  We consider a multi-dimensional billiard system in an (n+1)-dimensional
Euclidean space, the direct product of the "horizontal" hyperplane and the
"vertical" line. The hypersurface that determines the system is assumed to be
smooth and symmetric in all coordinate hyperplanes. Hence there exists a
periodic orbit $\gamma$ of period 2 moving along the "vertical" coordinate
axis. The question we ask is as follows. Is it possible to choose such a system
to have the dynamics locally (near $\gamma$) conjugated to the dynamics of a
linear map?
  Since the problem is local, the billiard hypersurface can be determined as
the graphs of the functions $\pm f$, where $f$ is even and defined in a
neighborhood of the origin on the "horizontal" coordinate hyperplane. We prove
that $f$ exists as a formal Taylor series in the non-resonant case and give
numerical evidence for convergence of the series.
