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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.

5 votes
Accepted

Asymptotic parametrization for negatively curved surfaces

As asked, the answer to the question is 'no'. The simply-connected cover $f:\mathbb{R}^2\to S$ of Sherck's first surface $S$ (which is defined in $\mathbb{R}^3$ by the equation $\mathrm{e}^{z} \cos x …
Robert Bryant's user avatar
10 votes
Accepted

Understanding exterior differential systems

Here's an expansion of my comment that the natural formulation of this problem as an EDS is on the coframe bundle $\pi: P\to M$, which, I hope, will be helpful. Also, because it will match my usual n …
Robert Bryant's user avatar
3 votes

Integrability of modified diagonalizable Jacobian

It has taken me a while to find time to write a more comprehensive answer to the above question. It turns out that for general dimension $N$, the overdetermined PDE system involved is not involutive, …
Robert Bryant's user avatar
6 votes
Accepted

Systems of (hyperbolic) 2nd order PDEs with lower order constraints

Yes, there is a standard procedure to analyze such systems, essentially, it is Cartan's method of prolongation combined with his theory of involutive systems. There are other approaches as well, but …
Robert Bryant's user avatar
1 vote
Accepted

Exterior differential systems on compact three-manifolds and Cartan-Kähler theory

In their comments to my first answer, the OP has clarified that they did not mean to regard the metric $h$ as a given, but, rather, an output of the problem of prescribing coframings by specifying the …
Robert Bryant's user avatar
3 votes

Exterior differential systems on compact three-manifolds and Cartan-Kähler theory

Let me phrase the problem as I understand the given data and then describe how the 'theory of exterior differential systems' would be applied. One starts with a compact Riemannian $3$-manifold $(M,h)$ …
Robert Bryant's user avatar
3 votes

Heat kernel of left-invariant metric on 3-sphere

I don't know a formula for $d(e^x,e^y)^2$, and I suspect that there is no simple formula, but the answer to Q2' is 'no'. The right hand side of (3) is linear in $Q^{-1}$, but it is not hard to see th …
Robert Bryant's user avatar
5 votes
Accepted

On a result of Cartan for homogeneous manifolds arising from a quotient of discrete subgroups

The result that you are looking for is not in Élie Cartan's 1936 book La topologie des groupes de Lie because it was not known to be true at the time the book was written. Indeed, as Cartan remarks i …
Robert Bryant's user avatar
8 votes
Accepted

Building a geodesic conjugate parameterization on catenoid

I thought about your problem and realized that there is no coordinate parametrization of the catenoid with the properties that you want. Here is my argument: First, note that, in the given $uv$-param …
Robert Bryant's user avatar
3 votes
Accepted

Method of characteristics with 2 dependent variables in 3 dimensions

The method of characteristics is a bit strange here because the equation is underdetermined, so one can't expect to be able to specify a solution by fixing initial data for $u$ and $v$ along a surface …
Robert Bryant's user avatar
5 votes
Accepted

Approximate isometric embeddings of surfaces

I think that the answer is 'yes' if $U$ is simply-connected, because there is a way to construct a candidate 'approximate surface' from 'approximate solutions' of Gauss and Codazzi, but a more useful …
Robert Bryant's user avatar
3 votes
Accepted

Hyperbolic system of PDEs with elliptic-like boundary contions

Here is an example for which there is no solution: Let $\Omega_1$ be defined by $x^2+y^2\le 1$ and $\Omega_1$ be defined by $X^2+Y^2\le R^2$, where $R>0$ is large. Take $Z(X,Y) = 0$. Then one is as …
Robert Bryant's user avatar
9 votes
Accepted

Existence of solution to linear inhomogeneous first order PDEs systems

You are correct that Cauchy-Kovalevskaya does not apply directly to this problem, but there are other theorems that give sufficient conditions, provided that you make certain basic regularity assumpti …
Robert Bryant's user avatar
13 votes
Accepted

Does the first Laplacian eigenfunction on a homogeneous space have a unique maximum?

The flat torus $\mathbb{T} = \mathbb{R}^2/\Lambda$ gives a counterexample: The first nontrivial eigenvalue is of the form $\lambda_1 = \xi_1^2+\xi_2^2$, where $\xi = (\xi_1,\xi_2)$ is a nonzero eleme …
Robert Bryant's user avatar
9 votes
Accepted

Solution existence in a pde system

Yes. Here is a general approach to this problem: Suppose that one has two functions a>0 and b on some interval $I\subset\mathbb{R}$ and one wants to know whether there is a solution $f$ to the syste …
Robert Bryant's user avatar

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