The S-procedure (also called as S-lemma) is a technique from V. A. Yakubovich that is used to relax a system of quadratic inequalities to a linear matrix inequality problem. It is used largely in control and optimization: a review of it is given in [1] and [2].
Does anyone know why the S-procedure is called this way? Is there a reason for why the letter 'S' in S-procedure? Unfortunately, I don't have access to the original paper ([3]) from Yakubovich.
References:
[1]: Derinkuyu, Kürşad, and Mustafa Ç. Pınar. "On the S-procedure and some variants." Mathematical Methods of Operations Research 64.1 (2006): 55-77.
[2]: Pólik, Imre, and Tamás Terlaky. "A survey of the S-lemma." SIAM review 49.3 (2007): 371-418.
[3]: Yakubovich, V. A. "S-procedure in nonlinear control theory." Vestnik Leningrad University: Mathematics 1 (1971): 62-77.