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 existence and uniqueness hold. In the one-dimensional case this has recently been achieved by Bressan and collaborators (see ,  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 . The possibility of generalizing this to relativistic and self-gravitating fluids was studied by Brauer . 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 flowing uniformly outwards. A theorem of this type has been proved by Grassin . There is also a global existence result due to Guo  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. Since, as has already been pointed out, we know so little about the global time evolution for a self-gravitating fluid ball, even in the Newtonian case, it is not possible to say anything rigorous about nonlinear stability at the present time. We can, however, make some statements about linear stability. 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 . (Cf. also  for the Newtonian problem.) The spectral properties of the linearized operator for general (i. e. non-spherically symmetric) perturbations in the Newtonian problem have been studied by Beyer . This could perhaps provide a basis for a stability analysis, but this has not been done.
|Local and Global Existence Theorems for the Einstein
Alan D. Rendall
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