Two Proofs of Haff’s Law for Dissipative Gases: the Use of Entropy and the Weakly Inelastic Regime

We revisit our recent contribution [2] and give two simpler proofs of the socalled Haff’s law for granular gases (with non-necessarily constant restitution coefficient). The first proof is based upon the use of entropy and asserts that Haff’s law holds whenever the initial datum is of finite entropy. The second proof uses only the moments of the solutions and holds in some weakly inelasticity regime which has to be clearly defined whenever the restitution coefficient is non-constant.