## Duke Mathematical Journal

### LS galleries, the path model, and MV cycles

#### Abstract

We give an interpretation of the path model of a representation (see [18]) of a complex semisimple algebraic group G in terms of the geometry of its affine Grassmannian. In this setting, the paths are replaced by Lakshmibai-Seshadri (LS) galleries in the affine Coxeter complex associated to the Weyl group of G. To explain the connection with geometry, consider a Demazure-Hansen-Bott-Samelson desingularization $\hat\Sigma(\lambda)$ of the closure of an orbit $G(\mathbb{C}[[t]])\cdot\lambda$ in the affine Grassmannian. The homology of $\hat\Sigma(\lambda)$ has a basis given by Białynicki-Birula (BB) cells, which are indexed by the T-fixed points in $\hat\Sigma(\lambda)$. Now the points of $\hat\Sigma(\lambda)$ can be identified with galleries of a fixed type in the affine Tits building associated to G, and the T-fixed points correspond in this language to combinatorial galleries of a fixed type in the affine Coxeter complex. We determine those galleries such that the associated cell has a nonempty intersection with $G(\mathbb{C}[[t]])\cdot\lambda$ (identified with an open subset of $\hat\Sigma(\lambda)$), and we show that the closures of the strata associated to LS galleries are exactly the Mirković-Vilonen (MV) cycles (see [25]) which form a basis of the representation V(λ) for the Langlands dual group $G^\vee$.

#### Article information

Source
Duke Math. J., Volume 127, Number 1 (2005), 35-88.

Dates
First available in Project Euclid: 4 March 2005

https://projecteuclid.org/euclid.dmj/1109963910

Digital Object Identifier
doi:10.1215/S0012-7094-04-12712-5

Mathematical Reviews number (MathSciNet)
MR2126496

Zentralblatt MATH identifier
1078.22007

#### Citation

Gaussent, S.; Littelmann, P. LS galleries, the path model, and MV cycles. Duke Math. J. 127 (2005), no. 1, 35--88. doi:10.1215/S0012-7094-04-12712-5. https://projecteuclid.org/euclid.dmj/1109963910

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