7.2 K -affine structure on smooth points [03VP]
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7.2 -affine structure on smooth points
Starting from this section till the end of the paper (except of the Section 11.7) we will assume the following
Zero Charactersistic Assumption.
is a complete non-archimedean local field such that
its residue field has characteristic zero.
Let be a -analytic manifold of dimension and we are given a continuous map , where is a topological space. Then carries a -affine structure (Theorem 1). Suppose that there is an open -analytic submanifold such that and there is a nonwhere vanishing analytic form . We are going to define a -affine function similarly to the definition of the function in Section 4.1. Namely, in local coordinates we consider the expression . This is an invertible function, and we define as . The independence on the choice of coordinates follows from the following lemma
Lemma 2
Let be two systems of invertible coordinates on for some connected open . Then
Proof: By Lemma 1 from Section 4.1 we know that as any invertible function can be written in form for some nonzero and a multi-index . Vectors form a basis of , as follows from the condition that form a coordinate system. Therefore, after applying the change of coordinates preserving form up to sign, we may assume that . The Jacobian matrix of the transformation is the identity matrix plus terms of size . Therefore its determinant has norm equal to 1.
Now we make the following
Constant Norm Assumption. The function
is locally constant.
Theorem 4
If the Constant Norm Assumption is satisfied then there is a -affine structure on compatible with the -affine structure (see Section 4.1).
Proof. Let us write in local coordinates . Define residue as the constant term in the Laurent expansion . It is easy to see that does not depend (up to a sign) on the choice of local coordinates. For non-vanishing everywhere satisfying Constant Norm Assumption we have . Therefore we have .
Let us return to the proof of the Theorem. Let be the sheaf of abelian groups consisting of such that . Then we have an exact sequence of sheaves
where denotes the constant sheaf with the fiber being the ring of integers of . Indeed we embed into as constant functions. The projection assigns to the function the linear part of the corresponding -affine function .
Notice that if is a connected domain then any can be written (non-canonically) as , where and in .
We define an epimorphism of sheaves by formula
Here and are understood as infinite convergent series (in order to make sense of them we use Zero Characteristic Assumption).
It is easy to see that is well-defined. Then the exact sequence of sheaves
defines a -affine structure on compatible with . This concludes proof of the Theorem.
Notice that the above proof gives an explicit construction of the -affine structure. We will denote it by . It is easy to see that this -affine structure does not change if we make a rescaling .