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P. F. Hsieh and Y. Sibuya, On the asymptotic integration of second order linear ordinary differential equations with polynomial coefficients, J. Math. Anal. Appl. 16 (1966), 84-103. MR 0200512 (34:403)

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M. A. Neumark, Lineare Differential-Operatoren, Akademie-Verlag, Berlin, 1960. MR 0216049 (35:6884)

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H. L. Turrittin, Solvable related equations pertaining to turning point problems, Proc. Sympos. Asymptotic Solutions of Differential Equations and Their Applications, pp. 27-52, Wiley, New York, 1964. MR 0192150 (33:377)