On isospectral arithmetical spaces
  We study the relationship between the arithmetic and the spectrum of the Laplacian for manifolds arising from congruent arithmetic subgroups of SL(1,D), where D is an indefinite quaternion division algebra defined over a number field F.   We give new examples of isospectral but non-isometric compact, arithmetically defined varieties, generalizing the class of examples constructed by Vigneras. These examples are based on an interplay between the simply connected and adjoint group and depend explicitly on the failure of strong approximation for the adjoint group. The examples can be considered as a geometric analogue and also as an application of the concept and results on L-indistinguishability for SL(1,D) due to Labesse and Langlands.   We verify that the Hasse-Weil zeta functions are equal for the examples of isospectral pair of Shimura varieties we construct giving further evidence for an archimedean analogue of Tate's conjecture, which expects that the spectrum of the Laplacian determines the arithmetic of such spaces. 
C. S. Rajan