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In mathematics, Kan complexes and Kan fibrations are part of the theory of simplicial sets. Kan fibrations are the fibrations of the standard model category for simplicial sets and are therefore of fundamental importance. Kan complexes are the fibrant objects in this model category. The name is in honor of Daniel Kan. ==Definition== For each ''n'' ≥ 0, recall that the standard -simplex, , is the representable simplicial set : corresponding to all the other faces of .〔See Goerss and Jardine, page 7〕 Horns of the form sitting inside look like the black V at the top of the image to the right. If is a simplicial set, then maps : correspond to collections of -simplices satisfying a compatibility condition. Explicitly, this condition can be written as follows. Write the -simplices as a list and require that : for all with .〔See May, page 2〕 These conditions are satisfied for the -simplices of sitting inside . A map of simplicial sets is a Kan fibration if, for any and , and for any maps and such that , there exists a map such that and . Stated this way, the definition is very similar to that of fibrations in topology (see also homotopy lifting property), whence the name "fibration". Using the correspondence between -simplices of a simplicial set and morphisms (a consequence of the Yoneda lemma), this definition can be written in terms of simplices. The image of the map can be thought of as a horn as described above. Asking that factors through corresponds to requiring that there is an -simplex in whose faces make up the horn from (together with one other face). Then the required map corresponds to a simplex in whose faces include the horn from . The diagram to the right is an example in two dimensions. Since the black V in the lower diagram is filled in by the blue -simplex, if the black V above maps down to it then the striped blue -simplex has to exist, along with the dotted blue -simplex, mapping down in the obvious way.〔May uses this simplicial definition; see page 25〕 A simplicial set ''X'' is called a Kan complex if the map from ''X'' to 1, the one-point simplicial set, is a Kan fibration. In the model category for simplicial sets, is the terminal object and so a Kan complex is exactly the same as a fibrant object. 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「Kan fibration」の詳細全文を読む スポンサード リンク
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