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Q-construction : ウィキペディア英語版
Q-construction
In algebra, Quillen's Q-construction associates to an exact category (e.g., an abelian category) an algebraic K-theory. More precisely, given an exact category ''C'', the construction creates a topological space B^+C so that \pi_0 (B^+C) is the Grothendieck group of ''C'' and, when ''C'' is the category of finitely generated projective modules over a ring ''R'', for i = 0, 1, 2, \pi_i (B^+C) is the ''i''-th K-group of ''R'' in the classical sense. (The notation "+" is meant to suggest the construction adds more to the classifying space ''BC''.) One puts
:K_i(C) = \pi_i(B^+C)
and call it the ''i''-th K-group of ''C''. Similarly, the ''i''-th K-group of ''C'' with coefficients in a group ''G'' is defined as the homotopy group with coefficients:
:K_i(C; G) = \pi_i(B^+ C; G).
The construction is widely applicable and is used to define an algebraic K-theory in a non-classical context. For example, one can define equivariant K-theory as \pi_
* of B^+ of the category of equivariant sheaves on a scheme.
Waldhausen's S-construction generalizes the Q-construction in a stable sense; in fact, the former, which uses a more general Waldhausen category, produces a spectrum instead of a space. Grayson's binary complex also gives a construction of algebraic K-theory for exact categories.〔Daniel R. Grayson, (Algebraic K-theory via binary complexes )〕 See also module spectrum#K-theory for a K-theory of a ring spectrum.
== Operations ==
Every ring homomorphism R \to S induces B^+P(R) \to B^+P(S) and thus K_i(P(R)) = K_i(R) \to K_i(S) where P(R) is the category of finitely generated projective modules over ''R''. One can easily show this map (called transfer) agrees with one defined in Milnor's ''Introduction to algebraic K-theory''. The construction is also compatible with the suspension of a ring (cf. Grayson).

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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