The expressive power of clocks
T.A. Henzinger, P. Kopke, H. Wong Toi, in:, 22nd International Colloquium on Automata, Languages and Programming , Springer, 1995, pp. 417–428.

Hybrid automata with finite bisimulations
T.A. Henzinger, in:, 22nd International Colloquium on Automata, Languages and Programming , Springer, 1995, pp. 324–335.