Lower Bound For The Ratios Of Eigenvalues Of Schrödinger Equations With Nonpositive Single-Barrier Potentials

Abstract: Horv\'ath and Kiss [Proc. Amer. Math. Soc., 2005] proved the upper bound estimate $\frac{\lambda {n}}{\lambda _{m}}\leq \frac{n^{2}}{m^{2}}$ $ (n>m\geq 1) $ for Dirichlet eigenvalue ratios of the Schr\"odinger problem $-y''+q(x)y=\lambda y$ with nonnegative and single-well potential $q$. In this paper, we prove that if $q(x)$ is a nonpositive, continuous and single-barrier potential, then $\frac{\lambda{n}}{\lambda_{m}}\geq \frac{n^{2}}{m^{2}}$ for $\lambda_n>\lambda_m \geq -2q^*$, where $q^{\ast}=\min{q(0), q(1)}$. In particular, if $q(x)$ satisfies the additional condition $\mid q^{\ast} \mid\leq \frac{\pi^{2}}{3}$, then $\lambda _{1}>0$ and $\frac{\lambda _{n}}{\lambda _{m}}\geq \frac{n^{2}%}{m^{2}}$ for $n>m\geq 1.$ For this result, we develop a new approach to study the monotonicity of the modified Pr\"ufer angle function.