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Base change map : ウィキペディア英語版
Base change map

In mathematics, the base change map relates the direct image and the pull-back of sheaves. More precisely, it is the following natural transformation of sheaves:
:g^
*(R^r f_
* \mathcal) \to R^r f'_
*(g'^
*\mathcal)
where f: X \to S, f':X' \to S', g':X' \to X, g:S' \to S are continuous maps between topological spaces that form a Cartesian square and \mathcal is a sheaf on ''X''.
In general topology, the map is an isomorphism under some mild technical conditions. An analogous result holds for étale cohomologies (with topological spaces replaced by sites), though more difficult. See proper base change theorem.
== General topology ==
If ''X'' is a Hausdorff topological space, ''S'' is a locally compact Hausdorff space and ''f'' is universally closed (i.e., X \times_S T \to T is closed for any continuous map T \to S), then
the base change map is an isomorphism. Indeed, we have: for s \in S,
:(R^r f_
* \mathcal)_s = \varinjlim H^r(U, \mathcal) = H^r(X_s, \mathcal), \quad X_s = f^(s)
and so for s = g(t)
:g^
* (R^r f_
* \mathcal)_t = H^r(X_s, \mathcal) = H^r(X'_t, g'^
* \mathcal) = R^r f'_
* (g'^
* \mathcal)_t.

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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