The following definition is the starting point of group theory.
A group is a set together with a function
called multiplication or binary operation and denoted simply by juxtaposition of that group, such that the following rules hold:
The law of composition is associative, that is,
For the given law of composition there exists a uniqueleft identity, that is there exists a unique such that .
For each , there exists an inverse of , that is an element of denoted such that.
Although these axioms to be satisfied by a group are quite brief, groups may be very complex, and the study of groups is not trivial. For instance, there exists a very complicated group, called the Monster group, which has roughly elements and the law of composition is so complicated that even modern computers have difficulty doing computations in this group.
There is a special type of groups (namely those that are commutative, i.e. the multiplication obeys the commutative law), which are named after the famous mathematician Niels Henrik Abel:
An Abelian group is a group such that its binary operation is commutative, that is,
Oftentimes, Abelian groups are written additively, that is, for the binary operation of and we write