On the chamber decomposition of a Hamiltonian torus action

Maarten Mol

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arXiv · 2609.30146

On the chamber decomposition of a Hamiltonian torus action

Maarten Mol

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Abstract

For a compact connected Hamiltonian $T$-space (with $T$ a torus), the components of the set of (relative) regular values of the momentum map are open convex polytopes, whose closures (called the chambers) form a polyhedral decomposition of the momentum polytope. In a previous paper we showed that there is a canonical affine stratification of the momentum polytope with the property that, while varying through a stratum, the fibers of the momentum map do not change as $T$-spaces. In this paper we show that the strata of this stratification are the relative interiors of the faces of the chambers. In doing so, we also give a proof of the fact that these faces form a polyhedral complex, which (it seems) was missing from the literature. We further extend the above to Hamiltonian $T$-spaces whose momentum map is proper as map into a convex set and, in the compact Kähler case, we point out a description of the above strata in terms of the $T_\mathbb{C}$-orbit closure polytopes.

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Maarten Mol. 2026-09-24. On the chamber decomposition of a Hamiltonian torus action. https://arxiv.org/abs/2609.30146

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