# Representations are determined by characters : Groups and Lie algebras

I know that any finite-dimensional complex representation of a finite group $$G$$ is determined by its characters. This is immediate, in view of the complete reducibility of this category modules.

My question is, do we need complete reducibility when we work in a category of modules over complex finite-dimensional semisimple Lie algebras in order to objects are characterized by their characters?

Thank you.

• Assume a module $M$ is determined up to isomorphism by its character $\chi$. Write $\chi = \chi_1 + \cdots + \chi_n$ as a sum of irreducibles. Let $M_i$ be a simple module affording $\chi_i$. The direct sum $M_1 \oplus \cdots \oplus M_n$ affords $\chi$ and by assumption is isomorphic to $M$. Hence $M$ is a direct sum of simple modules, so semisimple. Complete reducibility holds. Dec 28, 2019 at 7:41
• @JayTaylor please explain, why is it always possible to write a character (of a module over a Lie algebra) as a sum of characters of irreducibles? $\chi = \chi_1 + \cdots + \chi_n$? Dec 28, 2019 at 8:02
• Characters are additive in short exact sequences, so the character can't tell the difference between a direct sum and a nontrivial extension. In particular, the character of any module can be written as a sum of characters of simple modules (namely those of its Jordan-Hölder composition series). Dec 28, 2019 at 10:43
• @BertramArnold Such series exists for infinite-dimensional modules as well? because I remember for arbitrary highest weight modules of Kac-Moody algebras such series doesn't exist. Kindly explain to me more. thanks. Dec 28, 2019 at 13:29
• For finite dimensional algebras over algebraically closed fields, simple modules are characterized up to isomorphism by their characters. Probably algebraically closed is not needed. Dec 28, 2019 at 15:37

Doc, I am not sure what your question is, but the answer is yes. Whatever definition of character you are using, any two extensions of $$M$$ by $$N$$ will have the same character. Thus, a non-trivial extension has the character as $$M\oplus N$$. Bingo: non-isomorphic modules will have the same characters...
• @GA316 In this context, I believe it means a short exact sequence of the form $0\to M\to M'\to N\to 0$ With a split short exact sequence, $M'\cong M\oplus N$, but if you do not have complete reducibility, then there will exist non-split short exact sequences. Jan 6, 2020 at 8:17
• @Aaron Exactically, Aaron explained it well. A character is always a homomorphism from the Grothenideck group to some other group. Since $M^\prime$ and $M\oplus N$ give the same element of the Grothendieck group, they cannot be distinguished by a character... Jan 6, 2020 at 12:11