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In mathematics, especially in the group theoretic area of algebra, the projective linear group (also known as the projective general linear group or PGL) is the induced action of the general linear group of a vector space ''V'' on the associated projective space P(''V''). Explicitly, the projective linear group is the quotient group :PGL(''V'') = GL(''V'')/Z(''V'') where GL(''V'') is the general linear group of ''V'' and Z(''V'') is the subgroup of all nonzero scalar transformations of ''V''; these are quotiented out because they act trivially on the projective space and they form the kernel of the action, and the notation "Z" reflects that the scalar transformations form the center of the general linear group. The projective special linear group, PSL, is defined analogously, as the induced action of the special linear group on the associated projective space. Explicitly: :PSL(''V'') = SL(''V'')/SZ(''V'') where SL(''V'') is the special linear group over ''V'' and SZ(''V'') is the subgroup of scalar transformations with unit determinant. Here SZ is the center of SL, and is naturally identified with the group of ''n''th roots of unity in ''K'' (where ''n'' is the dimension of ''V'' and ''K'' is the base field). PGL and PSL are some of the fundamental groups of study, part of the so-called classical groups, and an element of PGL is called projective linear transformation, projective transformation or homography. If ''V'' is the ''n''-dimensional vector space over a field ''F'', namely the alternate notations PGL(''n'', ''F'') and PSL(''n'', ''F'') are also used. Note that PGL(''n'', ''F'') and PSL(''n'', ''F'') are equal if and only if every element of ''F'' has an ''n''th root in ''F''. As an example, note that but 〔Gareth A. Jones and David Silverman. (1987) Complex functions: an algebraic and geometric viewpoint. Cambridge UP. (Discussion of PSL and PGL on page 20 in google books )〕 this corresponds to the real projective line being orientable, and the projective special linear group only being the orientation-preserving transformations. PGL and PSL can also be defined over a ring, with an important example being the modular group, ==Name== The name comes from projective geometry, where the projective group acting on homogeneous coordinates (''x''0:''x''1: ... :''xn'') is the underlying group of the geometry.〔This is therefore PGL(''n'' + 1, ''F'') for projective space of dimension ''n''〕 Stated differently, the natural action of GL(''V'') on ''V'' descends to an action of PGL(''V'') on the projective space ''P''(''V''). The projective linear groups therefore generalise the case PGL(2, C) of Möbius transformations (sometimes called the Möbius group), which acts on the projective line. Note that unlike the general linear group, which is generally defined axiomatically as "invertible functions preserving the linear (vector space) structure", the projective linear group is defined ''constructively,'' as a quotient of the general linear group of the associated vector space, rather than axiomatically as "invertible functions preserving the projective linear structure". This is reflected in the notation: PGL(''n'', ''F'') is the group associated to GL(''n'', ''F''), and is the projective linear group of (''n''−1)-dimensional projective space, not ''n''-dimensional projective space. 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「projective linear group」の詳細全文を読む スポンサード リンク
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