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In theoretical physics, Eugene Wigner and Erdal İnönü have discussed〔 〕 the possibility to obtain from a given Lie group a different (non-isomorphic) Lie group by a group contraction with respect to a continuous subgroup of it. That amounts to a limiting operation on a parameter of the Lie algebra, altering the structure constants of this Lie algebra in a nontrivial (singular) manner, under suitable circumstances. For example, the Lie algebra of SO(3), , etc, may be rewritten by a change of variables , , , as : () = ε2 ''Y''3, () = ''Y''1, () = ''Y''2. The contraction limit trivializes the first commutator and thus yields the non-isomorphic algebra of the plane Euclidean group, . (This is isomorphic to the cylindrical group, describing motions of a point on the surface of a cylinder. It is the little group of null 4-vectors.) Specifically, the translation generators ''Y''1, ''Y''2, now generate the Abelian normal subgroup of E2 (cf. Group extension), the parabolic Lorentz transformations. Similar limits, of considerable application in physics (cf. Correspondence principles), contract * the de Sitter group to the Poincaré group ISO(3,1), as the de Sitter radius diverges: ; or *the Poincaré group to the Galilei group, as the speed of light diverges: ;〔Gilmore, Robert (2006). ''Lie Groups, Lie Algebras, and Some of Their Applications'' (Dover Books on Mathematics), ISBN 0486445291 〕 or *the Moyal bracket Lie algebra (equivalent to quantum commutators) to the Poisson bracket Lie algebra, in the classical limit as the Planck constant vanishes: . ==References== 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「group contraction」の詳細全文を読む スポンサード リンク
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