Almost inner derivations of Lie algebras KU Leuven
A classical problem in spectral geometry was to determine whether or not isospectral manifolds are necessarily isometric. It turns out that the answer to this question is negative and several counterexamples have been given. For the construction of continuous families of isospectral and non-isometric manifolds, class preserving automorphisms of nilpotent Lie groups were crucial.
An automorphism of a group is called class preserving if ...