Stability and approximations of eigenvalues and eigenfunctions for the Neumann Laplacian, Part 2

We extend the results obtained earlier in a joint paper with R. Banuelos, on the stability and approximations of the second Neumann eigenvalue and its corresponding eigenfunction, to the case when the second Neumann eigenvalue has multiplicity at least 2. We then show that our stability result can be applied to the Koch snowflake and the usual sequence of polygons approximating it from inside.