Consider the curves on the (x, y)-plane parametrized by λ and given by the equation, y 2 = x 5 − 5λx + 4λ.An easy computation shows that these curves are non-singular for λ ∈ C \ {0, 1}. On the other hand the projective closure of each of these curves is singular at the point [0 : 1 : 0]. Blowing up this point and another one infinitely near this one yields a resolution, C λ . The collection of C λ forms a family of smooth projective curves over the base C \ {0, 1}.Exercise 0.1. Prove that the projection of C λ onto the y = 0 axis is the only degree 2 morphism ontoA family of curves is called isotrivial if any two general members are isomorphic. For example the blow up of the projective plane at a single point, considered as a P 1 -bundle over P 1 , is an isotrivial family since all of its members are isomorphic to the projective line. Since the genus of a curve in a family is constant, and there is only one curve of genus zero, one cannot expect much more in this case. However, for higher genus curves one can have non-isotrivial families as the above example shows.
134Sándor J. Kovács projective curves of a given genus over a given base curve. Furthermore, if there is such a family, then the base curve satisfies a certain hyperbolic condition. (For definitions and a more precise formulation, see Section §1). The conjecture was confirmed by Parshin [57] for the case of a compact base and by Arakelov [3] in general. It was recently generalized to families of higher dimensional varieties. This generalization is our main topic.It will be advantageous to work with a compactification of the family. Considering families over a compact base curve B naturally leads to a slightly different view on the problem. Instead of smooth families over a non-compact base, we are looking at arbitrary families over a compact base, and consider the locus over which the family is smooth. It is fairly obvious that these are equivalent situations. An arbitrary family over a compact base gives a smooth family over some open subset and a smooth family over an open curve can be extended to a (not necessarily smooth) family over the projective closure of the curve cf.[28], III.9.8.One could ask what can be said about the singular members of the family. On the simplest level, how many are there? In fact, Szpiro has asked this: What is the lower bound on the number of singular members if B P 1 ? This is the same as asking how close the base of the smooth family can be to being compact. Beauville [6] gave the following answer to Szpiro's question: there are always at least three singular members and there are families with exactly three. In fact, Beauville's proof also shows that there is at least one singular member if the base curve is elliptic. In short, 2g(B) − 2 + δ > 0, where g(B) is the genus of the (compact) base curve and δ is the number of singular members of the family. In other words, the base of a smooth family must be hyperbolic.Notice that the existence of Kodaira surfaces shows that there are families over high genus curves wi...