On an open question in recovering Sturm-Liouville-type operators with delay

Abstract: In recent years, there appeared a considerable interest in the inverse spectral theory for functional-differential operators with constant delay. In particular, it is well known that specification of the spectra of two operators $\ell_j,$ $j=0,1,$ generated by one and the same functional-differential expression $-y''(x)+q(x)y(x-a)$ under the boundary conditions $y(0)=y^{(j)}(\pi)=0$ uniquely determines the complex-valued square-integrable potential $q(x)$ vanishing on $(0,a)$ as soon as $a\in[\pi/2,\pi).$ For many years, it has been a challenging {\it open question} whether this uniqueness result would remain true also when $a\in(0,\pi/2).$ Recently, a positive answer was obtained for the case $a\in[2\pi/5,\pi/2).$ In this paper, we give, however, a {\it negative} answer to this question for $a\in[\pi/3,2\pi/5)$ by constructing an infinite family of iso-bispectral potentials. Some discussion on a possibility of constructing a similar counterexample for other types of boundary conditions is provided, and new open questions are outlined.