  Let Gamma be a cocompact lattice in SO(1,n). A representation rho: Gamma \to
SO(2,n) is quasi-Fuchsian if it is faithfull, discrete, and preserves an
acausal subset in the boundary of anti-de Sitter space - a particular case is
the case of Fuchsian representations, ie. composition of the inclusions of
Gamma in SO(1,n) and of SO(1,n) in SO(2,n). We prove that if a representation
is Anosov in the sense of Labourie then it is also quasi-Fuchsian. We also show
that Fuchsian representations are Anosov : the fact that all quasi-Fuchsian
representations are Anosov will be proved in a second part by T. Barbot. The
study involves the geometry of locally anti-de Sitter spaces: quasi-Fuchsian
representations are holonomy representations of globally hyperbolic spacetimes
diffeomorphic to the product R \times Gamma\H^n and locally modeled on the
anti-de Sitter space.
