A note on the ultrahyperbolic wave equation in 3+3 dimensions
  The article concerns the relationship between the ultrahyperbolic wave operator and the Xi transform for flat space, introduced by Sparling (2007). The main theorem is that the wave operator and the Xi transform form an exact couple: the image of the wave operator is the kernel of the Xi transform and, vice versa, the image of the Xi transform is the kernel of the wave operator. 
Jonathan Holland
math-ph math.MP