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global section functor : ウィキペディア英語版
global section functor

Let ''X'' be a topological space, and \mathrm (X, \mathcal C) denote the category of sheaves with values in \mathcal C. Then the map that associates to a sheaf \mathcal F its global sections \Gamma(X,\mathcal F) is a covariant functor to \mathcal C.
If \mathcal C is the category of abelian groups, then this functor is left exact. This important remark leads to the notion of sheaf cohomology, ''via'' derived functors.
==Examples==

*Let }_X) = \mathbb Z^, i.e. the direct sum indexed by connected components of ''X''
*Let \mathcal O_X be the sheaf of holomorphic functions on the compact connected complex manifold ''X'', then by the maximum principle, global sections are constant, ''ie.'' \Gamma (X, \mathcal O_X) = \mathbb C
*Let \mathcal O (i), i\in \mathbb Z denote the twisting sheaves on the projective space \mathbb P_k^n, then \Gamma (X, \mathcal O(d)) = k_d(\ldots, X_n ) for d \ge 0, and 0 for d <0.

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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