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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.

6 votes
Accepted

Reference for $\epsilon$-regularity

First, it is straightforward to adapt Haslhofer's proof to higher dimensions, using the Sobolev inequality: For any compactly supported $u$ such that $\|\nabla u\|_2 < \infty$, $$ \|u\|_{\frac{2n}{n-2 …
Deane Yang's user avatar
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3 votes
Accepted

Well-posedness of PDE with $\partial_{tt}\Delta u$ - like term

Here's what I would try: Check that for each $t$ and $f \in S$, where $S$ is an appropriately chosen function space, there is a unique solution to the elliptic problem: $$ L\phi = f \text{ on }\Omega …
Deane Yang's user avatar
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8 votes
Accepted

Elliptic regularity on manifolds: Is this true?

Use a partition of unity to reduce the statement to a local one for a function compactly supported on a coordinate chart. At that point, any elliptic regularity theorem on an open domain in Rn can be …
Deane Yang's user avatar
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24 votes
Accepted

PDEs and algebraic varieties

What has been studied more extensively is the top order term, which is known as the principal symbol. Since it is a homogeneous polynomial of degree $d$, it defines a real projective algebraic variety …
Deane Yang's user avatar
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3 votes
Accepted

Reference request: inverse of differential operators

There are many possible answers to this question, depending on the type of the differential operator, the domain of the functions, and whether you want to impose any additional conditions such as spec …
Deane Yang's user avatar
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2 votes

Explicit solutions for linear system of PDEs with constant coefficients

ADDDED: Since equation (1) below is a first order linear ODE,it has an explicit solution (with an integral). It follows that the solution to the entire system of equations can be written explicitly as …
Deane Yang's user avatar
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15 votes

Applications of microlocal analysis?

Although microlocal analysis was developed originally exclusively for linear problems, it has played an increasingly important role in nonlinear PDE via what's known as paradifferential calculus. Ther …
Martin Sleziak's user avatar
21 votes

Does Ricci flow depend continuously on the initial metric?

If you use the right topology on the space of metrics, the answer is yes. Basically, this is always true and a consequence of the proof for any theorem on the existence, uniqueness, and regularity of …
Martin Sleziak's user avatar
3 votes
Accepted

How to prove the reverse Hölder inequality for Laplace equations?

The inequality is scale invariant and holds for a ball of any radius. It follows by a standard argument that is the inductive step in what's known as Moser iteration. The constant $C$ below can change …
Deane Yang's user avatar
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3 votes
Accepted

Do Laplace-Beltrami eigenfunctions vary continuously with the metric?

EDITED: Added clarification, as pointed out by @TerryTao. Let $g_1$, $g_2$ be Riemannian metrics and $\Delta_1$, $\Delta_2$ their respective Laplacians. Let $\lambda_1$ be an eigenvalue of $\Delta_1$ …
Deane Yang's user avatar
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1 vote

Local solvability and Cauchy-Kovalevskaya theorem for PDEs

The system you wrote down First, let's assume everything is smooth. \begin{align*} u_t &= v\\ v_{tt} &= u_x\\ \end{align*} is equivalent to the first order system \begin{align*} u_t &= v\\ v_t …
Deane Yang's user avatar
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4 votes
Accepted

Existence for an overdetermined system of PDEs

COMMENT: The answer below is just the proof of the Frobenius theorem (https://en.wikipedia.org/wiki/Frobenius_theorem_(differential_topology) applied to this specific case. The arguments below are als …
Deane Yang's user avatar
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6 votes

System of linear pde with non constant coefficients

If you switch the second and third rows of your system, the differential operator is the same as the linearization of equation (4.3) in Existence of elastic deformations with prescribed principal stra …
Deane Yang's user avatar
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1 vote

The Monge- Ampère equation with a non positive right hand side

In general nothing is known. Only local solvability is known for some special cases.
Deane Yang's user avatar
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6 votes
Accepted

Hyperbolic PDE in mathematics

Hyperbolic PDEs arise unexpectedly in some differential geometric questions involving prescribed data. What's weird in these cases is that there is no natural time coordinate in the PDEs. Here are som …
Deane Yang's user avatar
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