  The class of Divergence-free symmetric tensors is ubiquitous in Continuum
Mechanics. We show its invariance under projective transformations of the
independent variables. This action, which preserves the positiveness, extends
Sophus Lie's group analysis of Newtonian dynamics.When applied to models of gas
dynamics --~such as Euler system or Boltzmann equation,~-- in combination with
Compensated Integrability, this yields new dispersive estimates. The most
accurate one is obtained for mono-atomic gases. Then the space-time integral of
$t\rho^\frac1d p$ is bounded in terms of the total mass and moment of inertia
alone.

Analysis of PDEs;Classical Physics