I didn't find any reference on the subject Isospectral vs Isometry of the problem of Carolyn Gordon, David L. Webb and Scott Wolpert. Could anyone be able give me a book I may consult having a complete answer to their work? Precisely on the following problem :
Thanks in advance!


The problem is from an article by Marc Kac and is known as "can you hear the shape of a drum". The negative solution in high dimension was found by Milnor before Kac's paper, but getting examples for Kac's problem about planar domains was unsolved until the Gordon-Webb-Wolpert paper. Although the problem is geometric the solution is essentially group theoretic, using Sunada's method.
Rosenberg's book The Laplacian on a Riemannian Manifold has a proof of Sunada's theorem and might be an easier starting point.
https://en.wikipedia.org/wiki/Isospectral
https://en.wikipedia.org/wiki/Hearing_the_shape_of_a_drum