Shapovalov, Yuriy     Investigation of stability reserve of linear periodically-time-variable circuits by the frequency symbolic method [Текст] / Yuriy Shapovalov, Bohdan Mandziy, Dariya Bachyk // Computational problems of electrical engineering (CPEE'2015) : proc. 16th Intern. conf., Sept. 2-5, 2015, Lviv, Ukr. / Lviv Polytechnic Nat. Univ., Warsaw Univ. of Technology, Techn. Univ. of Lodz, Univ. of West Bohemia, Univ. of Zilina, Polish Acad. of Sciences ; [ed. S. Rendzinyak]. — Lviv : [Publ. House of Lviv Polytechnic], 2015. — P. 181-184.