Applications of Lie Semigroups in Analysis
J. Hilgert, in: K.H. Hofmann, et al. (Eds.), The Analytical and Topological Theory of Semigroups, De Gruyter, Berlin, 1990.

On the Causal Structure of Homogeneous Manifolds
J. Hilgert, K.H. Hofmann, Math. Scand. 67 (1990) 119–144.