Un sous-syst\`eme d'un syst\`eme plat de dimension diff\'erentielle 2 est plat. A subsystem of a flat system of differential dimension 2 is flat

Un sous-syst`eme de dimension diff'erentielle au plus 2 d’une extension plate est plate. Si un tel syst`eme plat est stationnaire, il admet des sorties plates ind'ependantes du temps. A subsystem of a flat system of differential dimension at most 2 is flat. Furthermore, if such a flat system is stationary, we show that there exist flat utputs not depending on the time.