Questions tagged [group-theory]

A group consists of a base set $G$ and a binary operation $\ast : G\times G\to G$, such that

1. $(a \ast b) \ast c = a \ast (b\ast c)$ for all $a,b,c\in G$ (associativity).
2. There is an identity or unit element $e\in G$ with $e\ast a = a\ast e = a$ for all $a\in G.$
3. For each element $a\in G$ there is an inverse element $a'$ such that $a\ast a' = a'\ast a = e$.

(Some authors include a fourth axiom, called closure, that states $\ast$ should be closed on $G$ (i.e., for all $a,b\in G$, we have $a\ast b\in G$); however, by stating that $\ast$ is a binary operation on $G$, this is implied.)

If additionally the commutative law $a \ast b = b\ast a$ for all $a,b\in G$ is satisfied, the group is called abelian or commutative.

The identity and inverses are always uniquely determined.

There are two main variants for the notation:

1. In multiplicative notation, the operation is denoted by $a\cdot b$ or just $ab$, the identity is often denoted by $1$, and the inverse of an $a\in G$ is denoted by $a^{-1}$.
2. For abelian groups often additive notation is used. Here, the operation is denoted by $a + b$, the identity by $0$ and the inverse of $a\in G$ by $-a$.

Group theory can also be seen as the mathematical theory of symmetries.

The historical roots of group theory include the study of symmetries of geometrical objects like the Platonic solids, and the study of roots of polynomial equations originated by Évariste Galois.