The Weakly Special Conjecture contradicts orbifold Mordell, and thus abc - Archive ouverte HAL
Pré-Publication, Document De Travail Année : 2024

The Weakly Special Conjecture contradicts orbifold Mordell, and thus abc

Résumé

Starting from an Enriques surface over Q(t) constructed by Lafon, we give the first examples of smooth projective weakly special threefolds which fiber over the projective line in Enriques surfaces (resp. K3 surfaces) with nowhere reduced, but non-divisible, fibres and general type orbifold base. We verify that these families of Enriques surfaces (resp. K3 surfaces) are non-isotrivial by computing their fundamental groups and studying the behaviour of local points along certain étale covers. The existence of the above threefolds implies that the Weakly Special Conjecture formulated in 2000 contradicts the Orbifold Mordell Conjecture, and hence the abc conjecture. Using these examples, we can also easily disprove several complex-analytic analogues of the Weakly Special Conjecture. Finally, the existence of such threefolds shows that Enriques surfaces and K3 surfaces can have non-divisible but nowhere reduced degenerations, thereby answering a question raised in 2005.

Dates et versions

hal-04729632 , version 1 (10-10-2024)

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Finn Bartsch, Frédéric Campana, Ariyan Javanpeykar, Olivier Wittenberg. The Weakly Special Conjecture contradicts orbifold Mordell, and thus abc. 2024. ⟨hal-04729632⟩
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