Algebraic character

Algebraic character

Algebraic character is a formal expression attached to a module in representation theory of semisimple Lie algebras that generalizes the character of a finite-dimensional representation and is analogous to the Harish-Chandra character of the representations of semisimple Lie groups.

Definition

Let mathfrak{g} be a semisimple Lie algebra with a fixed Cartan subalgebra mathfrak{h}, and let the abelian group A=mathbb{Z}mathfrak{h}^* consist of the (possibly infinite) formal integral linear combinations of e^{mu}, where muinmathfrak{h}^*, the (complex) vector space of weights. Suppose that V is a locally-finite weight module. Then the algebraic character of V is an element of Adefined by the formula:: ch(V)=sum_{mu}dim V_{mu}e^{mu}, where the sum is taken over all weight spaces of the module V.

Example

The algebraic character of the Verma module M_lambda with the highest weight lambda is given by the formula

: ch(M_{lambda})=frac{e^{lambda{prod_{alpha>0}(1-e^{-alpha})},

with the product taken over the set of positive roots.

Properties

Algebraic characters are defined for locally-finite weight modules and are "additive", i.e. the character of a direct sum of modules is the sum of their characters. On the other hand, although one can define multiplication of the formal exponents by the formula e^{mu}cdot e^{ u}=e^{mu+ u} and extend it to their "finite" linear combinations by linearity, this does not make A into a ring, because of the possibility of formal infinite sums. Thus the product of algebraic characters is well defined only in restricted situations; for example, for the case of a highest weight module, or a finite-dimensional module. In good situations, the algebraic character is "multiplicative", i.e., the character of the tensor product of two weight modules is the product of their characters.

Generalization

Characters also can be defined almost "verbatim" for weight modules over a Kac-Moody or generalized Kac-Moody Lie algebra.

See also

*Weyl-Kac character formula

References

*cite book|last = Weyl|first = Hermann|title = The Classical Groups: Their Invariants and Representations|publisher = Princeton University Press|date = 1946|isbn = 0691057567|url = http://books.google.com/books?id=zmzKSP2xTtYC|accessdate = 2007-03-26
*cite book|last = Kac|first = Victor G|title = Infinite-Dimensional Lie Algebras|publisher = Cambridge University Press|date = 1990|isbn = 0521466938|url = http://books.google.com/books?id=kuEjSb9teJwC|accessdate = 2007-03-26
*cite book|last = Wallach|first = Nolan R|coauthors = Goodman, Roe|title = Representations and Invariants of the Classical Groups|publisher = Cambridge University Press|date = 1998|isbn = 0521663482|url = http://books.google.com/books?vid=ISBN0521663482&id=MYFepb2yq1wC|accessdate = 2007-03-26


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