Borg-type theorem for the missing eigenvalue problem (Q1940685)

For the boundary value problem \[ -y''+q(x)y=\lambda y,\quad y'(0,\lambda)-h_0y(0,\lambda)=y'(1)-h_1y(1,\lambda)=0, \] the author proves a Borg-type theorem for a missing eigenvalue. He shows that two spectra missing one eigenvalue are sufficient to determine the potential \(q\) and the coefficients \(h_0\), \(h_1\).