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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.

31 votes
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Has it been proved that weak solutions to the Navier-Stokes equations are non-unique, and do...

In regards to the question of the "consensus" or "correctness", I will only point out that Tristan Buckmaster has had a proven record of studying nonuniqueness problems for low-regularity solutions in …
Willie Wong's user avatar
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28 votes
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Why don't we study hyperbolic equations as elliptic and parabolic equations?

Why we do not study such estimates for hyperbolic equations? Because they are false. Now: you may ask "why are they false?" This is a fairly deep question, and answers often involve discussion of p …
Willie Wong's user avatar
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22 votes
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Why don't existence and uniqueness for the Boltzmann equation imply the same for Navier-Stokes?

Okay, after figuring out which paper you were trying to link to in the third link, I decided that it is better to just give an answer rather then a bunch of comments. So... there are several issues at …
Willie Wong's user avatar
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19 votes

Method of characteristics for higher order PDEs in more than two variables

I hope to use this answer to convince you that in general the method of characteristics cannot work for higher order PDEs in more than 2 variables. Nevertheless, there are some ideas in PDEs that are, …
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19 votes
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Einstein field equations in perspectives from PDE and functional analysis

The statement It seems that the classical programme of the PDE community, i.e., (i) existence (ii) uniqueness (iii) regularity, heavily employing concepts from functional analysis, has not found prom …
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17 votes

Can an integral equation always be rewritten as a differential equation?

In general, no. An integral equation can be non-local, whereas a differential equation is local (in the sense that it can be described by a function over the jet-bundle). As an illustration Let $K(x) …
Willie Wong's user avatar
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16 votes

When to use more exciting function spaces than ordinary Sobolev spaces?

In many aspects of dispersive PDEs, the "optimal" function spaces are those adapted to the symbol of the linear evolution. They were introduced by Bourgain for the nonlinear Schroedinger equation (and …
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13 votes

What are the interesting cases of the generalized Korteweg-de Vries equation?

José is correct in his comment. Just to elaborate: in the linear case, one can easily study the equation using Fourier methods. Let $\tilde{u}$ denote the space-time Fourier transform and $\hat{u}$ de …
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11 votes
Accepted

Does the Poincaré inequality hold on annular domains?

I will prove the stronger result without the subtraction of $\bar{f}$. As we know $\int |f|^2 = \int |f - \bar{f}|^2 + \int |\bar{f}|^2$, the result without subtracting $\bar{f}$ would imply what you …
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11 votes
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leray schauder fixed point and schauder fixed point

Note that Leray-Schauder is usually proven by using the hypotheses to construct a mapping that satisfies the conditions of the Schauder fixed point theorem, and then appealing to the Schauder fixed p …
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11 votes
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Does harmonic map heat flow of a curve always fully converge to a geodesic?

The situation is actually quite complicated, it seems. In the case where the target manifold is real analytic, Leon Simon's results in Asymptotics for a Class of Non-Linear Evolution Equations, with …
Willie Wong's user avatar
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10 votes

Where do some "energy identities" in PDE theory come from?

First a comment: in the context of nonlinear wave and Klein-Gordon equations, the venerable "ABC method" of Cathleen Morawetz is literally "mucking around until you see something". (The A, B, and C re …
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9 votes
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Do we have Pohozaev's identity on compact manifolds without boundary?

To answer this question, it is better to understand Pohozaev's identity using the heuristic argument given in Berestycki, Henri; Lions, Pierre-Louis, Nonlinear scalar field equations. I: Existence of …
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8 votes

PDEs as a tool in other domains in mathematics

Another not-quite-yet connection which I learned from Lax's Hyperbolic PDE book: one can, technically speaking, extract the Riemann hypothesis from the scattering rates of certain "automorphic waves". …
8 votes
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Looking for references to study $U^p$ and $V^p$ spaces

You can take a look at Herbert Koch's contribution in Koch, Herbert; Tataru, Daniel; Vişan, Monica, Dispersive equations and nonlinear waves. Generalized Korteweg-de Vries, nonlinear Schrödinger, wave …
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