The moduli space of cactus flower curves and the virtual cactus group
Author:
Aleksei Ilin, Joel Kamnitzer, Yu Li, Piotr Przytycki, Leonid Rybnikov
Keyword:
Mathematics, Algebraic Geometry, Algebraic Geometry (math.AG), Group Theory (math.GR), Representation Theory (math.RT)
journal:
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date:
2023-08-13 16:00:00
Abstract
The space $ \mathfrak{ft}_n = \mathbb C^n/\mathbb C $ of $n$ points on the line modulo translation has a natural compactification $ \overline{\mathfrak{ft}}_n $ as a matroid Schubert variety. In this space, pairwise distances between points can be infinite; we call such a configuration of points a ``flower curve'', since we imagine multiple components joined into a flower. Within $ \mathfrak{ft}_n $, we have the space $ F_n = \mathbb C^n \setminus \Delta / \mathbb C $ of $ n$ distinct points. We introduce a natural compatification $ \overline F_n $ along with a map $ \overline F_n \rightarrow \overline{\mathfrak{ft}}_n $, whose fibres are products of genus 0 Deligne-Mumford spaces. We show that both $\overline{\mathfrak{ft}}_n$ and $\overline F_n$ are special fibers of $1$-parameter families whose generic fibers are, respectively, Losev-Manin and Deligne-Mumford moduli spaces of stable genus $0$ curves with $n+2$ marked points. We find combinatorial models for the real loci $ \overline{\mathfrak{ft}}_n(\mathbb R) $ and $ \overline F_n(\mathbb R) $. Using these models, we prove that these spaces are aspherical and that their equivariant fundamental groups are the virtual symmetric group and the virtual cactus groups, respectively. The deformation of $\overline F_n(\mathbb{R})$ to a real locus of the Deligne-Mumford space gives rise to a natural homomorphism from the affine cactus group to the virtual cactus group.