3:00 p.m., Friday (Jan. 25)
Math Annex 1100
University of Bologna
On coverings of stable curves
I will report on joint work with Dan Abramovich and Alessio Corti.
Compact Riemann surfaces, or smooth algebraic curves, are basic
objects in geometry. When studying families of smooth algebraic
curves of fixed genus one has to deal with the problem that these
families do not form a compact space; in other words, the limit
of a family of smooth curves is not necessarily smooth. It was
shown by Deligne and Mumford that we can get a compact space by
introducing some singular curves, called stable curves;
these have the mildest possible type of singularities, namely nodes.
One often wants to study Riemann surfaces with some additional
structure: for example, one might be interested in Riemann surfaces
with a specified topological cover, or a fixed element in the
first homology group. But then stable curves don't do the job,
because they do not have enough covers, and their first homology
group is too small.
It was discovered by Dan Abramovich and myself that in many cases
the natural limits are not given by bare stable curves, but by what
we call twisted curves. Very roughly, the idea is that one has to
substitute each singular point with the classifying space of a
I will start by reviewing the classical theory of stable curves
and explain its inadequacies, and conclude by giving some idea
of what twisted curves are, and why they give a good theory.