Group (mathematics)

A group is a mathematical structure consisting of set of elements combined with a binary operator which satisfies four conditions:

1. Closure: applying the binary operator to any two elements of the group produces a result which itself belongs to the group
2. Associativity: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle (AB)C = A(BC)} where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A} , $B$ and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle C} are any element of the group
3. Existence of Identity: there must exist an identity element $I$ such that $IA=AI=A$; that is, applying the binary operator to some element $A$ and the identity element $I$ leaves $A$ unchanged
4. Existence of Inverse: for each element $A$, there must exist an inverse Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A^{-1}} such that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle AA^{-1} = A^{-1}A = I}

A group with commutative binary operator is known as Abelian.

Example 1: the Klein four group consists of the set of formal symbols Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \{1, i, j, k \} } with the relations Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle i^{2} =j^{2}=k^{2}=1, \; ij=k, \; jk=i, \; ki=j. } All elements of the Klein four group (except the identity 1) have order 2. The Klein four group is isomorphic to Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Z_{2}\times Z_{2}} under mod addition.

Example 2: the set of complex numbers {1, -1, i,-i} under multiplication, where i is the square root of -1, the basis of the imaginary numbers. This group is isomorphic to Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Z_{4} } under mod addition.

Groups are the appropriate mathematical structures for any application involving symmetry.