# User:Rschwieb/Algebraic structure copy

In mathematics, and more specifically abstract algebra, the term algebraic structure generally refers to an arbitrary set with one or more binary operations defined on it. This idea effectively brings out the algebraic properties of the members of the set, and in turn defines an overall structure. Common examples of structures include groups, rings, fields and lattices. More complex algebraic structures can be defined by introducing multiple operations, different underlying sets, or by altering the defining axioms. Examples of more complex structures include vector spaces, modules and algebras.

The main motivation behind the study of algebraic structures is that many seemingly unrelated concepts can be related in terms of their algebraic properties. This provides links between them and gives a more in depth understanding of the mathematics involved. Although algebraic structures are mainly studied in pure mathematics, they do have applications in other fields, for example mathematical physics.

The properties of specific algebraic structures are studied in the branch known as abstract algebra. The general theory of algebraic structures has been formalized in universal algebra. Category theory is used to study the relationships between two or more classes of algebraic structures, often of different kinds. For example, Galois theory studies the connection between certain fields and groups, algebraic structures of two different kinds.

In a slight abuse of notation, the expression "structure" can also refer only to the operations on a structure, and not to the underlying set itself. For example, the group $(\mathbb Z, +)$ can be seen as a set $\mathbb Z$ which is equipped with an "algebraic structure", namely the operation $+$.

## Structures whose axioms are all identities

Universal algebra often considers classes of algebraic structures (such as the class of all groups), together with operations (such as products) and relations (such as "substructure") between these algebras. These classes are usually defined by "axioms", that is, a list of properties that all these structures have to share. If all axioms defining a class of algebras are "identities" , then the corresponding class is called variety (not to be confused with algebraic variety in the sense of algebraic geometry).

Identities are equations formulated using only the operations the structure allows, and variables that are tacitly universally quantified over the relevant universe. Identities contain no connectives, existentially quantified variables, or relations of any kind other than the allowed operations. The study of varieties is an important part of universal algebra.

An algebraic structure in a variety may be understood as the quotient algebra of term algebra (also called "absolutely free algebra") divided by the equivalence relations generated by a set of identities. So, a collection of functions with given signatures generate a free algebra, the term algebra T. Given a set of equational identities (the axioms), one may consider their symmetric, transitive closure E. The quotient algebra T/E is then the algebraic structure or variety. Thus, for example, groups have a signature containing two operators: the multiplication operator m, taking two arguments, and the inverse operator i, taking one argument, and the identity element e, a constant, which may be considered to be an operator taking zero arguments. Given a (countable) set of variables x, y, z, etc. the term algebra is the collection of all possible terms involving m, i, e and the variables; so for example, m(i(x), m(x,m(y,e))) would be an element of the term algebra. One of the axioms defining a group is the identity m(x, i(x)) = e; another is m(x,e) = x. These equations induce equivalence classes on the free algebra; the quotient algebra then has the algebraic structure of a group.

All structures in this section are elements of naturally defined varieties. Some of these structures are most naturally axiomatized using one or more nonidentities, but are nevertheless varieties because there exists an equivalent axiomatization, one perhaps less perspicuous, composed solely of identities. Algebraic structures that are not varieties are described in the following section, and differ from varieties in their metamathematical properties.

In this section and the following one, structures are listed in approximate order of increasing complexity, operationalized as follows:

• Simple structures requiring but one set, the universe S, are listed before composite ones requiring two sets;
• Structures having the same number of required sets are then ordered by the number of binary operations (0 to 4) they require. Incidentally, no structure mentioned in this entry requires an operation whose arity exceeds 2;
• Let A and B be the two sets that make up a composite structure. Then a composite structure may include 1 or 2 functions of the form A × AB or A × BA;
• Structures having the same number and kinds of binary operations and functions are more or less ordered by the number of required unary and 0-ary (distinguished elements) operations, 0 to 2 in both cases.

The indentation structure employed in this section and the one following is intended to convey information. If structure B is under structure A and more indented, then all theorems of A are theorems of B; the converse does not hold.

Ringoids and lattices can be clearly distinguished despite both having two defining binary operations. In the case of ringoids, the two operations are linked by the distributive law; in the case of lattices, they are linked by the absorption law. Ringoids also tend to have numerical models, while lattices tend to have set-theoretic models.

Simple structures: No binary operation:

• Set: a degenerate algebraic structure having no operations.
• Pointed set: S has one or more distinguished elements, often 0, 1, or both.
• Unary system: S and a single unary operation over S.
• Pointed unary system: a unary system with S a pointed set.

Group-like structures:

One binary operation. The binary operation can be indicated by any symbol, or with no symbol (juxtaposition) as is done for ordinary multiplication of real numbers. For nonassociative operations, it becomes necessary to indicate the order of operations with parentheses. For monoids, boundary algebras, and sloops, S is a pointed set.

• Magma or groupoid: S and a single binary operation over S.
• Steiner magma: A commutative magma satisfying x(xy) = y.
• Squag: an idempotent Steiner magma.
• Sloop: a Steiner magma with distinguished element 1, such that xx = 1.
• Semigroup: an associative magma.
• Monoid: a unital semigroup.
• Band: a semigroup of idempotents.
• Semilattice: a commutative band. The binary operation can be called either meet or join.
• Boundary algebra: a unital semilattice (equivalently, an idempotent commutative monoid) with a unary operation, complementation, denoted by enclosing its argument in parentheses, giving rise to an inverse element that is the complement of the identity element. The identity and inverse elements bound S. Also, x(xy) = x(y) holds.

Three binary operations. Quasigroups are listed here, despite their having 3 binary operations, because they are (nonassociative) magmas. Quasigroups feature 3 binary operations only because establishing the quasigroup cancellation property by means of identities alone requires two binary operations in addition to the group operation.

• Quasigroup: a cancellative magma. Equivalently, ∀x,yS, ∃!a,bS, such that xa = y and bx = y.
• Loop: a unital quasigroup with a unary operation, inverse.
• Moufang loop: a loop in which a weakened form of associativity, (zx)(yz) = z(xy)z, holds.
• Group: an associative loop.

Lattice: Two or more binary operations, including meet and join, connected by the absorption law. S is both a meet and join semilattice, and is a pointed set if and only if S is bounded. Lattices often have no unary operations. Every true statement has a dual, obtained by replacing every instance of meet with join, and vice versa.

• Bounded lattice: S has two distinguished elements, the greatest lower bound and the least upper bound. Dualizing requires replacing every instance of one bound by the other, and vice versa.
• Complemented lattice: a lattice with a unary operation, complementation, denoted by postfix " ' ", giving rise to an inverse element. That element and its complement bound the lattice.
• Modular lattice: a lattice in which the modular identity holds.
• Distributive lattice: a lattice in which each of meet and join distributes over the other. Distributive lattices are modular, but the converse does not hold.
• Kleene algebra: a bounded distributive lattice with a unary operation whose identities are x"=x, (x+y)'=x'y', and (x+x')yy'=yy'. See "ring-like structures" for another structure having the same name.
• Boolean algebra: a complemented distributive lattice. Either of meet or join can be defined in terms of the other and complementation.
• Interior algebra: a Boolean algebra with an added unary operation, the interior operator, denoted by postfix " ' " and obeying the identities x'x=x, x"=x, (xy)'=x'y', and 1'=1.
• Heyting algebra: a bounded distributive lattice with an added binary operation, relative pseudo-complement, denoted by infix " ' ", and governed by the axioms x'x=1, x(x'y) = xy, x'(yz) = (x'y)(x'z), (xy)'z = (x'z)(y'z).

Ringoids: Two binary operations, addition and multiplication, with multiplication distributing over addition. Semirings are pointed sets.

• Semiring: a ringoid such that S is a monoid under each operation. Each operation has a distinct identity element. Addition also commutes, and has an identity element that annihilates multiplication.
• Commutative semiring: a semiring with commutative multiplication.
• Ring: a semiring with a unary operation, additive inverse, giving rise to an inverse element -x, which when added to x, yields the additive identity element. Hence S is an abelian group under addition.
• Rng: a ring lacking a multiplicative identity.
• Commutative ring: a ring with commutative multiplication.
• Boolean ring: a commutative ring with idempotent multiplication, equivalent to a Boolean algebra.
• Kleene algebra: a semiring with idempotent addition and a unary operation, the Kleene star, denoted by postfix * and obeying the identities (1+x*x)x*=x* and (1+xx*)x*=x*. See "Lattice-like structures" for another structure having the same name.

N.B. The above definition of ring does not command universal assent. Some authorities employ "ring" to denote what is here called a rng, and refer to a ring in the above sense as a "ring with identity."

Modules: Composite Systems Defined over Two Sets, M and R: The members of:

1. R are scalars, denoted by Greek letters. R is a ring under the binary operations of scalar addition and multiplication;
2. M are module elements (often but not necessarily vectors), denoted by Latin letters. M is an abelian group under addition. There may be other binary operations.

The scalar multiplication of scalars and module elements is a function RxMM which commutes, associates (∀r,sR, ∀xM, r(sx) = (rs)x ), has 1 as identity element, and distributes over module and scalar addition. If only the pre(post)multiplication of module elements by scalars is defined, the result is a left (right) module.

• Free module: a module having a free basis, {e1, ... en}⊂M, where the positive integer n is the dimension of the free module. For every vM, there exist κ1, ..., κnR such that v = κ1e1 + ... + κnen. Let 0 and 0 be the respective identity elements for module and scalar addition. If r1e1 + ... + rnen = 0, then r1 = ... = rn = 0. (Note that the class of free modules over a given ring R is in general not a variety.)

Vector spaces, closely related to modules, are defined in the next section.

## Structures with some axioms that are not identities

The structures in this section are not axiomatized with identities alone, so the classes considered below are not varieties. Nearly all of the nonidentities below are one of two very elementary kinds:

1. The starting point for all structures in this section is a "nontrivial" ring, namely one such that S≠{0}, 0 being the additive identity element. The nearest thing to an identity implying S≠{0} is the nonidentity 0≠1, which requires that the additive and multiplicative identities be distinct.
2. Nearly all structures described in this section include identities that hold for all members of S except 0. In order for an algebraic structure to be a variety, its operations must be defined for all members of S; there can be no partial operations.

Structures whose axioms unavoidably include nonidentities are among the most important ones in mathematics, e.g., fields and vector spaces. Moreover, much of theoretical physics can be recast as models of multilinear algebras. Although structures with nonidentities retain an undoubted algebraic flavor, they suffer from defects varieties do not have. For example, neither the product of integral domains nor a free field over any set exist.

Arithmetics: Two binary operations, addition and multiplication. S is an infinite set. Arithmetics are pointed unary systems, whose unary operation is injective successor, and with distinguished element 0.

• Robinson arithmetic. Addition and multiplication are recursively defined by means of successor. 0 is the identity element for addition, and annihilates multiplication. Robinson arithmetic is listed here even though it is a variety, because of its closeness to Peano arithmetic.
• Peano arithmetic. Robinson arithmetic with an axiom schema of induction. Most ring and field axioms bearing on the properties of addition and multiplication are theorems of Peano arithmetic or of proper extensions thereof.

Field-like structures: Two binary operations, addition and multiplication. S is nontrivial, i.e., S≠{0}.

The following structures are not varieties for reasons in addition to S≠{0}:

Composite Systems: Vector Spaces, and Algebras over Fields. Two Sets, M and R, and at least three binary operations.

The members of:

1. M are vectors, denoted by lower case letters. M is at minimum an abelian group under vector addition, with distinguished member 0.
2. R are scalars, denoted by Greek letters. R is a field, nearly always the real or complex field, with 0 and 1 as distinguished members.

Three binary operations.

Four binary operations.

• Algebra over a field: An algebra over a ring except that R is a field instead of a commutative ring.
• Jordan algebra: a Jordan ring except that R is a field.
• Lie algebra: an algebra over a field respecting the Jacobi identity, whose vector multiplication, the Lie bracket denoted [u,v], anticommutes, does not associate, and is nilpotent.
• Associative algebra: an algebra over a field, or a module, whose vector multiplication associates.
• Linear algebra: an associative unital algebra with the members of M being matrices. Every matrix has a dimension nxm, n and m positive integers. If one of n or m is 1, the matrix is a vector; if both are 1, it is a scalar. Addition of matrices is defined only if they have the same dimensions. Matrix multiplication provides the algebra multiplication. Let matrix A be nxm and matrix B be ixj. Then AB is defined if and only if m=i; BA, if and only if j=n. There also exists an mxm matrix I and an nxn matrix J such that AI=JA=A. If u and v are vectors having the same dimensions, they have an inner product, denoted 〈u,v〉. Hence there is an orthonormal basis; see inner product space above. There is a unary function, the determinant, from square (nxn for any n) matrices to R.
• Commutative algebra: an associative algebra whose vector multiplication commutes.

Composite Systems: Multilinear algebras. Two sets, V and K. Four binary operations:

1. The members of V are multivectors (including vectors), denoted by lower case Latin letters. V is an abelian group under multivector addition, and a monoid under outer product. The outer product goes under various names, and is multilinear in principle but usually bilinear. The outer product defines the multivectors recursively starting from the vectors. Thus the members of V have a "degree" (see graded algebra below). Multivectors may have an inner product as well, denoted uv: V×VK, that is symmetric, linear, and positive definite; see inner product space above.
2. The properties and notation of K are the same as those of R above, except that K may have −1 as a distinguished member. K is usually the real field, as multilinear algebras are designed to describe physical phenomena without complex numbers.
3. The multiplication of scalars and multivectors, V×KV, has the same properties as the multiplication of scalars and module elements that is part of a module.
• Graded algebra: an associative algebra A with a direct sum decomposition resulting in assignment of "degrees" to elements in distinct summands of the decomposition. The idea is that if the degrees of two elements a and b are known, then the grade of ab is known, and so the location of the product ab is determined in the decomposition.
• Tensor algebra: a vector space V gives rise to "the most general associative algebra containing V". The tensor algebra of a vector space also carries the structure of a graded algebra, where the gradings are determined by tensor types.
• Clifford algebra: a vector space V with a bilinear form Q: V×VK gives rise to a graded algebra known as a Clifford algebra. The special case Q=0 yields the exterior algebra of V (see below.)
• Exterior algebra (also Grassmann algebra): a Clifford algebra with Q=0 produces a graded algebra whose product is the anticommutative exterior product, denoted by infix ∧. This product is also sometimes called the outer product or wedge product. V has an orthonormal basis v1v2 ∧ ... ∧ vk = 0 if and only if v1, ..., vk are linearly dependent. Multivectors also have an inner product.
• Geometric algebra: a Clifford algebra (typically over the field of real numbers) whose bilinear form is nondegenerate is called a geometric algebra. It is called "geometric" because the product provides a framework for geometric problems. The geometric product of parallel multivectors commutes, that of orthogonal vectors anticommutes. The product of a scalar with a multivector commutes. vv yields a scalar.

## Examples

Some recurring universes: N=natural numbers; Z=integers; Q=rational numbers; R=real numbers; C=complex numbers.

N is a pointed unary system, and under addition and multiplication, is both the standard interpretation of Peano arithmetic and a commutative semiring.

Boolean algebras are at once semigroups, lattices, and rings. They would even be abelian groups if the identity and inverse elements were identical instead of complements.

Group-like structures

Lattices

Ring-like structures

• The set R[X] of all polynomials over some coefficient ring R is a ring.
• 2x2 matrices with matrix addition and multiplication form a ring.
• If n is a positive integer, then the set Zn = Z/nZ of integers modulo n (the additive cyclic group of order n ) forms a ring having n elements (see modular arithmetic).
• Sets of hypercomplex numbers were early prototypes of algebraic structures now called rings.

Integral domains

• Z under addition and multiplication is an integral domain.

Fields

Algebraic structures can also be defined on sets with added structure of a non-algebraic nature, such as a topology. The added structure must be compatible, in some sense, with the algebraic structure.

## Category theory

The discussion above has been cast in terms of elementary abstract and universal algebra. Category theory is another way of reasoning about algebraic structures (see, for example, Mac Lane 1998). A category is a collection of objects with associated morphisms. Every algebraic structure has its own notion of homomorphism, namely any function compatible with the operation(s) defining the structure. In this way, every algebraic structure gives rise to a category. For example, the category of groups has all groups as objects and all group homomorphisms as morphisms. This concrete category may be seen as a category of sets with added category-theoretic structure. Likewise, the category of topological groups (whose morphisms are the continuous group homomorphisms) is a category of topological spaces with extra structure. A forgetful functor between categories of algebraic structures "forgets" a part of a structure.

There are various concepts in category theory that try to capture the algebraic character of a context, for instance