In Robinet et al. (2000), an instability for shock waves in gas dynamics is exhibited.
The fluid was assumed to obey the polytropic perfect gas pressure law. We show in
this note that such an instability does not occur, as proved in the extensive work of
Majda (1983). Two arguments are developed: we give first a mathematical result that
is violated by the conclusions of Robinet et al. (2000). Then we detail the results of
Robinet et al. (2000) and show why they do not yield any conclusions. Finally, we
develop a general calculation and show that an instability cannot occur.