The similarity problem for indefinite Sturm-Liouville operators and the HELP inequality

TL;DR

This paper establishes new criteria for the similarity of indefinite Sturm-Liouville operators to self-adjoint operators and the validity of the HELP inequality in singular cases, using Weyl-Titchmarsh m-functions and coefficient behavior.

Contribution

It provides the first criteria in terms of m-functions for singular cases and links the similarity problem to the HELP inequality validity, extending classical results.

Findings

Criteria for the validity of the HELP inequality in singular cases.

Conditions for the similarity of indefinite Sturm-Liouville operators to self-adjoint operators.

Application of results to the two-way diffusion and Fokker-Planck equations.

Abstract

We study two problems. The first one is the similarity problem for the indefinite Sturm-Liouville operator \[ A=-(\sgn\, x)\frac{d}{wdx}\frac{d}{rdx} \] acting in $L^2_{w}(-b,b)$. It is assumed that $w,r\in L^1_{\loc}(-b,b)$ are even and positive a.e. on $(-b,b)$. The second object is the so-called HELP inequality \[(\int_{0}^b\frac{1}{\tilde{r}}|f'|\, dx)^2 \le K^2 \int_{0}^b|f|^2\tilde{w}\,dx\int_{0}^b\Big|\frac{1}{\tilde{w}}\big(\frac{1}{\tilde{r}}f'\big)'\Big|^2\tilde{w}\, dx, \] where the coefficients $\tilde{w},\tilde{r}\in L^1_{\loc}[0,b)$ are positive a.e. on $(0,b)$. Both problems are well understood when the corresponding Sturm-Liouville differential expression is regular. The main objective of the present paper is to give criteria for both the validity of the HELP inequality and the similarity to a self-adjoint operator in the singular case. Namely, we establish new criteria formulated in terms of the behavior of the corresponding Weyl-Titchmarsh $m$-functions at 0 and at $\infty$. As a biproduct of this result we show that both problems are closely connected. Namely, the operator $A$ is similar to a self-adjoint one precisely if the HELP inequality with $\tilde{w}=r$ and $\tilde{r}=w$ is valid. Next we characterize the behavior of $m$-functions in terms of coefficients and then these results enable us to reformulate the obtained criteria in terms of coefficients. Finally, we apply these results for the study of the two-way diffusion equation, also known as the time-independent Fokker-Plank equation.