in control theory, many articles refer to the following paper: V. A. Yakubovich, S-procedure in nonlinear control theory, Vestnik Leningrad University, 4 (1977), pp. 73-93. english translation; original Russian publication in Vestnik Leningradskogo Universiteta, Seriya Matem- atika (1971), 62-77.
Unfortunately I wasn't able so far to find it anywhere on the internet/my library. Can anyone provide my with a link or copy of this paper? That would be of great help!