An overview on some results concerning the transport equation and its applications to conservation laws
Gianluca Crippa Laura V. Spinolo
We provide an informal overview on the theory of transport equa- tions with non smooth velocity fields, and on some applications of this theory to the well-posedness of hyperbolic systems of conservation laws.
keywords: BV functions. Continuity equation conservation laws
Leaf superposition property for integer rectifiable currents
Luigi Ambrosio Gianluca Crippa Philippe G. Lefloch
We consider the class of integer rectifiable currents without boundary in $\R^n\times\R$ satisfying a positivity condition. We establish that these currents can be written as a linear superposition of graphs of finitely many functions with bounded variation.
keywords: Metric spaces valued $BV$ functions Multi-valued functions. Integer rectifiable currents Currents in metric spaces Cartesian currents
Strong continuity for the 2D Euler equations
Gianluca Crippa Elizaveta Semenova Stefano Spirito
We prove two results of strong continuity with respect to the initial datum for bounded solutions to the Euler equations in vorticity form. The first result provides sequential continuity and holds for a general bounded solution. The second result provides uniform continuity and is restricted to Hölder continuous solutions.
keywords: stability renormalized solutions. vorticity continuity Euler equations
Flows of vector fields with point singularities and the vortex-wave system
Gianluca Crippa Milton C. Lopes Filho Evelyne Miot Helena J. Nussenzveig Lopes
The vortex-wave system is a version of the vorticity equation governing the motion of 2D incompressible fluids in which vorticity is split into a finite sum of Diracs, evolved through an ODE, plus an $L^p$ part, evolved through an active scalar transport equation. Existence of a weak solution for this system was recently proved by Lopes Filho, Miot and Nussenzveig Lopes, for $p>2$, but their result left open the existence and basic properties of the underlying Lagrangian flow. In this article we study existence, uniqueness and the qualitative properties of the (Lagrangian flow for the) linear transport problem associated to the vortex-wave system. To this end, we study the flow associated to a two-dimensional vector field which is singular at a moving point. We first observe that existence and uniqueness of the regular Lagrangian flow are ensured by combining previous results by Ambrosio and by Lacave and Miot. In addition we prove that, generically, the Lagrangian trajectories do not collide with the point singularity. In the second part we present an approximation scheme for the flow, with explicit error estimates obtained by adapting results by Crippa and De Lellis for Sobolev vector fields.
keywords: renormalized solutions singular vector fields. vortex-wave system continuity and transport equations regular Lagrangian flows Euler equations
Logarithmic estimates for continuity equations
Maria Colombo Gianluca Crippa Stefano Spirito
The aim of this short note is twofold. First, we give a sketch of the proof of a recent result proved by the authors in the paper [7] concerning existence and uniqueness of renormalized solutions of continuity equations with unbounded damping coefficient. Second, we show how the ideas in [7] can be used to provide an alternative proof of the result in [6,9,12], where the usual requirement of boundedness of the divergence of the vector field has been relaxed to various settings of exponentially integrable functions.
keywords: Lagrangian flows Continuity and transport equations renormalized solutions. well-posedeness

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