We consider the Sturm–Liouville equation \renewcommand{\theequation}{1.\arabic{equation}} \begin{equation} y^{\prime\prime}(x)+\{\lambda - q(x)\}y(x) = 0\quad (0 \le x < \infty) \end{equation} with a boundary condition at $x = 0$ which can be either the Dirichlet condition \begin{equation} y(0) = 0 \end{equation} or the Neumann condition \begin{equation} y^\prime(0) = 0. \end{equation} As usual, $\lambda$ is the complex spectral parameter with $0 \le \arg \lambda < 2\pi$ , and the potential $q$ is real-valued and locally integrable in $[0, \infty)$ .