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Partial differential equations (PDEs): Existence and uniqueness, regularity, boundary conditions, linear and non-linear operators, stability, soliton theory, integrable PDEs, conservation laws, qualitative dynamics.

2 votes
Accepted

Constant in Poincaré Inequality

This is a fairly standard stuff. Suppose that the Stokes operator $A=-\Delta$ is defined on smooth divergence-free vector fields $u$ which satisfy the standard no-slip boundary condition $$\qquad\qqua …
Andrey Rekalo's user avatar
9 votes

PDEs as a tool in other domains in mathematics

PDEs are massively used in the theory of harmonic maps. My personal favourite is a nice theorem by Lemaire and Sacks-Uhlenbeck. Theorem. Suppose $M$ is a compact Riemann surface, possibly with bo …
3 votes
Accepted

Invertibility of the Laplacian

A careful exposition can be found in "Analytic Semigroups and Semilinear Initial Boundary Value Problems" by Kazuaki Taira. He considers the mixed boundary value problem $$\begin{cases} A u=f & \mb …
Andrey Rekalo's user avatar
12 votes

Harmonic Functions

$f$ is harmonic under the weaker assumption that it is just continuous. Multiplying the identity $$f(x+\delta,y+\delta)+f(x-\delta,y+\delta)+u(x-\delta,y-\delta) + f(x+\delta,y-\delta)-4f(x,y)=0$$ wi …
Andrey Rekalo's user avatar
4 votes

Trace space and Neumann boundary condition

If you are interested in $L^p$-theory, you are probably looking for solutions belonging to a Sobolev class $H^{s,p}(\Omega)$ with some $s>0$ and $p>1$. In this case, the Besov space $B^{s-1-1/p,p}(\p …
Andrey Rekalo's user avatar
46 votes

Counterexamples in PDE

Scheffer has shown that there is a nontrivial weak solution $u(x,t)\in L^2(\mathbb R^2\times\mathbb R)$ to the incompressible Euler equations in 2D $$\begin{cases} \frac{\partial u}{\partial t}+\nabla …
4 votes

Can the "physical argument" for the existence of a solution to Dirichlet's problem be made i...

The electrostatic intuition does lead to a correct mathematical formulation of the Dirichlet problem. Let's consider an electric charge distribution of two thin layers (one layer is positive and the …
Andrey Rekalo's user avatar
19 votes
Accepted

Is there a mathematically precise definition of turbulence for solutions of Navier-Stokes?

There is probably no universally accepted mathematical definition of turbulence. (By the way, is there a physical one?) Moreover, the prevailing definitions seem to be highly volatile and time-depend …
Andrey Rekalo's user avatar
13 votes

Where was/is Compensated Compactness used?

Compensated compactness helps when one needs to find the limit of $u_n \cdot v_n$, where the sequences of vector fields $u_n$ and $v_n$ converge weakly in $L^2$: $u_n\rightharpoonup u$, $v_n\righth …
Andrey Rekalo's user avatar
33 votes
5 answers
3k views

How to define a differential form on a fractal?

It is well known how to construct a Laplacian on a fractal using the Dirichlet forms (see e.g. the survey article by Strichartz). This implies, in particular, that a fractal can be "heated", i.e. one …
Andrey Rekalo's user avatar