Fluid dynamics is an ancient science incredibly alive today. Modern
technol- ogy and new needs require a deeper knowledge of the behavior of
real fluids, and new discoveries or steps forward pose, quite often,
challenging and diffi- cult new mathematical {:: oblems. In this
framework, a special role is played by incompressible nonviscous
(sometimes called perfect) flows. This is a mathematical model
consisting essentially of an evolution equation (the Euler equation) for
the velocity field of fluids. Such an equation, which is nothing other
than the Newton laws plus some additional structural hypo- theses, was
discovered by Euler in 1755, and although it is more than two centuries
old, many fundamental questions concerning its solutions are still open.
In particular, it is not known whether the solutions, for reasonably
general initial conditions, develop singularities in a finite time, and
very little is known about the long-term behavior of smooth solutions.
These and other basic problems are still open, and this is one of the
reasons why the mathe- matical theory of perfect flows is far from being
completed. Incompressible flows have been attached, by many
distinguished mathe- maticians, with a large variety of mathematical
techniques so that, today, this field constitutes a very rich and
stimulating part of applied mathematics.