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• Print publication year: 2016
• Online publication date: August 2016

# 2 - The Structure of Algebras for Which Every Module Is Semisimple 15

## Summary

In this chapter, we present the Artin–Wedderburn structure theorem for semisimple algebras and its immediate consequences. This theorem is the ringtheoretic manifestation of the module theoretic hypothesis of semisimplicity that was introduced in Chapter 1, and it shows that the kind of algebras that can arise when all modules are semisimple is very restricted. The theorem applies to group algebras over a field in which the group order is invertible (as a consequence of Maschke's Theorem), but since the result holds in greater generality we will assume we are working with a finite-dimensional algebra A over a field k.

Schur's Lemma andWedderburn's Theorem

Possibly the most important single technique in representation theory is to consider endomorphism rings. It is the main technique of this chapter, and we will see it in use throughout this book. The first result is basic and will be used time and time again.

Theorem (Schur's Lemma). Let A be a ring with a 1 and let S1 and S2 be simple A-modules. Then HomA(S1, S2) = 0 unless S1S2, in which case the endomorphism ring EndA(S1) is a division ring. If A is a finite-dimensional algebra over an algebraically closed field k, then every A-module endomorphism of S1 is multiplication by some scalar. Thus, EndA(S1) ≅ k in this case.

Proof. Suppose θ : S1S2 is a nonzero homomorphism. Then 0 ≠ θ (S1) ⊆ S2, so θ (S1) = S2 by simplicity of S2, and we see that θ is surjective. Thus, Ker θS1, so Ker θ = 0 by simplicity of S1, and θ is injective. Therefore, θ is invertible, S1S2, and EndA(S1) is a division ring.

If A is a finite-dimensional k-algebra and k is algebraically closed then S1 is a finite-dimensional vector space. Let θ be an A-module endomorphism of S1 and let λ be an eigenvalue of θ. Now (θλI) : S1S1 is a singular endomorphism of A-modules, so θλI = 0 and θ = λI.