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2. I. T. Kiguradze, Some Singular Boundary-Value Problems for Ordinary Differential Equations [in Russian], Tbilisi University, Tbilisi (1975).
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4. T. I. Kodlyuk, V. A. Mikhailets, and N. V. Reva, "Limit theorems for one-dimensional boundary-value problems," Ukr. Mat. Zh., 65, No. 1, 70-81 (2013)
5. English translation: Ukr. Math. J., 65, No. 1, 77-90 (2013).