A sharp bound for the slope of double cover fibrations

Details A sharp bound for the slope of double cover fibrations A sharp bound for the slope of double cover fibrations

2005-10-07

arxiv.org/abs/math/0510144v2

Michigan Math. J. 56 (2008), 551-561

Mathematics

Algebraic Geometry

13 pages, revised version

Scientific paper

Let f: X->B be a fibred surface of genus g whose general fibre is a double cover of a smooth curve of genus gamma. We show that, for g > 4gamma+1, the number 4(g-1)/(g-gamma) is a sharp lower bound for the slope of f, proving a conjecture of Barja. Moreover, we give a characterisation of the fibred surfaces that reach the bound. In the case g = 4gamma+1 we obtain the same sharp bound under the assumption that the involutions on the general fibres glue to a global involution on X.

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