# Asymptotics for solution of transport equation and characteristics

Consider the transport equation $$u_t(t,x) + v(t,x) \cdot \nabla u(t,x) = 0.$$ Suppose that the solution of the characteristic equation $$\dot X(t) = v(t,X(t))$$ decays to zero as $$t \to \infty$$. What happens to the solution $$u$$ of the PDE as $$t \to \infty$$? Does it also decay to zero or to the Dirac delta as the weak solution formula $$\int_{\mathbb R^N} \phi u(t,x)dx = \int_{\mathbb R^N} \phi(X(t,x))u_0(x)dx \qquad \phi \in C^\infty_c$$ suggests?

• V is invompressible if its divergence is 0 everywhere. Such flows cannot have sinks as required by your 2nd eq. Jun 28, 2020 at 16:34
• @PiyushGrover Thanks. Can you show why if $\mathrm{div} v \neq 0$ then the solution of the ODE cannot decay to zero?
– Zac
Jun 28, 2020 at 16:39
• I am saying the opposite, that is if div v=0, then ODE cannot decay to 0 for all initial conditions.. Just take tiny circle around origin and apply divergence thm. Since all traj. are going into that circle, the line integral will be non-zero, but the area integral is 0 if div.v=0. Jun 28, 2020 at 16:44
• @PiyushGrover This counterexample is not clear to me: where are you applying the divergence theorem? Let's start over: if div v = 0, is it possible to prove that $X(t) > c \ge 0$ for every $t>0$?
– Zac
Jun 28, 2020 at 16:55
• Take a 2D example with 0 divergence. $\dot{x}=x$,$\dot{y}=-y$. See what you get. Jun 28, 2020 at 21:20

Let me change slightly your notations with the flow $$\psi (t,y)$$ defined by $$\dot \psi(t,y)=v(t, \psi(t,y)),\quad \psi(0,y)=y.$$ The solution $$u$$ is constant along the integral curves of the vector field, i.e. $$u(t,\psi(t,y))=u(0, y).$$ Using the inverse function theorem you can introduce $$\phi(t,x)$$ to be a first integral defined by $$x=\psi(t,y)\Longleftrightarrow y=\phi(t,x).$$ It is possible locally and let us assume that we can do that globally. Then we have $$u(t,x)=u(t=0, \phi(t,x))=u_{0}(\phi(t,x)).$$ Assuming for instance that the initial datum $$u_{0}$$ is compactly supported or decays at infinity, you will get decay for the solution $$u$$ whenever $$\phi(t,x)$$ goes to infinity when $$t\rightarrow+\infty$$. The natural condition for decay of $$u$$ whenever the Cauchy datum $$u_{0}$$ is say compactly supported is that the first integral (which is the inverse function of the flow) goes to infinity with $$t$$.
• Thanks! I have some additional questions: 1. Does $\psi \to 0$ imply $\phi \to \infty$ as $t \to \infty$? 2. What happens in general, for example for measure initial data? Do we have convergence to the Dirac delta?
• @Zac Since $x=\psi(t,y)$ is equivalent to $y=\phi(t,x)$, we have $x=\psi(t,\phi(t,x))$. Assuming $x\not=0$ we must have that $\phi(t,x)$ goes to infinity, otherwise under a mild continuity assumption, $x=\psi (+\infty,0)=0$. Jun 29, 2020 at 10:20
• @Zac Well, under some conditions of regularity and behavior at infinity, the solution of the transport equation is given via the first integral above with the formula $u=u_0(\phi(t,x))$, where $u_0$ is the Cauchy datum. To prove convergence to the Dirac mass at $x=0$, you take $u_0=\delta_0$ which is indeed well localized; I guess that the arguments sketched above show that $\phi(t,x)$ goes to infinity for $x\not=0$, so that $u=u_0(\phi(t,x))=0$, proving that the limit distribution is supported at 0. Jun 29, 2020 at 16:28