\Gamma-extensions of the spectrum of an orbifold
  We introduce the \Gamma-extension of the spectrum of the Laplacian of a Riemannian orbifold, where \Gamma is a finitely generated discrete group. This extension, called the \Gamma-spectrum, is the union of the Laplace spectra of the \Gamma-sectors of the orbifold, and hence constitutes a Riemannian invariant that is directly related to the singular set of the orbifold. We compare the \Gamma-spectra of known examples of isospectral pairs and families of orbifolds and demonstrate that it many cases, isospectral orbifolds need not be \Gamma-isospectral. We additionally prove a version of Sunada's theorem that allows us to construct pairs of orbifolds that are \Gamma-isospectral for any choice of \Gamma. 
Carla Farsi, Emily Proctor, and Christopher Seaton
math.DG math.AT