Abstract
Let \({\mathcal {H}}\) be a Coxeter hyperplane arrangement in n-dimensional Euclidean space. Assume that the negative of the identity map belongs to the associated Coxeter group W. Furthermore assume that the arrangement is not of type \(A_1^n\). Let K be a measurable subset of the Euclidean space with finite volume which is stable by the Coxeter group W and let a be a point such that K contains the convex hull of the orbit of the point a under the group W. In a previous article the authors proved the generalized pizza theorem: that the alternating sum over the chambers T of \({\mathcal {H}}\) of the volumes of the intersections \(T\cap (K+a)\) is zero. In this paper we give a dissection proof of this result. In fact, we lift the identity to an abstract dissection group to obtain a similar identity that replaces the volume by any valuation that is invariant under affine isometries. This includes the cases of all intrinsic volumes. Apart from basic geometry, the main ingredient is a theorem of the authors where we relate the alternating sum of the values of certain valuations over the chambers of a Coxeter arrangement to similar alternating sums for simpler subarrangements called 2-structures introduced by Herb to study discrete series characters of real reduced groups.
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Notes
The pseudo-root systems of types \(B_n\) and \(C_n\) are identical after normalizing the lengths of the roots.
In fact, it follows from Theorem 4.1 that \(K_{\mathcal {Z}}(H)\longrightarrow K_{\mathcal {Z}}(V)\) is injective, so we get an isomorphism from \(K_{\mathcal {Z}}(H)\) to \(K'_{\mathcal {Z}}(V)\).
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Acknowledgements
The authors thank Dominik Schmid for introducing them to the Pizza Theorem, and Ramon van Handel for dispelling some of their misconceptions and lending them a copy of [21]. They made extensive use of Geogebra to understand the 2-dimensional situation and to produce some of the figures. This work was partially supported by the LABEX MILYON (ANR-10-LABX-0070) of Université de Lyon, within the program “Investissements d’Avenir” (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR), and by Princeton University. This work was also partially supported by grants from the Simons Foundation (#429370 and #854548 to Richard Ehrenborg and #422467 to Margaret Readdy). The third author was also supported by NSF grant DMS-2247382.
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Ehrenborg, R., Morel, S. & Readdy, M. Pizza and 2-Structures. Discrete Comput Geom 70, 1221–1244 (2023). https://doi.org/10.1007/s00454-023-00600-2
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DOI: https://doi.org/10.1007/s00454-023-00600-2
Keywords
- Coxeter arrangements
- 2-Structures
- Dissections
- Pizza theorem
- Reflection groups
- Intrinsic volumes
- Pseudo-root systems
- Bolyai–Gerwien Theorem