  A special version of multi--dimensional simple waves given in [G. Boillat,
{\it J. Math. Phys.} {\bf 11}, 1482-3 (1970)] and [G.M. Webb, R. Ratkiewicz, M.
Brio and G.P. Zank, {\it J. Plasma Phys.} {\bf 59}, 417-460 (1998)] is employed
for fully relativistic fluid and plasma flows. Three essential modes: vortex,
entropy and sound modes are derived where each of them is different from its
nonrelativistic analogue. Vortex and entropy modes are formally solved in both
the laboratory frame and the wave frame (co-moving with the wave front) while
the sound mode is formally solved only in the wave frame at ultra-relativistic
temperatures. In addition, the surface which is the boundary between the
permitted and forbidden regions of the solution is introduced and determined.
Finally a symmetry analysis is performed for the vortex mode equation up to
both point and contact transformations. Fundamental invariants and a form of
general solutions of point transformations along with some specific examples
are also derived.
