# User:Daviddaved/"Can One Hear the Shape of a Drum?"

From Wikibooks, open books for an open world

Let be the Dirichlet eigenvalues for the domain , that is, the eigenvalues of the Dirichlet problem for the Laplacian:

Since the system above is self-adjoint, the eigenvalues are positive. Two domains are said to be isospectral if they have the same Laplacian eigenvalues.

One can state the discrete analog of the question on graphs...