# nLab BGG resolution

Contents

### Context

#### Representation theory

representation theory

geometric representation theory

# Contents

## Idea

A BGG resolution is a certain projective resolution of certain representations.

More in detail, give a finite dimensional semisimple complex Lie algebra $\mathfrak{g}$ with Cartan subalgebra $\mathfrak{h}$, and a positive root system, and let $P^+\subset \mathfrak{h}^*$ be the set of dominant integral weights. Then for every $\lambda\in P^+$ consider the corresponding finite dimensional left $U(\mathfrak{g})$-module $L(\lambda)$. Certain resolutions of $L(\lambda)$ are defined in a series of papers of Bernstein, Gel’fand and Gel’fand (BBG 7x) and are now called BGG resolutions. There are also generalizations, e.g. for Kac-Moody algebras.

These resolutions have a natural incarnation in terms of complexes of sections of tractor bundles over flag varieties or more generally over homogeneous parabolic Klein geometries. As such, there are generalizations of the construction to more general parabolic Cartan geometries, called curved BGG sequences. See for instance (Calderbank-Diemer 00, theorem 3.6).

BGG resolutions may be used to construct resolutions of sheaves of constant functions on Klein geometries/coset space $G/H$ that are more efficient (smaller) that the general resolution given by the de Rham complex (the Poincare lemma). In this way BGG resolutions are used notably for computation in Leray spectral sequences as they appear in Penrose transforms (Baston-Eastwood 89, chapter 8).

## Properties

### Curved $A_\infty$-Structure

Under a cup product the BGG sequence becomes a curved A-infinity algebra. (Calderbank-Diemer 00, section 6)

## References

### Original case over flag variety

• eom: Alvany Rocha, BGG resolution; wikipedia, category O

• I.N. Bernstein, I. M. Gelfand, S. I. Gelfand, Structure of representations generated by vectors of highest weight, Funkts. Anal. Prilozh. 5: 1 (1971) pp. 1–9; A certain category of -modules, Funkts. Anal. Prilozh. 10: 2 (1976) pp. 1–8; Differential operators on the base affine space and a study of $\mathfrak{g}$-modules), I.M. Gelfand (ed.), Lie groups and their representations, Proc. Summer School on Group Representations, Janos Bolyai Math. Soc.& Wiley (1975) pp. 39–64

• James E. Humphreys, Representations of semisimple Lie algebras in the BGG category $\mathcal{O}$, 2008, pdf

• M. Falk, V. Schechtman, A. Varchenko, BGG resolutions via configuration spaces, arxiv/1309.7811

We study the blow-ups of configuration spaces. These spaces have a structure of what we call an Orlik-Solomon manifold; it allows us to compute the intersection cohomology of certain flat connections with logarithmic singularities using some Aomoto type complexes of logarithmic forms. Using this construction we realize geometrically the $sl_2$ Bernstein - Gelfand - Gelfand resolution as an Aomoto complex.

• Sergey Arkhipov, A new construction of the semi-infinite BGG resolution, q-alg/9605043

Discussion in the context of the Penrose transform includes

• Michael Eastwood, Variations on the de Rham complex, Notices Amer. Math. Soc. 46 (1999), no. 11, 1368–1376 pdf

• R. J. Baston, M. G. Eastwood, The Penrose transform, Oxford Univ. Press, New York, 1989; MR92j:32112

### Curved generalization to parabolic Cartan geometries

The generalization from coset spaces to parabolic Cartan geometries (curved BGG sequences) is discussed in

Last revised on February 17, 2018 at 16:17:46. See the history of this page for a list of all contributions to it.