Lower Bound For The Ratios Of Eigenvalues Of Schrödinger Equations With Nonpositive Single-Barrier Potentials
Horváth and Kiss [Proc. Amer. Math. Soc., 2005] proved the upper bound estimate $\frac{λ_{n}}{λ_{m}}\leq \frac{n^{2}}{m^{2}}$ $ (n>m\geq 1) $ for Dirichlet eigenvalue ratios of the Schrödinger problem $-y''+q(x)y=λy$ 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 $\frac{λ_{n}}{λ_{m}}\geq \frac{n^{2}}{m^{2}}$ for $λ_n>λ_m \geq -2q^*$, where $q^{\ast}=\min\{q(0), q(1)\}$. In particular, if $q(x)$ satisfies the additional condition $\mid q^{\ast} \mid\leq \frac{π^{2}}{3}$, then $λ_{1}>0$ and $\frac{λ_{n}}{λ_{m}}\geq \frac{n^{2}%}{m^{2}}$ for $n>m\geq 1.$ For this result, we develop a new approach to study the monotonicity of the modified Prüfer angle function.