A Vectorial Inverse Nodal Problem

Consider the vectorial Sturm-Liouville problem:   −y′′(x) + P (x)y(x) = λIdy(x) Ay(0) + Idy ′(0) = 0 By(1) + Idy ′(1) = 0 where P (x) = [pij(x)]i,j=1 is a continuous symmetric matrix-valued function defined on [0, 1], and A and B are d×d real symmetric matrices. An eigenfunction y(x) of the above problem is said to be of type (CZ) if any isolated zero of its component is a nodal point of y(x). We show that when d = 2, there are infinitely many eigenfunctions of type (CZ) if and only if (P (x), A,B) are simultaneously diagonalizable. This indicates that (P (x), A,B) can be reconstructed when all except a finite number of eigenfunctions are of type (CZ). The results supplement a theorem proved by Shen-Shieh (the second author) for Dirichlet boundary conditions. The proof depends on an eigenvalue estimate, which seems to be of independent interest.