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In geometry, a point group is a group of geometric symmetries (isometries) that keep at least one point fixed. Point groups can exist in a Euclidean space with any dimension, and every point group in dimension ''d'' is a subgroup of the orthogonal group O(''d''). Point groups can be realized as sets of orthogonal matrices ''M'' that transform point ''x'' into point ''y'': : ''y'' = ''Mx'' where the origin is the fixed point. Point-group elements can either be rotations (determinant of ''M'' = 1) or else reflections, or improper rotations (determinant of ''M'' = −1). Discrete point groups in more than one dimension come in infinite families, but from the crystallographic restriction theorem and one of Bieberbach's theorems, each number of dimensions has only a finite number of point groups that are symmetric over some lattice or grid with that number. These are the crystallographic point groups. ==Chiral and achiral point groups, reflection groups== Point groups can be classified into ''chiral'' (or purely rotational) groups and ''achiral'' groups. The chiral groups are subgroups of the special orthogonal group SO(''d''): they contain only orientation-preserving orthogonal transformations, i.e., those of determinant +1. The achiral groups contain also transformations of determinant −1. In an achiral group, the orientation-preserving transformations form a (chiral) subgroup of index 2. Finite Coxeter groups or ''reflection groups'' are those point groups that are generated purely by a set of reflectional mirrors passing through the same point. A rank ''n'' Coxeter group has ''n'' mirrors and is represented by a Coxeter-Dynkin diagram. Coxeter notation offers a bracketed notation equivalent to the Coxeter diagram, with markup symbols for rotational and other subsymmetry point groups. Reflection groups are necessarily achiral (except for the trivial group containing only the identity element). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Point group」の詳細全文を読む スポンサード リンク
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