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In mathematics, the Krull–Schmidt theorem states that a group subjected to certain finiteness conditions on chains of subgroups, can be uniquely written as a finite direct product of indecomposable subgroups. ==Definitions== We say that a group ''G'' satisfies the ascending chain condition (ACC) on subgroups if every sequence of subgroups of ''G'': : is eventually constant, i.e., there exists ''N'' such that ''G''''N'' = ''G''''N''+1 = ''G''''N''+2 = ... . We say that ''G'' satisfies the ACC on normal subgroups if every such sequence of normal subgroups of ''G'' eventually becomes constant. Likewise, one can define the descending chain condition on (normal) subgroups, by looking at all decreasing sequences of (normal) subgroups: : Clearly, all finite groups satisfy both ACC and DCC on subgroups. The infinite cyclic group satisfies ACC but not DCC, since (2) > (2)2 > (2)3 > ... is an infinite decreasing sequence of subgroups. On the other hand, the -torsion part of (the quasicyclic ''p''-group) satisfies DCC but not ACC. We say a group ''G'' is indecomposable if it cannot be written as a direct product of non-trivial subgroups ''G'' = ''H'' × ''K''. 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「Krull–Schmidt theorem」の詳細全文を読む スポンサード リンク
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