We study several examples of surfaces with p g = q = 2 and maximal Albanese dimension that are endowed with an irrational fibration.
IntroductionThe classification of surfaces S of general type with χ( S ) = 1, i.e. p g (S) = q(S), is currently an active area of research, see for instance [BCP06]. In this case some wellknown results imply p g ≤ 4 . While surfaces with p g = q = 4 and p g = q = 3 have been completely described in One of the most useful techniques used to understand the geometry of an algebraic surface S is the study of its fibrations f : S −→ C, where C is a smooth curve. If g(C) ≥ 1 we say that the f is irrational, and if all its smooth fibres are isomorphic we say that f is isotrivial. The study of irrational fibrations on surfaces with p g = q = 2 was started by Zucconi in [Z03a]. Later on, however, it was found that Zucconi's results were incomplete: see [Pe11], where the first author deals with the isotrivial case.If the image of the Albanese map of S is a curve then everything is known: S is a socalled generalized hyperelliptic surface, i.e. a quotient S = (C 1 × C 2 )/G by the diagonal action of a finite group G on C 1 × C 2 such that the Galois morphism C 1 −→ C 1 /G is unramified and C 2 /G ∼ = 1 , see [Ca00, Z03b, Pe11]. Therefore it only remains to investigate the case where S has maximal Albanese dimension, i.e. its Albanese map α: S −→ Alb(S) is generically finite; in this situation the base of any irrational fibration on S is an elliptic curve E (see Proposition 1.3). An important invariant of the fibration f is the push-forward of the relative canonical bundle f * ω S/E (see for instance [BHPV03, Chapter III]), and the aim of this paper is to explicitly calculate this invariant in many different examples related to our previous work on the subject, see [PP13a, PP13b, PP14].This article is organized as follows. In Section 1 we fix the notation and the terminology and we state some technical results needed in the sequel of the paper. Moreover, by using methods borrowed from [Fu78a,Fu78b,Ba00,CD13], we deduce the following structure result, see Propositions 1.6 and 1.7. This theorem corrects and extends Zucconi's results quoted above. For instance, in [Z03a] only the case where r = 1 is considered, and the existence of non-isotrivial irrational fibrations is overlooked. See Remark 1.9 for more details.In Section 2 we provide several examples with r = 1 and r ≥ 2, both in the isotrivial case (Examples 2.2, 2.3, 2.4, 2.5) and in the non-isotrivial one (Examples 2.6, 2.10, 2.11, 2.12). In particular, we show the existence of surfaces S with p g