REFERENCES
- DAVIES , M. J. , 1971 , Int. J. Control , 14 , 175 – 181 .
- HERMES , H. , 1907 , J. cliff. Equations , 3 , 256 .
- SNOW , D. R. , 1967 , in Advances in Control Systems , ( New York Academic Press ), p. 133 .
- † Communicated by Dr. A. J. Fuller