In this paper different types of stabilities (global, local) with respect to instantaneous perturbations and permanent source produced time harmonic perturbations are presented in case of a spatially developing gas flow. Some types of instabilities (global absolute, local convective) are also presented. For this purpose the Euler equations linearized at the constant gas flow are used. It is shown for instance, that the constant gas flow is global absolutely unstable with respect to some

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... neous and some permanent source produced time harmonic perturbations. The locally convective instability is also proven with respect to some instantaneous and permanent source produced time harmonic perturbations.