### 4.1 Hydrodynamics

Solutions of the classical (compressible) Euler equations typically develop singularities, i.e. discontinuities of
the basic fluid variables, in finite time [322]. Some of the results of [322] were recently generalized to the
case of a relativistic fluid [167]. The proofs of the development of singularities are by contradiction and so
do not give information about what happens when the smooth solution breaks down. One of the things that
can happen is the formation of shock waves and it is known that, at least in certain cases, solutions can be
extended in a physically meaningful way beyond the time of shock formation. The extended solutions only
satisfy the equations in the weak sense. For the classical Euler equations there is a well-known
theorem on global existence of weak solutions in one space dimension which goes back to [157].
This has been generalized to the relativistic case. Smoller and Temple treated the case of an
isentropic fluid with linear equation of state [323] while Chen analysed the cases of polytropic
equations of state [84] and flows with variable entropy [85]. This means that there is now an
understanding of this question in the relativistic case similar to that available in the classical
case.
In space dimensions higher than one there are no general global existence theorems. For a long time
there were also no uniqueness theorems for weak solutions even in one dimension. It should be
emphasized that weak solutions can easily be shown to be non-unique unless they are required to
satisfy additional restrictions such as entropy conditions. A reasonable aim is to find a class
of weak solutions in which both existence and uniqueness hold. In the one-dimensional case
this has recently been achieved by Bressan and collaborators (see [68, 70, 69] and references
therein).

It would be desirable to know more about which quantities must blow up when a singularity forms in
higher dimensions. A partial answer was obtained for classical hydrodynamics by Chemin [83]. The
possibility of generalizing this to relativistic and self-gravitating fluids was studied by Brauer [66]. There is
one situation in which a smooth solution of the classical Euler equations is known to exist for all time. This
is when the initial data are small and the fluid initially is flowing uniformly outwards. A theorem of this
type has been proved by Grassin [161]. There is also a global existence result due to Guo [164] for an
irrotational charged fluid in Newtonian physics, where the repulsive effect of the charge can suppress the
formation of singularities.

A question of great practical interest for physics is that of the stability of equilibrium stellar models.
The linear stability of a large class of static spherically symmetric solutions of the Einstein-Euler equations
within the class of spherically symmetric perturbations has been proved by Makino [240] (cf. also [228] for
the Newtonian problem). A nonlinear stability result for solutions of the Euler-Poisson system was proved
in [276] under the assumption of global existence. The spectral properties of the linearized operator for
general (i.e. non-spherically symmetric) perturbations in the Newtonian problem have been studied by
Beyer [54]. This could perhaps provide a basis for a stability analysis, but this has not been
done.