Gerritzen–van der Put's position conjecture for split degenerate superelliptic curves

Let SPK1S \subset \mathbb{P}_K^1 be the set of fixed points of generators of a pp-Whittaker group Γ0\Gamma_0, and let BPK1\mathcal{B} \subset \mathbb{P}_K^1 be its image under reduction modulo Γ0\Gamma_0, namely the set of branch points of the resulting superelliptic curve. For a finite subset APK1A \subset \mathbb{P}_K^1 of cardinality at least 33, its position is the combinatorial data of the tree T(A)T(A) together with the map from AA to the vertices of T(A)T(A) determined by the components containing the reductions of its points; two equally large subsets have the same position when a bijection between them is compatible with an isomorphism of their trees.

Gerritzen–van der Put's conjecture. With this setup, the (2g+2)(2g+2)-element subsets SPK1S \subset \mathbb{P}_K^1 and BPK1\mathcal{B} \subset \mathbb{P}_K^1 have the same position.

The conjecture proposes that the configuration of the fixed points and the configuration of the branch points agree combinatorially. The paper disproves it by a counterexample.

Sources & referencesView supporting material

Primary source

Jeffrey Yelton, “Branch points of split degenerate superelliptic curves II: on a conjecture of Gerritzen and van der Put”, arXiv:2407.11303 (2024).

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