ScalingStacks

7.2 K -affine structure on smooth points [03VP]

Original official author HTML, exact retained edition. Historical TeX conversion verdicts remain unchanged. Cited-edition alignment and mathematical self-containment are not assessed.

Complete original source context · Original author HTML

7.2 KK-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. KK is a complete non-archimedean local field such that its residue field has characteristic zero.

Let XX be a KK-analytic manifold of dimension nn and we are given a continuous map π:X→B\pi:X\to B, where BB is a topological space. Then Bs​mB^{sm} carries a 𝐙{\bf Z}-affine structure (Theorem 1). Suppose that there is an open KK-analytic submanifold U⊂XU\subset X such that π−1​(Bs​m)⊂U\pi^{-1}(B^{sm})\subset U and there is a nonwhere vanishing analytic form Ω∈Γ⁡(U,ΩXn)\Omega\in\Gamma(U,\Omega_{X}^{n}). We are going to define a 𝐙{\bf Z}-affine function V​a​l​(Ω)Val(\Omega) similarly to the definition of the function V​a​l​(φ)Val(\varphi) in Section 4.1. Namely, in local coordinates (z1,…,zn)(z_{1},...,z_{n}) we consider the expression φ:=Ω/⋀1≤i≤n(d​zi/zi)\varphi:=\Omega/\bigwedge_{1\leq i\leq n}(dz_{i}/z_{i}). This is an invertible function, and we define V​a​l​(Ω)Val(\Omega) as V​a​l​(φ)Val(\varphi). The independence on the choice of coordinates follows from the following lemma

Lemma 2

Let (zi)i=1,…,n,(zi′)i=1,…,n(z_{i})_{i=1,\dots,n},\,\,(z^{\prime}_{i})_{i=1,\dots,n} be two systems of invertible coordinates on π−1​(U)\pi^{-1}(U) for some connected open U⊂Bs​mU\subset B^{sm}. Then

|(⋀1≤i≤n(d​zi/zi))/(⋀1≤i≤n(d​zi′/zi′))|x=1​∀x∈π−1​(U).\left|\left({\textstyle\bigwedge_{1\leq i\leq n}(dz_{i}/z_{i})}\right)/\left({\textstyle\bigwedge_{1\leq i\leq n}(dz^{\prime}_{i}/z^{\prime}_{i})}\right)\right|_{x}=1\,\,\,\forall x\in\pi^{-1}(U)\,\,.

Proof: By Lemma 1 from Section 4.1 we know that zi′z_{i}^{\prime} as any invertible function can be written in form ci​zI(i)​(1+o⁡(1))c_{i}z^{I^{(i)}}(1+o(1)) for some nonzero ci∈Kc_{i}\in K and a multi-index I(i)∈𝐙nI^{(i)}\in{\bf Z}^{n}. Vectors I(1),…,I(n)I^{(1)},\dots,I^{(n)} form a basis of 𝐙n{\bf Z}^{n}, as follows from the condition that z1′,…,zn′z_{1}^{\prime},\dots,z_{n}^{\prime} form a coordinate system. Therefore, after applying the change of coordinates zi↦ci​zI(i)z_{i}\mapsto c_{i}z^{I^{(i)}} preserving form ⋀id​zi/zi\bigwedge_{i}dz_{i}/z_{i} up to sign, we may assume that zi′=(1+o⁡(1))​ziz_{i}^{\prime}=(1+o(1))z_{i}. The Jacobian matrix of the transformation (zi)→(zi′)(z_{i})\to(z_{i}^{\prime}) is the identity matrix plus terms of size o⁡(1)o(1). Therefore its determinant has norm equal to 1. ■\blacksquare

Now we make the following
Constant Norm Assumption. The function V​a​l​(φ)Val(\varphi) is locally constant.

Theorem 4

If the Constant Norm Assumption is satisfied then there is a KK-affine structure on Bs​mB^{sm} compatible with the 𝐙{\bf Z}-affine structure A​f​f𝐙,Bs​mc​a​nAff_{{{\bf Z}},B^{sm}}^{can} (see Section 4.1).

Proof. Let us write in local coordinates Ω=φ⁡(z1,…​zn)​⋀1≤i≤nd​zizi\Omega=\varphi(z_{1},...z_{n})\bigwedge_{1\leq i\leq n}{dz_{i}\over z_{i}}. Define residue R​e​s​(Ω)∈KRes(\Omega)\in K as the constant term φ0\varphi_{0} in the Laurent expansion φ⁡(z1,…​zn)=∑I∈𝐙nφI​zI\varphi(z_{1},...z_{n})=\sum_{I\in{\bf Z}^{n}}\varphi_{I}z^{I}. It is easy to see that R​e​s​(Ω)Res(\Omega) does not depend (up to a sign) on the choice of local coordinates. For non-vanishing everywhere Ω\Omega satisfying Constant Norm Assumption we have exp⁡(−V​a​l​(φ))=|φ|=|φ0|\exp(-Val(\varphi))=|\varphi|=|\varphi_{0}|. Therefore we have R​e​s​(Ω)≠0Res(\Omega)\neq 0.

Let us return to the proof of the Theorem. Let FF be the sheaf of abelian groups F⊂π∗​(𝒪X×)F\subset\pi_{\ast}({\cal O}_{X}^{\times}) consisting of ff such that V​a​l​(f)=0Val(f)=0. Then we have an exact sequence of sheaves

0→K×/𝒪K×→π∗​(𝒪X×)/F→(TX∗)𝐙→0,0\to K^{\times}/{\cal O}_{K}^{\times}\to\pi_{\ast}({\cal O}_{X}^{\times})/F\to(T_{X}^{\ast})^{{\bf Z}}\to 0\,\,\,,

where 𝒪K{\cal O}_{K} denotes the constant sheaf with the fiber being the ring of integers of KK. Indeed we embed K×/𝒪K×K^{\times}/{\cal O}_{K}^{\times} into π∗​(𝒪X×)/F\pi_{\ast}({\cal O}_{X}^{\times})/F as constant functions. The projection π∗​(𝒪X×)/F→(TX∗)𝐙\pi_{\ast}({\cal O}_{X}^{\times})/F\to(T_{X}^{\ast})^{{\bf Z}} assigns to the function ff the linear part of the corresponding 𝐙{\bf Z}-affine function V​a​l​(f)Val(f).

Notice that if U⊂Bs​mU\subset B^{sm} is a connected domain then any f∈Γ⁡(U,F)f\in\Gamma(U,F) can be written (non-canonically) as f=a⁡(1+r)f=a(1+r), where a∈𝒪K×a\in{\cal O}_{K}^{\times} and r=o⁡(1)r=o(1) in π−1​(U)\pi^{-1}(U).

We define an epimorphism of sheaves pΩ:F↠𝒪K×p_{\Omega}:F\twoheadrightarrow{\cal O}_{K}^{\times} by formula

pΩ​(f)=pΩ​(a⁡(1+r))=a​exp⁡(R​e​s​(Ω​log⁡(1+r))R​e​s​(Ω)).p_{\Omega}(f)=p_{\Omega}(a(1+r))=a\,\exp\left({Res(\Omega\,\log(1+r))\over{Res(\Omega)}}\right)\,\,.

Here exp\exp and log\log are understood as infinite convergent series (in order to make sense of them we use Zero Characteristic Assumption).

It is easy to see that pΩp_{\Omega} is well-defined. Then the exact sequence of sheaves

1→K×→π∗​(𝒪X×)/ker⁡(pΩ)→(TX∗)𝐙→11\to K^{\times}\to\pi_{\ast}({\cal O}_{X}^{\times})/\ker(p_{\Omega})\to(T_{X}^{\ast})^{{\bf Z}}\to 1

defines a KK-affine structure on Bs​mB^{sm} compatible with A​f​f𝐙,Bs​mc​a​nAff_{{{\bf Z}},B^{sm}}^{can}. This concludes proof of the Theorem. ■\blacksquare

Notice that the above proof gives an explicit construction of the KK-affine structure. We will denote it by A​f​fK,Bs​mΩAff_{K,B^{sm}}^{\Omega}. It is easy to see that this KK-affine structure does not change if we make a rescaling Ω↦c​Ω,c∈K×\Omega\mapsto c\Omega,c\in K^{\times}.

Original mathematics by the credited authors. Source-backed reader collection; mathematical self-containment is not assessed.