Sum formula for Kloosterman sums and fourth moment of the Dedekind   zeta-function over the Gaussian number field
  We prove the Kloosterman-Spectral sum formula for PSL(2,Z[i])\PSL(2,C), and apply it to derive an explicit spectral expansion for the fourth power moment of the Dedekind zeta function of the Gaussian number field. This sum formula allows the extension of the spectral theory of Kloosterman sums to all algebraic number fields. 
Roelof W. Bruggeman, Yoichi Motohashi