Verma modules can be used to prove that an irreducible highest weight module with highest weight is finite-dimensional, if and only if the weight is dominant and integral. Their homomorphisms correspond to invariant differential operators over flag manifolds.
Definition of Verma modules
The definition relies on a stack of relatively dense notation. Let be a field and denote the following:
- , a semisimple Lie algebra over , with universal enveloping algebra .
- , a Borel subalgebra of , with universal enveloping algebra .
- , a Cartan subalgebra of . We do not consider its universal enveloping algebra.
- , a fixed weight.
To define the Verma module, we begin by defining some other modules:
- , the one-dimensional -vector space (i.e. whose underlying set is itself) together with a -module structure such that acts as multiplication by and the positive root spaces act trivially. As is a left -module, it is consequently a left -module.
- Using the Poincaré–Birkhoff–Witt theorem, there is a natural right -module structure on by right multiplication of a subalgebra. is naturally a left -module, and together with this structure, it is a -bimodule.
Now we can define the Verma module (with respect to ) as
which is naturally a left -module (i.e. a representation of ). The Poincaré–Birkhoff–Witt theorem implies that the underlying vector space of is isomorphic to
where is the Lie subalgebra generated by the negative root spaces of .
Verma modules, considered as -modules, are highest weight modules, i.e. they are generated by a highest weight vector. This highest weight vector is (the first is the unit in and the second is the unit in the field , considered as the -module ) and it has weight .
Verma modules are weight modules, i.e. is a direct sum of all its weight spaces. Each weight space in is finite-dimensional and the dimension of the -weight space is the number of possibilities how to obtain as a sum of positive roots (this is closely related to the so-called Kostant partition function).
Verma modules have a very important property: If is any representation generated by a highest weight vector of weight , there is a surjective -homomorphism That is, all representations with highest weight that are generated by the highest weight vector (so called highest weight modules) are quotients of
The Verma module itself is irreducible if and only if none of the coordinates of in the basis of fundamental weights is from the set .
The Verma module is called regular, if its highest weight λ is on the affine Weyl orbit of a dominant weight . In other word, there exist an element w of the Weyl group W such that
where is the affine action of the Weyl group.
The Verma module is called singular, if there is no dominant weight on the affine orbit of λ. In this case, there exists a weight so that is on the wall of the fundamental Weyl chamber (δ is the sum of all fundamental weights).
Homomorphisms of Verma modules
For any two weights a non-trivial homomorphism
Each homomorphism of Verma modules is injective and the dimension
for any . So, there exists a nonzero if and only if is isomorphic to a (unique) submodule of .
There exists a nonzero homomorphism if and only if there exists
a sequence of weights
such that for some positive roots (and is the corresponding root reflection and is the sum of all fundamental weights) and for each is a natural number ( is the coroot associated to the root ).
if and only if
in the Bruhat ordering of the Weyl group.
be a sequence of -modules so that the quotient B/A is irreducible with highest weight μ. Then there exists a nonzero homomorphism .
An easy consequence of this is, that for any highest weight modules such that
there exists a nonzero homomorphism .
if and only if
There exists an exact sequence of -homomorphisms
where n is the length of the largest element of the Weyl group.
A similar resolution exists for generalized Verma modules as well. It is denoted shortly as the BGG resolution.
Recently, these resolutions were studied in special cases, because of their connections to invariant differential operators in a special type of Cartan geometry, the parabolic geometries. These are Cartan geometries modeled on the pair (G, P) where G is a Lie group and P a parabolic subgroup).
- Bernstein I.N., Gelfand I.M., Gelfand S.I., Structure of Representations that are generated by vectors of highest weight, Functional. Anal. Appl. 5 (1971)
- Verma N., Structure of certain induced representations of complex semisimple Lie algebras, Bull. Amer. Math. Soc. 74 (1968)
- Bernstein I. N., Gelfand I. M., Gelfand S. I., Differential Operators on the Base Affine Space and a Study of g-Modules, Lie Groups and Their Representations, I. M. Gelfand, Ed., Adam Hilger, London, 1975.
- For more information, see: Eastwood M., Variations on the de Rham complex, Notices Amer. Math. Soc, 1999 - ams.org. Calderbank D.M., Diemer T., Differential invariants and curved Bernstein-Gelfand-Gelfand sequences, Arxiv preprint math.DG/0001158, 2000 - arxiv.org . Cap A., Slovak J., Soucek V., Bernstein-Gelfand-Gelfand sequences, Arxiv preprint math.DG/0001164, 2000 - arxiv.org 
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