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2. G. S. Larionov, “Investigation of oscillations of relaxing systems by the averaging method,” Mekh. Polimer., No. 4 (1969).
3. J. Prüss, Evolutionary Integral Equations and Applications (Birkhäuser, Switzerland, 1993).
4. V. V. Vlasov and N. A. Rautian, Spectral Analysis of Functional-Differential Equations (MAKS, Moscow, 2016) [in Russian].
5. K. Rektorys, Variational Methods in Mathematics, Science, and Engineering, 2nd ed. transl. from the Czech (Reidel, Dordrecht, 1980).