Group
From AoPSWiki
A group 
is a set of elements together with an operation 
(the dot is frequently suppressed, so 
is written instead of 
) satisfying the following conditions:
- For all
, 
(associativity).
- There exists an element
so that for all 
, 
(identity).
- For any , there exists
so that 
( inverses).
(Equivalently, a group is a monoid with inverses.)
Note that the group operation need not be commutative. If the group operation is commutative, we call the group an abelian group (after the Norwegian mathematician Niels Henrik Abel).
Groups frequently arise as permutations or symmetries of collections of objects. For example, the rigid motions of 
that fix a certain regular 
-gon is a group, called the dihedral group and denoted in some texts 
(since it has 
elements) and in others 
(since it preserves a regular -gon). Another example of a group is the symmetric group 
of all permutations of 
.
See Also
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