1. Bellman, R., Kalaba, R., Selected Papers on Mathematical Trends in Control Theory, (Dover Publications, New York 1964)
2. Miller, Kempster B. (Unfortunately it has so far proved impossible to locate a copy of this work in any British library and no further reference can be given).
3. Blake, G.G.: History of Radio Telegraphy and Telephony, (Radio Press, London 1926), p. 260