In abstract algebra, an algebraic structure consists of a set together with a collection of operations or relations defined on it which satisfy certain axioms. When there are no ambiguities, mathematicians usually identify the set with the algebraic structure. For example, a group (G,*) is usually referred simply as a group G. If there are only relations and no operations, we speak of a relational structure.
Algebraic structures are defined by the operations, relations and axioms that underlie them. The following is a partial list of algebraic structures:
(Although some mathematicians would not count the following as algebraic structures, we include them for completeness)
- Set: a set can itself be thought of as a degenerate algebraic structure, one that has zero operations defined on it
- Pointed set: a set S with a distinguished element s of S
- Unary system: a set S with a unary operation, i.e. a function S → S
- Pointed unary system: a unary system with a distinguished element (such objects occur in discussions of the Peano axioms)
- Magma or groupoid: a set with a single binary operation
- Quasigroup: a magma in which division is always possible
- Loop: a quasigroup with an identity element
- Semigroup: an associative magma
- Monoid: a semigroup with an identity element
- Group: a monoid in which every element has an inverse, or equivalently, an associative loop
- Abelian group: a commutative group
- Archimedean group: a linearly ordered group for which the Archimedean property holds
- Semiring: similar to a ring, but without additive inverses
- Ring: a set with an abelian group operation as addition, together with a monoid operation as multiplication, satisfying distributivity
- Commutative ring: a ring whose multiplication is commutative
- Division ring: a ring with 0 ≠ 1 in which each non-zero element has an inverse
- Field: a commutative division ring
- Kleene algebra: an idempotent semiring with additional unary operator (the Kleene star); these are modeled on regular expressions
- Module over a given ring R: a set with an abelian group operation as addition, together with an additive unary operation of scalar multiplication for every element of R, with an associativity condition linking scalar multiplication to multiplication in R
- Vector space: a module over a field
- Algebra: a module or vector space together with a bilinear operation as multiplication
- Associative algebra: an algebra whose multiplication is associative
- Commutative algebra: an associative algebra whose multiplication is commutative
- Graded algebra: an algebra with a "grading"
- Lie algebra: a non-associative algebra important in geometry
- Clifford algebra: an associative algebra determined by quadratic form on a vector space
- Lattice: a set with two commutative, associative, idempotent operations satisfying the absorption law
- Boolean algebra: a distributive, complemented lattice
Those statements that apply to all algebraic structures collectively are investigated in the branch of mathematics known as universal algebra.
Algebraic structures can also be defined on sets with additional non-algebraic structures, such as topological spaces. For example, a topological group is a topological space with a group structure such that the operations of multiplication and taking inverses are continuous; a topological group has both a topological and an algebraic structure. Other common examples are topological vector spaces and Lie groups.
Every algebraic structure has its own notion of homomorphism, a function that is compatible with the given operation(s). In this way, every algebraic structure defines a category. For example, the category of groups has all groups as objects and all group homomorphisms as morphisms. This category, being a concrete category, may be regarded as a category of sets with extra structure in the category-theoretic sense. Similarly, the category of topological groups (with continuous group homomorphisms as morphisms) is a category of topological spaces with extra structure.
There are various concepts in category theory that try to capture the algebraic character of a context, for instance
functors and categories.
See also: signature (universal algebra)cs:Algebraická struktura de:Algebraische Struktur fr:Structure algébrique it:Struttura algebrica he:מבנה אלגברי nl:Algebraïsche structuur ja:代数的構造 pt:Estrutura algébrica ru:Алгебраическая система sv:Algebraisk struktur uk:Алгебраїчна система zh:代数结构