Orbifold Fano bound conjecture for locally soluble fibres

Let π:XP1\pi:X\to\mathbb{P}^1 be a standard fibration, meaning a smooth, proper, geometrically irreducible variety over Q\mathbb{Q} with dominant morphism to P1\mathbb{P}^1 and geometrically integral generic fibre. For each closed point D(P1)(1)D\in(\mathbb{P}^1)^{(1)}, let mDm_D be the minimum multiplicity of the irreducible components of π1(D)\pi^{-1}(D), and define

π=D(P1)(1)(11mD)[D].\partial_\pi=\sum_{D\in(\mathbb{P}^1)^{(1)}}\left(1-\frac{1}{m_D}\right)[D].

Orbifold Fano bound conjecture. If the Q\mathbb{Q}-divisor (KP1+π)-(K_{\mathbb{P}^1}+\partial_\pi) is ample, then, for every ε>0\varepsilon>0,

Nloc(π,B)=Oε(B2degπ+ε).N_{\text{loc}}(\pi,B)=O_{\varepsilon}\left(B^{2-\deg\partial_\pi+\varepsilon}\right).

This conjecture predicts a power-saving upper bound governed by the degree of the orbifold boundary. The paper presents it as a proposed extension of known sparsity results to standard fibrations with multiple fibres; the excerpt does not state that it has been proved or disproved.

Sources & referencesView supporting material

Primary source

Tim Browning, Julian Lyczak and Arne Smeets, “Paucity of rational points on fibrations with multiple fibres”, arXiv:2310.01135 (2023).

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