Lorentz invariance relations between parton distributions and the   Wandzura-Wilczek approximation
  The violation of the so-called Lorentz invariance relations between parton distribution functions is considered in a model independent way. It is shown that these relations are not violated in a generalized Wandzura-Wilczek approximation, indicating that numerically their violation may be small. 
A. Metz, P. Schweitzer, T. Teckentrup