To Dennis Sullivan on the occasion of his 70th birthday CURVATURES OF SOBOLEV METRICS ON DIFFEOMORPHISM GROUPS

Many conservative partial differential equations correspond to geodesic equations on groups of diffeomorphisms. Stability of their solutions can be studied by examining sectional curvature of these groups: negative curvature in all sections implies exponential growth of perturbations and hence suggests instability, while positive curvature suggests stability. In the first part of the paper we survey what we currently know about the curvature-stability relation in this context and provide detailed calculations for several equations of continuum mechanics associated to Sobolev H and H energies. In the second part we prove that in most cases (with some notable exceptions) the sectional curvature assumes both signs. AMS Subject Classification (2000): 53C21, 58D05, 58D17.