Invariant Integrability Criterion for the Equations of Hydrodynamical   Type
  Invariant integrability criterion for the equations of hydrodynamical type is found. This criterion is written in the form of vanishing for some tensor which is derived from the velocities matrix of hydrodynamical equations. 
M.V. Pavlov (Landau Institute for Theor. Physics., Moscow, Russia),   R.A. Sharipov (Dep. of Math., Bashkir State Univ., Ufa...