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Exponential asymptotics for an eigenvalue of a problem involving parabolic cylinder functions. (English) Zbl 0759.34054
The authors consider the problem of finding the imaginary part of the “eigenvalue” $\lambda$ of the problem $y''(x)+(\lambda+\varepsilon x\sp 2)y(x)=0$ on $(0,\infty)$, $y'(0)+hy(0)=0$, $\lim\sb{x\to\infty} W[y(x),\exp({i\over 2}\varepsilon\sp{1/2} x\sp 2)]=0$, where $\varepsilon\geq 0$, $h$ is a positive constant, $W$ is the Wronskian. They obtain the asymptotic formula (1) $\text{Im} \lambda\sim-2h\sp 2\exp[-{h\sp 2\pi \over 2\sqrt{\varepsilon}}]$ as $\varepsilon\to 0+$. This result is obtained by two different methods, both of which use results which the authors establish on asymptotics of the parabolic cylinder function and on exponentially improved asymptotics of a quotient of gamma functions. The first method consists of construction of a linear combination of parabolic cylinder functions that satisfy the boundary condition at infinity. Insertion into the boundary condition at the origin yields (1). The second method consists of construction of the Titchmarsh-Weyl function $m(\lambda)$ and determination of its pole approximated by (1). A problem similar to the one in this paper with potential $\varepsilon x$ instead of $\varepsilon x\sp 2$ has been studied by {\it R. B. Paris} and {\it A. D. Wood} [IMA J. Appl. Math. 43, No. 3, 273-384 (1989; Zbl 0695.34057)]. Problems of the type studied in this article have arisen in optical tunnelling and in the theory of surface waves.

34L15Eigenvalues, estimation of eigenvalues, upper and lower bounds for OD operators
34B20Weyl theory and its generalizations
34E20Asymptotic singular perturbations, turning point theory, WKB methods (ODE)
34B24Sturm-Liouville theory
33B15Gamma, beta and polygamma functions
33E99Other special functions
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