Algebraic structure
In abstract algebra, an algebraic structure consists of a set together with one or more operations on the set which satisfy certain axioms. In case there are no ambiguities, we usually identify the set with the algebraic structure. For example, a group (G,*) is usually referred simply as a group G.Depending on the operations and axioms, the algebraic structures get their names. The following is a partial list of algebraic structures:
- 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
- Ring: a set with an abelian group operation as addition, together with a monoid operation as multiplication, satisfying distributivity
- Field: a ring in which the non-zero elements form an abelian group under multiplication
- 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
- Kleene algebra: two binary operations and one unary operator, modeled on regular expressions
- Lattice: a set with two commutative, associative, idempotent operations satisfying the absorption law
- Boolean algebra: a bounded, distributive, complemented lattice
- Set: although some mathematicians would not count it, a set can itself be thought of as a degenerate algebraic structure, one that has zero operations defined on it
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.
