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Let $K$ be a field and let $v:K\to G\sqcup\{+\infty\}$ be a valuation on $K$, where $G$ is a totally ordered abelian group. For each $\alpha$, define $V_\alpha=v^{-1}((\alpha,+\infty])$. Then $\{V_\alpha\}_{\alpha\in G}$ is an open neighborhood basis at zero of a unique topology $\mathscr{T}_v$ on $K$ (Bourbaki, Commutative Algebra, Ch. VI, § 5, no. 1). We can thus complete $K$ analytically (i.e., through the means of Cauchy filters; or Cauchy sequences if $\mathscr{T}_v$ is first-countable) and obtain a topological ring $\hat{K}$. Since $\mathscr{T}_v$ is well-behaved, $\hat{K}$ is actually a field (ibid., no. 3, Proposition 5 (a), or the link below for a proof when $\mathscr{T}_v$ is first-countable).

Recently a person told me that in the case $v$ is a discrete valuation one can construct $\hat{K}$ algebraically, via the following idea: if $\mathfrak{m}$ denotes the ideal of $v$, i.e., $\mathfrak{m}=v^{-1}((0,+\infty])$ (it is the maximal ideal of the local ring $A=v^{-1}([0,+\infty])$), then one consider the following inverse limit on topological abelian groups $$ \label{inv_lim}\tag{1} \hat{K}=\varprojlim_{n\geq 0} K/\mathfrak{m}^n $$ (where $\mathfrak{m}^0=A$). Note that this inverse limit a priori is not a ring, for $K/\mathfrak{m}^n$ are not rings, as $\mathfrak{m}^n$ is not an ideal of $K$. The same person told me that only a posteriori one can define ad-hoc a multiplication on $\hat{K}$, so that $\hat{K}$ becomes a topological field.

My only question is:

Where can I find the details of this construction? (Book, article, or link)

It is not that I don't believe in the completion of a field (I'm comfortable with the analytic completion via Cauchy sequences, and I understand why it's a topological field [ref]). However, I've been unable to find any mention on the literature of the inverse-limit approach to the completion of a field with respect to a discrete valuation, nor searching in Google or here in MSE.

  • As you're referring to an "ad hoc multiplication", one can do this I think: Fix a uniformiser $\pi$. Let $\hat{a},\hat{b} \in \varprojlim K/\mathfrak{m}^n$, where $\hat{a}=(a_n)_{n \in \Bbb N}$ one checks that $v(a_n)$ is eventually constant if $\hat{a} \neq 0$. Thus there exists a $m$ such that $\pi^ma_n \in A$ for all $n$. Do the same for $\hat{b}$. Now one can interpret $\pi^m\hat{a}$ and $\pi^k\hat{b}$ as elements of $\widehat{A}$, which does have a well-defined multiplication. Divide the result by $\pi^{m+k}$ and check that this doesn't depend on any choices. Not really a new construction – Lukas Heger Oct 14 '23 at 15:20
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    @LukasHeger $\def\m{\mathfrak{m}}$Yes, but one is yet to show that $\hat{K}$ becomes a topological field with that definition. The fact that $\hat{K}$ is a ring seems to be straightforward. I guess one should be able to show that $\hat{K}$ is also a field by defining $a_n'=0$ if $a_n=0$ and $a_n'=f^{-1}+\m^n$ for some $f\in a_n$, if $a_n\neq 0$, and then showing that $(a_n')\in\varprojlim K/\m^n$ is an inverse of $\hat{a}$. Also, there's the issue of showing that the product $\hat{K}\times\hat{K}\to\hat{K}$ and the map $(-)^{-1}:\hat{K}\to\hat{K}$ are continuous. – Elías Guisado Villalgordo Oct 14 '23 at 18:57
  • That's why I posted this as a comment and not as an answer. – Lukas Heger Oct 15 '23 at 01:04

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