Nodal count of eigenfunctions as index of instability

Zeros of vibrational modes have been fascinating physicists for several centuries. Mathematical study of zeros of eigenfunctions goes back at least to Sturm, who showed that, in dimension d=1, the n-th eigenfunction has n-1 zeros. Courant showed that in higher dimensions only half of this is true, namely zero curves of the n-th eigenfunction of the Laplace operator on a compact domain partition the domain into at most n parts (which are called nodal domains).
It recently transpired that the difference between this upper bound and the actual value can be interpreted as an index of instability of a certain energy functional with respect to suitably chosen perturbations. We will discuss two examples of this phenomenon:
(1) stability of the nodal partitions of a domain in R^d with respect to a perturbation of the partition boundaries and
(2) stability of a graph eigenvalue with respect to a perturbation by magnetic field. In both cases, the nodal defect of the eigenfunction coincides with the Morse index of the energy functional at the corresponding critical point.

Based on preprints arXiv:1107.3489 (joint with P.Kuchment and U.Smilansky) and arXiv:1110.5373.