We introduce a variational time discretization for the multi-dimensional gas dynamics equations, in the spirit of minimizing movements for curves of maximal slope. Each timestep requires the minimization of a functional measuring the acceleration of fluid elements, over the cone of monotone transport maps. We prove convergence to measure-valued solutions for the pressureless gas dynamics and the compressible Euler equations. For one space dimension, we obtain sticky particle solutions for the pressureless case.Density and pressure define the specific thermodynamical entropy, given asin the case of polytropic gases. We assume thatso that the entropy density σ = ̺S is well-defined as a measure. Definition 1.1 (Internal Energy). Let U (r, S) := κe S r γ for all r 0 and S ∈ Ê, where κ > 0 and γ > 1 are constants. Then we define the internal energy U[̺, σ] := ˆÊ d U r(x), S(x) dx if ̺ = rL d and σ = ̺S, ∞ otherwise, for all pairs of measures (̺, σ) ∈ P(Ê d ) × M + (Ê d ).Since we are only interested in solutions with finite energy, the density must be absolutely continuous with respect to the Lebesgue measure, thus ̺(t, ·) = r(t, ·) L d . In this case, we define P (r, S) = U ′ (r, S)r − U (r, S) (here ′ denotes differentiation with respect to r), and the pressure term in (1.13) takes the form p(t, ·) = P r(t, ·), S(t, ·) L d for all t ∈ [0, ∞).(1.7)Moreover, combining (1.6) and (1.7) with (1.1), we obtain that(1.8)Equivalently, the specific entropy S must be constant along characteristics:(1.9)Formally, system (1.1) is equivalent to the one where the energy equation is replaced by (1.8) (or even (1.9)). But since the solutions to the compressible Euler equationsthe Lipschitz seminorm, with W 2 the Wasserstein distance; see (2.1).Definition 1.3. We denote by M Ent (Ê d ) the space of non-negative Borel measures with finite second moments and total variation equal to Ent ∈ [0, ∞), endowed with as suitably rescaled Wasserstein distance; see Definition 2.1.We will assume that the total momentum vanishes initially, which implies that the total momentum vanishes for all t > 0. This is not a restriction as the hyperbolic conservation law (1.1) is invariant under transformations to a moving reference frame in the absence of boundaries. The momentum map t → m t = ̺ t v t takes values in a convex set of Ê d -valued Borel measures whose total variations are uniformly bounded, as a consequence of a bound on the total energy. On this set, the narrow convergence of measures is metrized by the Monge-Kantorovich norm: Definition 1.4. We denote by Lip(Ê d ; Ê N ) the vector space of Lipschitz continuous maps ζ :We denote by BL(Ê d ; Ê N ) the subspace of bounded functions in Lip(Ê d ; Ê N ). It is a Banach space when equipped with the bounded Lipschitz normLet BL 1 (Ê d ; Ê N ) be the space of all ζ ∈ BL(Ê d ; Ê N ) with ζ BL(Ê d ) 1.
FABIO CAVALLETTI, MARC SEDJRO, AND MICHAEL WESTDICKENBERGWe denote by M K (Ê d ; Ê N ) the space of Ê N -valued Borel measures m with zero mean and finite first moment, equipped with the Monge-Kanto...