A group is a set paired with an operation on the set. As such, a group can be conceptualized as an ordered pair , where is a set, and is an operation.

A set and operation is a group if and only if it satisfies the following properties:

  1. Identity elementThere exists an , called an identity element, such that , for all
  2. InversesFor each , there exists an , called an inverse of , such that
  3. Associativity — For all
  4. Closure — For all

Whenever the group operation is , the operation of group elements , , is often abbreviated as simply a juxtaposition of the group elements, .

Important Results

From the given criterion for a group, the following properties can be shown for any group  :

  • There exists exactly one identity element;
  • For each , there exists exactly one inverse of , and henceforth is referred to as (proof)
  • For each
  • Groups have the cancellation property: For all implies , and implies .

Optional Properties

A group is:

A group with a partial order on it is a partially ordered group if for all , if , then and (translation invariance). It is a totally ordered group if in addition is a total order.

Community content is available under CC-BY-SA unless otherwise noted.