## 11.1 Introduction

A reference is the lectures by Serre in the Seminaire Cartan, see . Serre in turn refers to [Deuring] and [ANT]. We changed some of the proofs, in particular we used a fun argument of Rieffel to prove Wedderburn's theorem. Very likely this change is not an improvement and we strongly encourage the reader to read the original exposition by Serre.

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