
A Bank-Laine function is an entire function \(E(z)\) such that \(E'(z)=\pm 1\) at every zero of \(E(z)\). These functions are related to linear complex differential equations: Given \(A(z)\) entire, let \(f_1\), \(f_2\) be linearly independent solutions of (1) \(w''+A(z)w=0\), normalized such that their Wronskian is \(=1\). Then \(E(z)=f_1(z)f_2(z)\) is a Bank-Laine function satisfying \(4A=(E'/E)^2-2E''/E-1/E^2\). Conversely, if \(E(z)\) is Bank-Laine, then \(A\) is entire and \(E\) is the product of two normalized linearly independent solutions of~(1). Basic results and examples of Bank-Laine functions may be found in [Trans. Am. Math. Soc. 273, 351-363 (1982; Zbl 0505.34026)]. Moreover, \textit{L. Shen} [Proc. Am. Math. Soc. 95, 544-546 (1985; Zbl 0596.30048)] observed that if \((a_n)\) is a complex sequence of distinct points tending to \(\infty\), then there exists a Bank-Laine function with zero-sequence \((a_n)\). However, this function may be of infinite order, even if the exponent of convergence of \((a_n)\) is finite. Non-existence results for such Bank-Laine functions of finite order have been offered in [\textit{S. M. Elzaidi}, Complex Variables, Theory Appl. 38, No. 3, 201-220 (1999)] and the present paper as well. Perhaps the most interesting part of this paper is that one addressing non-trivial examples of Bank-Laine functions [see also \textit{J. K. Langley}, Arch. Math. 71, No. 3, 233-239 (1998; Zbl 0930.30028)]. The following result will be proved: Let \((c_n)\) be a positive sequence tending to \(+\infty\). Then there exists a Bank-Laine function \(E(z)=e^z\prod_{n=1}^\infty(1-z/\alpha_n)\) with \(|\alpha_n|>c_n\) for each~\(n\). Moreover, \(E\) is of order \(=1\), and its zeros have exponent of convergence \(\lambda(E)=0\). Concerning the corresponding equation~(1), \(A\) is transcendental and \(f_1\) has no zeros. The result means that there exist Bank-Laine functions of finite order with arbitrary sparse zero-sequences. Perhaps we should add that a complete understanding of the Bank-Laine functions would also solve the Bank-Laine conjecture which asks whether \(\max(\lambda(f_1),\lambda(f_2))=\infty\) provided \(f_1\), \(f_2\) are linearly independent solutions of~(1) with \(A(z)\) entire of non-integer finite order.
Oscillation, growth of solutions to ordinary differential equations in the complex domain, Entire and meromorphic solutions to ordinary differential equations in the complex domain, Value distribution of meromorphic functions of one complex variable, Nevanlinna theory
Oscillation, growth of solutions to ordinary differential equations in the complex domain, Entire and meromorphic solutions to ordinary differential equations in the complex domain, Value distribution of meromorphic functions of one complex variable, Nevanlinna theory
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