Hearing shapes of drums - mathematical and physical aspects of   isospectrality
  In a celebrated paper '"Can one hear the shape of a drum?"' M. Kac [Amer. Math. Monthly 73, 1 (1966)] asked his famous question about the existence of nonisometric billiards having the same spectrum of the Laplacian. This question was eventually answered positively in 1992 by the construction of noncongruent planar isospectral pairs. This review highlights mathematical and physical aspects of isospectrality. 
O. Giraud and K. Thas
math-ph math.MP nlin.CD quant-ph