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Semi-Simple Lie Algebra Representations

  1. Oct 29, 2007 #1
    I'm trying to prove that any representation of a semisimple Lie algebra can be uniquely decomposed into irreducible representations.

    I have seen some sketches of proofs that show that any representation [tex] \phi [/tex] of a semisimple Lie algebra which acts on a finite-dimensional complex vector space [tex] V [/tex] is completely reducible (i.e. [tex] V = V_1 \oplus V_2 \oplus \cdots \oplus V_k [/tex], such that the restriction of [tex] \phi [/tex] to each [tex] V_i [/tex] is irreducible). But how do we know that this decomposition is unique?
    Last edited: Oct 29, 2007
  2. jcsd
  3. Oct 29, 2007 #2

    matt grime

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    You show that in two decompositions

    [tex]V\cong \oplus V_i \cong W_j[/tex]

    that one of the summands V_r is isomorphic to one W_s, wlog V_1 and W_1, then use induction on V/V_1.
  4. Oct 29, 2007 #3

    So two representations [tex] \phi [/tex] and [tex] \phi ' [/tex] for [tex] \mathfrak{g} [/tex] are said to be equivalent if there is an isomorphism [tex] E [/tex] between the underlying vector spaces such that [tex] E \phi (X) = \phi' (X) E [/tex], [tex] \forall X \in \mathfrak{g} [/tex]. How do we know that the restriction of a representation to a given [tex] V_i [/tex] above is equivalent to a restriction of that representation to a given [tex] W_i [/tex]?
    Note that, by irreducible, I mean a vector space with no invariant subspaces under the given representation (except zero and itself).
    Last edited: Oct 29, 2007
  5. Oct 30, 2007 #4

    matt grime

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    The identity map on V restricts to the identity map on V_1. This must then factor through one of the W_i, wlog W_1, hence V_1 is isomorphic to W_1.
  6. Oct 31, 2007 #5
    Could you be a little more explicit?
    Note that I'm trying to prove this for an arbitrary representation.
  7. Oct 31, 2007 #6

    matt grime

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    I know what you're trying to do; I was explicit; where do you see me do something not for an arbitrary representation? You want to show that V_1 is isomorphic to W_1 (after reordering the indices of the W_i). That is what I did.

    1. You want to find an isomrphism from V_1 to W_1, that is maps f,g such that gf= Id on V_1 and fg=Id on W_1. That is the definition of isomorphism.

    2. Consider Id on V. This maps \oplus V_i to \oplus W_j, and back again.

    3. Look at V_1 mapping under Id to \oplus W_j.

    4. The image must lie in at least one of the W_j. WLOG W_1.

    5. So we have a map, Id restricted to V_1, call it f, that maps to W_1.

    6. Call Id restricted to W_1 g.

    7. What is gf? It is Id on V_1.

    I really can't be any more explicit. In fact that is precisely what I wrote before.
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