Erratum: "derivation of amplitude equations and analysis of side-band instabilities in two-layer flows" (vol 5, pg 2738, 1993)

In the amplitude equations, one further term should have been taken into account. This term is formally of higher order when the original scaling of variables is used. However, the analysis of sideband instabilities involves various rescalings, and for one of the cases the term becomes of the same order as others. It therefore affects the criteria for sideband instability.