文摘
In V. Barcilon (J Math Anal Appl 93:222-34, 1983) two boundary value problems were considered generated by the differential equation of a string $$y^{\prime\prime} + \lambda p(x)y = 0, \,\, 0 \leq x \leq L with continuous real function p(x) (density of the string) and the boundary conditions y(0) =? y(L)?=?0 for the first problem and \({y^{\prime}(0) = y(L) = 0}\) for the second one. In the above paper the following formula was stated $$p(0) = \frac{1}{L^2\mu_1} \mathop{\prod} \limits_{n=1}^{\infty}\frac{\lambda_n^2}{\mu_n \mu_{n+1}} \quad \quad \quad (**)$$ where \({\{\lambda_k\}_{k=1}^{\infty}}\) is the spectrum of the first boundary value problem and \({\{\mu_k\}_{k=1}^{\infty}}\) of the second one. A rigorous proof of (**) was given in C.-L. Shen (Inverse Probl 21:635-55, 2005) under the more restrictive conditions of piecewise continuity of \({p^{\prime}(x)}\) . In this paper (**) was deduced using $$p(0)=\lim\limits_{\lambda\to +\infty} \left(\frac{\phi(L,-\lambda)} {\lambda^{\frac{1}{2}} \psi(L,-\lambda)} \right)^2 \quad \quad \quad \quad (\ast\ast\ast)$$ where \({\phi(x,\lambda)}\) is the solution of (*) which satisfies the boundary conditions \({\phi(0) - 1 = \phi^{\prime}(0) = 0 \,\,{\rm and}\,\, \psi(x,\lambda)}\) is the solution of (*) which satisfies \({\psi(0) = \psi^{\prime}(0) - 1 = 0}\) . In our paper we prove that (***) is true for the so-called M.G. Krein strings which may have any nondecreasing mass distribution function M(x) with finite nonzero \({M^{\prime}(0)}\) . Also we show that (**) is true for a wide class of strings including those for which M(x) is a singular function, i.e. \({M^{\prime}(x) = p(x)\mathop{=} \limits^{a.e.}0}\) .