1. Le plan projectif des octaves et les sphères comme espaces homogènes;Borel;C. R. Acad. Sci. Paris,1950
2. Theory of Lie Groups;Chevalley,1946
3. Über Systeme von linearen partiellen Differentialgleichungen erster Ordnung;Chow;Math. Ann.,1939
4. A consequence of controllability;Elliott;J. Differential Equation,1971
5. D. Elliott and T. J. Tarn, Controllability and observability for bilinear systems, (unpublished).