Uniconvergence theorems for Sturm--Liouville operators with potentials from Sobolev space $W_2^{-1}[0,π]$

Abstract

We consider a Sturm--Liouville $Ly=-y''+q(x)y$ in space $L_2[0,π]$ with potential from Sobolev space $W_2^{-1}[0,π]$. Moreover, we assume, that $q=u'$, where $u\in L_2[0,π]$. We consider Direchlet boundary conditions $y(0)=y(π)=0$, although we can treat a boundary conditions of Sturm type. It is known, that operators of such class have a discrete spectr with only accumulation point $+\infty$ and the system $\{y_k\}_1^\infty$ of eigen and associated functions is a Riesz basis in $L_2[0,π]$. Moreover, this basis is a Hilbert--Schmidt perturbation of the basis $\{sin(kx)\}_1^\infty$. In this paper we prove the uniconvergence theorem: for any element $f\in L_2[0,π]$ the sequence $P_nf-S_nf\to0$ as $n\to\infty$ in $C[0,π]$ (here $P_n$ and $S_n$ are the Riesz projectors to $\{y_k\}_1^n$ and $\{\sin(kt)\}_1^n$ respectively).