The wider influence of the work of V. A. Yakubovich

The views of Roger W. Brockett on the work of V.A. Yakubovich in the field of modern control theory are discussed. V.A. Yakubovich was known to interconnect the development of technological, mathematical and scientific ideas. Various ways of thinking about the relationship between transfer functions and differential equations were developed but the lemma pioneered by Yakubovich was dominant. It was observed that rational positive real functions are important part in network synthesis because a driving point impedance is positive real and every positive real function can be realized as a driving point impedance of passive network. The subject of stability of periodic Hamiltonian systems was also included by Professor Yakubovich because the subject appeared to be one in which further development was possible.