1. I. Amemiya and M. Ozawa, ‘‘Non-existence of finite order solutions of $$w^{\prime\prime}+e^{-z}w^{\prime}+Q(z)w=0$$,’’ Hokkaido Math. J. 10, 1–17 (1981).
2. L. Bieberbach, Theorie der gewöhnlichen Differentialgleichungen (Springer, Berlin, 1965).
3. Z. X. Chen, ‘‘The growth of solutions of $$f^{\prime\prime}+e^{-z}f^{\prime}+Q(z)f=0$$ where the order $$(Q)=1$$,’’ Sci. China, Ser. A 45, 290–300 (2002).
4. Z.-X. Chen, ‘‘On the hyper-order of solutions of higher order differential equations,’’ Chin. Ann. Math., Ser. B 24, 501–508 (2003).
5. Z.-X. Chen, ‘‘On the growth of solutions of a class of higher order differential equations,’’ Acta Math. Sci., Ser. B 24, 52–60 (2004).