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Preliminaries: Let $X$ be a projective smooth curve (scheme of finite type, integral and of dimension $1$) over a perfect field $F$. Let $K=K(X)$ be the function field of $X$ and for a closed point $P$ let $K_P$ be the completion of $K$ with respect to the discrete valuation $v_P$ at $\mathcal O_{X,P}$.

By following "Tate - differential and residues on curves" one can define in a very abstract way the residue at a closed point $P$ as a map:

$$\text{res}_P:\Omega^1_{K|F}\rightarrow F$$ and moreover it is not difficult to prove the following properties :

  1. Since $F$ is perfect, $\Omega^1_{K|F}$ has dimension $1$ over $K$. Therefore by chosing an uniformizer parameter $t\in\mathcal O_{X,P}$ for $v_P$ it is possible to write every non-zero differential form as $\omega=fdt$ where $f\in K^\times$

  2. Let $\omega=fdt$, then $\text{res}_P(\omega)=\text{Tr}_{k(P)|F}(a_{-1,f})$ where: $a_{-1,f}$ is the coefficient of $t^{-1}$ in the expansion of $f$ as Laurent power series and $k(P)$ is the residue field at $P$.


Is it possible to define a (complete) residue map: $$\text{res}_P:\Omega^1_{K_P|F}\rightarrow F$$

such that the properties $1.$ and $2.$ hold? I'd like to have a representation $\omega=gdt$ with $g\in K_P$ and $\text{res}_P(\omega)=\text{Tr}_{k(P)|F}(a_{-1,g})$.

It seems that the key point is that $\Omega^1_{K|F}$ is one dimensional, but what about $\Omega^1_{K_P|F}$?

EDIT: As a comment suggests, it should be better to work with $\Omega:=K_P\otimes_K\Omega^1_{K|F}$, so my question can be modified in the following way: Is it possible to extend $\text{res}_P$ on $\Omega$ in order to obtain the property 2. for the elements of the form $gdt$ with $g\in K_P$? One could define directly $\text{res}_P(\omega)=\text{Tr}_{k(P)|F}(a_{-1,g})$ but now I'm not sure about the independence from the choice of $t$.

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    $\begingroup$ The field extension $K_P/F$ has infinite transcendence degree. So the $K_P$-module of relative differentials $\Omega^1_{K_P|F}$ is infinitely generated. Are you sure you would not prefer to work with he $K_P$-module $K_P\otimes_K \Omega^1_{K|F}$? That still includes elements of the form $g\ dt$ for $t\in K$ a uniformizing element at $p$ and $g\in K_P$ arbitrary. $\endgroup$ Commented Mar 3, 2016 at 11:24
  • $\begingroup$ You are right $K_P\otimes_K\Omega^1_{K|F}$ is a more reasonable module. Is it possible to extend the residue map to this module? $\endgroup$
    – Dubious
    Commented Mar 3, 2016 at 11:33
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    $\begingroup$ Yes, it is possible to extend it, for instance because $\Omega$ is the completion of $\Omega_{K|F}$ with respect to the topology defined by $v_P$, and $res_P$ is continuous in this topology (as follows immediately from the explicit formula). $\endgroup$
    – t3suji
    Commented Mar 3, 2016 at 15:28
  • $\begingroup$ @t3suji, this seems a very good explanation. If you write a quite complete answer, I will accept it. $\endgroup$
    – Dubious
    Commented Mar 3, 2016 at 18:22

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