Fundamental groups of moduli spaces of real weighted stable curves
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arXiv
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| Format: | Preprint |
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2025
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| _version_ | 1866916666147864576 |
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| author | Levinson, Jake Liu, Haggai |
| author_facet | Levinson, Jake Liu, Haggai |
| contents | The ordinary and $S_n$-equivariant fundamental groups of the moduli space $\overline{M_{0,n+1}}(\mathbb{R})$ of real $(n+1)$-marked stable curves of genus $0$ are known as \emph{cactus groups} $J_n$ and have applications both in geometry and the representation theory of Lie algebras. In this paper, we compute the ordinary and $S_n$-equivariant fundamental groups of the Hassett space of weighted real stable curves $\overline{M_{0,\mathcal{A}}}(\mathbb{R})$ with $S_n$-symmetric weight vector $\mathcal{A} = (1/a, \ldots, 1/a, 1)$, which we call \emph{weighted cactus groups} $J_n^a$. We show that $J_n^a$ is obtained from the usual cactus presentation by introducing braid relations, which successively simplify the group from $J_n$ to $S_n \rtimes \mathbb{Z}/2\mathbb{Z}$ as $a$ increases. Our proof is by decomposing $\overline{M_{0,\mathcal{A}}}(\mathbb{R})$ as a polytopal complex, generalizing a similar known decomposition for $\overline{M_{0,n+1}}(\mathbb{R})$. In the unweighted case, these cells are known to be cubes and are `dual' to the usual decomposition into associahedra (by the combinatorial type of the stable curve). For $\overline{M_{0,\mathcal{A}}}(\mathbb{R})$, our decomposition instead consists of products of permutahedra. The cells of the decomposition are indexed by weighted stable trees, but `dually' to the usual indexing. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2503_23253 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Fundamental groups of moduli spaces of real weighted stable curves Levinson, Jake Liu, Haggai Algebraic Geometry Algebraic Topology Combinatorics 14H10, 14P25, 05E14, 51M20, 14F35 The ordinary and $S_n$-equivariant fundamental groups of the moduli space $\overline{M_{0,n+1}}(\mathbb{R})$ of real $(n+1)$-marked stable curves of genus $0$ are known as \emph{cactus groups} $J_n$ and have applications both in geometry and the representation theory of Lie algebras. In this paper, we compute the ordinary and $S_n$-equivariant fundamental groups of the Hassett space of weighted real stable curves $\overline{M_{0,\mathcal{A}}}(\mathbb{R})$ with $S_n$-symmetric weight vector $\mathcal{A} = (1/a, \ldots, 1/a, 1)$, which we call \emph{weighted cactus groups} $J_n^a$. We show that $J_n^a$ is obtained from the usual cactus presentation by introducing braid relations, which successively simplify the group from $J_n$ to $S_n \rtimes \mathbb{Z}/2\mathbb{Z}$ as $a$ increases. Our proof is by decomposing $\overline{M_{0,\mathcal{A}}}(\mathbb{R})$ as a polytopal complex, generalizing a similar known decomposition for $\overline{M_{0,n+1}}(\mathbb{R})$. In the unweighted case, these cells are known to be cubes and are `dual' to the usual decomposition into associahedra (by the combinatorial type of the stable curve). For $\overline{M_{0,\mathcal{A}}}(\mathbb{R})$, our decomposition instead consists of products of permutahedra. The cells of the decomposition are indexed by weighted stable trees, but `dually' to the usual indexing. |
| title | Fundamental groups of moduli spaces of real weighted stable curves |
| topic | Algebraic Geometry Algebraic Topology Combinatorics 14H10, 14P25, 05E14, 51M20, 14F35 |
| url | https://arxiv.org/abs/2503.23253 |