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A. E. Zernov and Y. V. Kuzina, “Qualitative Investigation of the Singular Cauchy Problem <em class="a-plus-plus">F</em>(<em class="a-plus-plus">t</em>, <em class="a-plus-plus">x</em>, <em class="a-plus-plus">x</em>′) = 0, <em class="a-plus-plus">x</em&gt;(0) = 0”, Ukr. Mat. Zhurn., vol. 55, no. 12, pp. 1720-1723, Dec. 2003.