On an inverse spectral problem and a generalized Sturm's nodal theorem for nonlinear boundary value problems

We consider an inverse optimization spectral problem for the Sturm-Liouville operator $$\mathcal{L}[q] u:=-u''+q(x)u$$ subject to the separated boundary conditions. In the main result, we prove that this problem is related to the existence of solutions of boundary value problems for the nonlinear equations of the form $$-u"+q_0(x) u=\lambda u+\sigma u^3$$ with $\sigma=1$ or $\sigma=-1$. The key outcome of this relationship is a generalized Sturm's nodal theorem for the nonlinear boundary value problems.