Orbifold Fano bound conjecture for locally soluble fibres

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Let π:X→P1\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 π−1(D)\pi^{-1}(D), and define

∂π=∑D∈(P1)(1)(1−1mD)[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(B2−deg⁡∂π+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.

References

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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