Modified Xiao conjecture for the relative irregularity

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Let f ⁣:S→Bf\colon S\to B be a fibred surface, meaning a fibration from a smooth projective surface SS to a smooth projective curve BB, with general fibre of genus g≥2g\geq 2. Write qfq_f for the relative irregularity and call ff non-isotrivial when its smooth fibres are not all mutually isomorphic. Modified Xiao conjecture. For every non-isotrivial fibred surface f ⁣:S→Bf\colon S\to B of genus g≥2g\geq 2, one has

qf≤⌈g+12⌉.q_f\leq \left\lceil \frac{g+1}{2} \right\rceil.

This modifies Xiao's original bound, which was disproved by Pirola and by later examples of Albano and Pirola. The supplied status evidence records that the modified inequality is also contradicted by those counterexamples, although they satisfy equality for the modified bound.

References

Primary source

Vìctor González-Alonso, Lidia Stoppino and Sara Torelli, “On the rank of the flat unitary summand of the Hodge bundle”, arXiv:1709.05670 (2018).

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