# Algebraic structure

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, we 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.

Depending on the operations, relations and axioms, the algebraic structures get their names. The following is a partial list of algebraic structures:

Simple 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 SS
• Pointed unary system: a unary system with a distinguished element (such objects occur in discussions of the Peano axioms)

Group-like structures

Ring-like structures

Modules

• 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

Algebras

Lattices

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.

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