Weyl's theorem on complete reducibility

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In algebra, Weyl's theorem on complete reducibility is a fundamental result in the theory of Lie algebra representations. Let \mathfrak{g} be a semisimple Lie algebra over a field of characteristic zero. The theorem states that every finite-dimensional module over \mathfrak{g} is semisimple as a module (i.e., a direct sum of simple modules.)

The theorem is a consequence of Whitehead's lemma (see Weibel's homological algebra book). Weyl's original proof was analytic in nature: it famously used the unitarian trick.

A Lie algebra is called reductive if its adjoint representation is semisimple. Thus, the theorem says that a semisimple Lie algebra is reductive. (But this can be seen more directly.)

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