Spahn the fundamental group and Galois theory (changes)

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Let $o$ be a Dedekind domain, let $K:=Quot(o)$ denote its quotient field, let $L/K$ be a finite separable field ~~extension,~~ extension ~~let~~ of degree $O n \supset o$ ~~O\supset~~ n o , be let ~~the~~ ~~integral~~ ~~closure~~ of $O \supset o$ O\supset o in be the integral closure of $L o$ L o . ~~Then~~ in $O L$ O L. Then $O$ is in particular a Dedekind domain

Let for

o\stackrel{i}{\to} O\to L

$f:=Spec(i):Spec(O)\to Spec(o)$ be the induced map between the ring spectra . calledramified covering

Let $p\in Spec(o)$ be a maximal prime ideal. Then the ideal $pO$ in $O$ has a unique product decomposition

pO=P_1^{e_1}\dots P_r^{e_r}

with different $P_i\in Spec(O)$ . It is custom to introduce some classifying vocabulary depending on the kind of this decomposition and the degree of the field extension $f_i:=[O/P_i:o/p]$ : the $e_i$ are called ramification indices and the $f_i$ are called inertia degrees (see e.g. the German wikipedia for more details). These satisfy the fundamental identity $\Sigma_i e_i f_i =n$ and every point $p\in Spec(O)$ has $\le n$ preimages. If $p$ has $\lt n$ preimages then $p$ is called ramified.

Let now $L/K$ be galois then the Galois automorphisms $\sigma\in Gal(L/K):=Aut_K(L)$ (i.e. the automorphisms of $L$ which restrict to the identity on $K$ ) induce automorphisms of schemes $Spec(\sigma)$ and (since $\sigma$ fixes $o$ ) the diagram

\array{ Spec(O)&\stackrel{\Spec(\sigma)}{\to}&\Spec(O) \\ \downarrow^f &\swarrow \\ Spec(o) }

commutes. $\tau:=\Spec(\sigma)$ is an example of a cover automorphism (also called cover transformation or Deck transformation). Since $Spec:Rings\to Schemes$ is an equivalence of categories we have an isomorphism

Gal(L/K)\simeq Aut_{Spec(o)}(Spec(O))

where the object to the right we have the group of cover automorphism.

One can show that there is a maximal unramified extension ( $e_i=1$ and the extensions $O/P_i:o/p$ being separable) $\tilde K$ of $K$ and the scheme $\tilde Y:=Spec(\tilde o)$ where $\tilde o$ denotes the integral closure of $o$ in $\tilde K$ . Then $f:\tilde Y\to Y:=Spec(O)$ satisfies the axioms of a universal covering and consequently we define on the side of schemes the fundamental group

\pi_1/Y):=Aut_Y(\tilde Y)\simeq Gal(\tilde K/ K)

References

Jürgen Neukirch, algebraic number theory, I.§13.6

Last revised on August 25, 2012 at 23:22:01. See the history of this page for a list of all contributions to it.