Four-divergence as a paravector operator. Invariance of the wave   equation under orthogonal paravector transformation
  The article presents four identities containing the spatio-temporal differential operator also known as four-divergence. These equations are used to prove the invariance of wave equation under orthogonal paravector transformations. Moreover, the transformation of equations containing spatio-temporal differential operators under the rotation of reference frame has been presented. 
Jozef Radomanski
math-ph math.MP