Theorem 11.3.3 (0747)—The Stacks project

Table of contents
Part 1: Preliminaries
Chapter 11: Brauer groups
Section 11.3: Wedderburn's theorem
Theorem 11.3.3 (cite)

previous lemma

Simple finite algebras over a field are matrix algebras over a skew field.

Theorem 11.3.3. Let $A$ be a simple finite $k$-algebra. Then $A$ is a matrix algebra over a finite $k$-algebra $K$ which is a skew field.

Proof. We may choose a simple submodule $M \subset A$ and then the $k$-algebra $K = \text{End}_ A(M)$ is a skew field, see Lemma 11.3.2. By Lemma 11.3.1 we see that $A = \text{End}_ K(M)$. Since $K$ is a skew field and $M$ is finitely generated (since $\dim _ k(M) < \infty $) we see that $M$ is finite free as a left $K$-module. It follows immediately that $A \cong \text{Mat}(n \times n, K^{op})$. $\square$

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Comment #1367 by Herman Rohrbach on March 20, 2015 at 13:26

Suggested slogan: Simple finite algebras over a field are matrix algebras over a skew field.

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6 comment(s) on Section 11.3: Wedderburn's theorem

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