# Difference between revisions of "Group"

### From Online Dictionary of Crystallography

BrianMcMahon (talk | contribs) m (Tidied translations.) |
(not necessarily true for molecular point groups (ex. cylindrical molecules like acetylene)) |
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A group ''G'' may have a finite or infinite number of elements. In the first case, the number of elements of ''G'' is the '''order''' of ''G''. In the latter case, ''G'' is called an '''infinite group'''. | A group ''G'' may have a finite or infinite number of elements. In the first case, the number of elements of ''G'' is the '''order''' of ''G''. In the latter case, ''G'' is called an '''infinite group'''. | ||

− | Examples of infinite groups are [[space group]]s and their translation subgroups, whereas [[point group]] | + | Examples of infinite groups are [[space group]]s and their translation subgroups, whereas [[point group|crystallographic point groups]] are finite groups. |

==See also== | ==See also== |

## Latest revision as of 15:46, 30 November 2018

زمرة (*Ar*). Groupe (*Fr*). Gruppe (*Ge*). Gruppo (*It*). 群 (*Ja*). Группа (*Ru*). Grupo (*Sp*).

A set *G* equipped with a binary operation *: *G × G* → *G*, assigning to a pair *(g,h)* the product *g*h* is called a **group** if:

- The operation is
*associative*, i.e.*(a*b)*c = a*(b*c).* -
*G*contains an*identity element*(*neutral element*)*e*:*g*e = e*g = g*for all*g*in*G*. - Every
*g*in*G*has an*inverse element**h*for which*g*h = h*g = e*. The inverse element of*g*is written as*g*^{−1}.

Often, the symbol for the binary operation is omitted. The product of the elements *g* and *h* is then denoted by the concatenation *gh*.

The binary operation need not be commutative, *i.e.* in general one will have *g*h ≠ h*g*. In the case that *g*h = h*g* holds for all *g,h* in *G*, the group is an Abelian group.

A group *G* may have a finite or infinite number of elements. In the first case, the number of elements of *G* is the **order** of *G*. In the latter case, *G* is called an **infinite group**.
Examples of infinite groups are space groups and their translation subgroups, whereas crystallographic point groups are finite groups.

## See also

- Chapter 1.1 of
*International Tables for Crystallography, Volume A*, 6th edition