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つづき

Period rings and comparison isomorphisms in arithmetic geometry
The general strategy of p-adic Hodge theory, introduced by Fontaine, is to construct certain so-called period rings[3] such as BdR, Bst, Bcris, and BHT which have both an action by GK and some linear algebraic structure and to consider so-called Dieudonne modules

D_{B}(V)=(B\otimes _{\mathbf {Q} _{p}}V)^{G_{K}}}
(where B is a period ring, and V is a p-adic representation) which no longer have a GK-action, but are endowed with linear algebraic structures inherited from the ring B.
In particular, they are vector spaces over the fixed field E:=B^{G_{K}}}E:=B^{{G_{K}}}.[4] This construction fits into the formalism of B-admissible representations introduced by Fontaine.
For a period ring like the aforementioned ones B? (for ? = HT, dR, st, cris), the category of p-adic representations Rep?(K) mentioned above is the category of B?-admissible ones, i.e. those p-adic representations V for which

\dim _{E}D_{B_{\ast }}(V)=\dim _{\mathbf {Q} _{p}}V}
or, equivalently, the comparison morphism

\alpha _{V}:B_{\ast }\otimes _{E}D_{B_{\ast }}(V)\longrightarrow B_{\ast }\otimes _{\mathbf {Q} _{p}}V}
is an isomorphism.

This formalism (and the name period ring) grew out of a few results and conjectures regarding comparison isomorphisms in arithmetic and complex geometry:

If X is a proper smooth scheme over C, there is a classical comparison isomorphism between the algebraic de Rham cohomology of X over C and the singular cohomology of X(C)
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