pointwise manner and multiplication via endomorphism composition. Using these operations, the set of endomorphisms of an abelian group forms a (unital) ring, with the zero map as additive identity and the identity map as multiplicative identity.**

In mathematics, the **exterior product** or **wedge product** of vectors is an algebraic construction used in geometry to study areas, volumes, and their higher-dimensional analogues. The exterior product of two vectors and , denoted by , is called a bivector and lives in a space called the *exterior square*, a vector space that is distinct from the original space of vectors. The magnitude of can be interpreted as the area of the parallelogram with sides and , which in three dimensions can also be computed using the cross product of the two vectors. More generally, all parallel plane surfaces with the same orientation and area have the same bivector as a measure of their oriented area. Like the cross product, the exterior product is anticommutative, meaning that for all vectors and , but, unlike the cross product, the exterior product is associative.

**Ring theory is the branch of mathematics in which rings are studied: that is, structures supporting both an addition and a multiplication operation. This is a glossary of some terms of the subject.**

**In mathematics, a bialgebra over a field K is a vector space over K which is both a unital associative algebra and a counital coassociative coalgebra. The algebraic and coalgebraic structures are made compatible with a few more axioms. Specifically, the comultiplication and the counit are both unital algebra homomorphisms, or equivalently, the multiplication and the unit of the algebra both are coalgebra morphisms.**

In the mathematical field of representation theory, a **Lie algebra representation** or **representation of a Lie algebra** is a way of writing a Lie algebra as a set of matrices in such a way that the Lie bracket is given by the commutator. In the language of physics, one looks for a vector space together with a collection of operators on satisfying some fixed set of commutation relations, such as the relations satisfied by the angular momentum operators.

** In mathematics, a Hopf algebra, named after Heinz Hopf, is a structure that is simultaneously an algebra and a coalgebra, with these structures' compatibility making it a bialgebra, and that moreover is equipped with an antiautomorphism satisfying a certain property. The representation theory of a Hopf algebra is particularly nice, since the existence of compatible comultiplication, counit, and antipode allows for the construction of tensor products of representations, trivial representations, and dual representations.**

**In mathematics, the tensor algebra of a vector space V, denoted T(V) or T^{•}(V), is the algebra of tensors on V with multiplication being the tensor product. It is the free algebra on V, in the sense of being left adjoint to the forgetful functor from algebras to vector spaces: it is the "most general" algebra containing V, in the sense of the corresponding universal property.**

**In algebra, a flat module over a ring R is an R-module M such that taking the tensor product over R with M preserves exact sequences. A module is faithfully flat if taking the tensor product with a sequence produces an exact sequence if and only if the original sequence is exact.**

**In mathematics, the symmetric algebraS(V) on a vector space V over a field K is a commutative algebra over K that contains V, and is, in some sense, minimal for this property. Here, "minimal" means that S(V) satisfies the following universal property: for every linear map f from V to a commutative algebra A, there is a unique algebra homomorphism g : S(V) → A such that f = g ∘ i, where i is the inclusion map of V in S(V).**

**In mathematics, a sesquilinear form is a generalization of a bilinear form that, in turn, is a generalization of the concept of the dot product of Euclidean space. A bilinear form is linear in each of its arguments, but a sesquilinear form allows one of the arguments to be "twisted" in a semilinear manner, thus the name; which originates from the Latin numerical prefix sesqui- meaning "one and a half". The basic concept of the dot product – producing a scalar from a pair of vectors – can be generalized by allowing a broader range of scalar values and, perhaps simultaneously, by widening the definition of a vector.**

**The representation theory of groups is a part of mathematics which examines how groups act on given structures.**

**In mathematics, Tannaka–Krein duality theory concerns the interaction of a compact topological group and its category of linear representations. It is a natural extension of Pontryagin duality, between compact and discrete commutative topological groups, to groups that are compact but noncommutative. The theory is named after Tadao Tannaka and Mark Grigorievich Krein. In contrast to the case of commutative groups considered by Lev Pontryagin, the notion dual to a noncommutative compact group is not a group, but a category of representations Π(G) with some additional structure, formed by the finite-dimensional representations of G.**

**In differential geometry, a Lie algebra-valued form is a differential form with values in a Lie algebra. Such forms have important applications in the theory of connections on a principal bundle as well as in the theory of Cartan connections.**

**In mathematics, the tensor product of modules is a construction that allows arguments about bilinear maps to be carried out in terms of linear maps. The module construction is analogous to the construction of the tensor product of vector spaces, but can be carried out for a pair of modules over a commutative ring resulting in a third module, and also for a pair of a right-module and a left-module over any ring, with result an abelian group. Tensor products are important in areas of abstract algebra, homological algebra, algebraic topology, algebraic geometry, operator algebras and noncommutative geometry. The universal property of the tensor product of vector spaces extends to more general situations in abstract algebra. It allows the study of bilinear or multilinear operations via linear operations. The tensor product of an algebra and a module can be used for extension of scalars. For a commutative ring, the tensor product of modules can be iterated to form the tensor algebra of a module, allowing one to define multiplication in the module in a universal way.**

**In mathematics, a vector-valued differential form on a manifold M is a differential form on M with values in a vector space V. More generally, it is a differential form with values in some vector bundle E over M. Ordinary differential forms can be viewed as R-valued differential forms.**

**The intent of this article is to highlight the important points of the derivation of the Navier–Stokes equations as well as its application and formulation for different families of fluids.**

**In mathematics, a separable algebra is a kind of semisimple algebra. It is a generalization to associative algebras of the notion of a separable field extension.**

**In mathematics, a zonal spherical function or often just spherical function is a function on a locally compact group G with compact subgroup K that arises as the matrix coefficient of a K-invariant vector in an irreducible representation of G. The key examples are the matrix coefficients of the spherical principal series, the irreducible representations appearing in the decomposition of the unitary representation of G on L^{2}(G/K). In this case the commutant of G is generated by the algebra of biinvariant functions on G with respect to K acting by right convolution. It is commutative if in addition G/K is a symmetric space, for example when G is a connected semisimple Lie group with finite centre and K is a maximal compact subgroup. The matrix coefficients of the spherical principal series describe precisely the spectrum of the corresponding C* algebra generated by the biinvariant functions of compact support, often called a Hecke algebra. The spectrum of the commutative Banach *-algebra of biinvariant L^{1} functions is larger; when G is a semisimple Lie group with maximal compact subgroup K, additional characters come from matrix coefficients of the complementary series, obtained by analytic continuation of the spherical principal series.**

**In mathematics, given an action of a group scheme G on a scheme X over a base scheme S, an equivariant sheafF on X is a sheaf of -modules together with the isomorphism of -modules**

**Artin, Michael (1999). "Noncommutative Rings" (PDF).****Bourbaki, N. (1989).***Algebra I*. Springer. ISBN 3-540-64243-9.**Cohn, P.M. (2003).***Further Algebra and Applications*(2nd ed.). Springer. ISBN 1852336676. Zbl 1006.00001.**Nathan Jacobson, Structure of Rings****James Byrnie Shaw (1907) A Synopsis of Linear Associative Algebra, link from Cornell University Historical Math Monographs.****Ross Street (1998)***Quantum Groups: an entrée to modern algebra*, an overview of index-free notation.**Waterhouse, William (1979),***Introduction to affine group schemes*, Graduate Texts in Mathematics,**66**, Berlin, New York: Springer-Verlag, doi:10.1007/978-1-4612-6217-6, ISBN 978-0-387-90421-4, MR 0547117

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