I thinks there is some problem related to the solution of a 2 order differential equation given by Sturm-Liouville system which is nontrivial.
It is well-know that the power of Sturm-Liouville theory see wiki, is due to it is some kind of “spectral decomposition” in the solution space.
Two kind of problem is interesting, one is the eigenvalue estimate, both upper bound and lower bound, this already investigated in ESTIMATING THE EIGENVALUES OF STURM-LIOUVILLE. PROBLEMS BY APPROXIMATING THE DIFFERENTIAL EQUATION.
I post two problem here, this is a product due to a random walk along the boundary of topology and the analysis,
Problem 1.
Fix a set $latex A=\{k_1<k_2<…<k_l\}$, is there a 2 order ordinary differential equation given by Sturm–Liouville theory such that the eigenfunction $latex f_{k}$ is periodic if and only if $latex k\in A$?
There is also some weak version of this and a infinity version of this.
Of course we have the following map, from the high order ordinary differential equation to the 1 order differential equation in high dimension. But the key point is that it is not a bijection! The Frobenius condition play a crucial role.
Problem 2.
There is a homotopy in the moduli space of differential equation, and we could define a direct product operator in this space, and we consider the topology defamation of the eigenfunction, could there be some equality, one side of it explain the topology information, the other side explain the spectral (or analysis) information?
There is another interesting problem.