Invariants for systems of two linear hyperbolic-type equations by   complex methods
  Invariants of general linear system of two hyperbolic partial differential equations (PDEs) are derived under transformations of the dependent and independent variables by real infinitesimal method earlier. Here a subclass of the general system of linear hyperbolic PDEs is investigated for the associated invariants, by complex as well as real methods. The complex procedure relies on the correspondence of systems of PDEs with the base complex equation. Complex invariants of the base complex PDEs are shown to reveal invariants of the corresponding systems. A comparison of all the invariant quantities obtained by complex and real methods for this class, is presented which shows that the complex procedure provides a few invariants different from those extracted by real symmetry analysis. 
A. Aslam, M. Safdar and F. M. Mahomed
math.CA