We construct examples of non-isotrivial algebraic families of smooth complex
projective curves over a curve of genus 2. This solves a problem from Kirby’s list of
problems in low-dimensional topology. Namely, we show that 2 is the smallest
possible base genus that can occur in a 4–manifold of non-zero signature which is
an oriented fiber bundle over a Riemann surface. A refined version of the
problem asks for the minimal base genus for fixed signature and fiber genus.
Our constructions also provide new (asymptotic) upper bounds for these
numbers.
Received: 24 May 2001
Revised: 7 February 2002
Accepted: 26 February 2002
Published: 27 February 2002
Proposed: Dieter Kotschick
Seconded: Walter Neumann, Gang Tian