Geometry of nodal sets and domains for eigenfunctions of Schroedinger operators

Perturbations of the Laplacian are known as Schrodinger operators. We pose a question about perturbations of eigenfunctions from multiple eigenspaces and answer the question in the case of the two-sphere. This is used to extend a previously known result about nodal sets of spherical harmonics to eigenfunctions of Schrodinger operators on the two-sphere. We also review some of the classical and recent results about nodal sets and domains for eigenfunctions of the Laplacian.