An Exact, Three-Dimensional, Time-Dependent Wave Solution in Local   Keplerian Flow
  We present an exact three-dimensional wave solution to the shearing sheet equations of motion. The existence of this solution argues against transient amplification as a route to turbulence in unmagnetized disks. Moreover, because the solution covers an extensive dynamical range in wavenumber space, it is an excellent test of the dissipative properties of numerical codes. 
Steven A. Balbus and John F. Hawley