# Group theory

In mathematics and abstract algebra, group theory studies a type of algebraic structure called a group. Group theory is often used in mathematics as a starting point for the study of many algebraic structures, and of addition and multiplication of numbers. Because group theory is also useful for studying symmetry in nature and abstract systems, it has many applications in physics and chemistry.

## Definition

A group is a set (collection) G whose members are called elements. The elements can be numbers of some kind, or other abstract objects. The elements can even be material objects. There is also a binary operation which combines any two elements of G and results in another element of G. This new element may be different from either of the original two, but it does not have to be. It only has to be an element of G. To be a group, the following four conditions must be true of G and the operation defined over G:

• Closure: It is required to check if the suggested operation is actually an operation on the set. If an operation is used on any elements in the group, the element that is formed will also be part of the group.
• For all a, b in G, the result of the operation ab is also in G.
• Identity element: One element of the group is special. It is called the identity element. If the operation is used with the identity element and another element, the other element will not change.
• There exists an element e in G, such that for all elements a in G, the equation ea = ae = a holds.
• Associativity: When there are many operations, it does not matter in what order they are done, the result will be the same.
• For all a, b and c in G, the equation (ab) • c = a • (bc) holds.
• Inverse element: Every element in the group has another element in the group known as its inverse. When the operation is performed between them, the result is the identity element.
• For each a in G, there exists an element b in G such that ab = ba = e, where e is the identity element.

An important fact about groups is that when two elements are combined using the operation, the order of the elements matters. Switching the order of two combined elements typically gives a different result. This means that ab = ba does not hold. If the order does not matter and the previous equation does hold, the group is called an abelian group.

## Examples

One example of a group is the set of integers (called Z) with the addition operator +. This group is called G = (Z, +). Each required condition is true of G, so G is a group.

• Closure: For any two integers a, b in G, a + b is also in G.
• Identity element: For any integer a in G, a + 0 = a. So 0 is the identity element of G.
• Associativity: For any three integers a, b, and c in G, (a + b) + c = a + (b + c).
• Inverse element: For any integer a in G, there is a b = -a in G such that a + b = 0.

The addition operator is also commutative – the order of a and b does not matter. For any two integers a and b in G, a + b = b + a. Thus, G is also an abelian group.