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2. Yu. B. Zelinskii, I. Yu. Vygovskaya, and M. V. Stefanchuk, "Generalized convex sets and the problem of shadow," Ukr. Mat. Zh., 67, No. 12, 1658-1666 (2015)
3. English translation: Ukr. Math. J., 67, No. 12, 1874-1883 (2016).
4. Yu. B. Zelins'kyi and M. V. Stefanchuk, "Generalization of the shadow problem," Ukr. Mat. Zh., 68, No. 6, 757-762 (2016)
5. English translation: Ukr. Math. J., 68, No. 6, 862-867 (2016).