  Let L be a finite extension of Q_p and d a positive integer. A conjecture,
due to C. Breuil and P. Schneider, says that the existence of invariant norms
on certain locally algebraic representations of GL_{d+1}(L) should be
equivalent to the existence of certain (d+1)-dimensional de Rham
representations of Gal(\bar{L}/L). We prove the easy direction of this
conjecture: the existence of invariant norms implies the existence of
admissible filtrations, by generalizing an idea of M.Emerton.
