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5. O. A. Boichuk and E. V. Panasenko, “Weakly nonlinear boundary-value problems for differential equations in Banach spaces in the critical case,” Nelin. Kolyvannya, 13, No. 4, 483–496 (2010); English translation : Nonlin. Oscillat., 13, No. 4, 515–529 (2011).