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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.

4 votes
1 answer
273 views

Can every diffeomorphism be rescaled into a volume preserving one?

This is a cross-post. Let $D \subseteq \mathbb{R}^2$ be the closed unit disk, and let $f:D \to D$ be a diffeomorphism. Does there exist a smooth $h \in C^{\infty}(D)$ such that $h\cdot f$ is an ar …
9 votes
0 answers
287 views

Is there a variational interpretation for the equation $\operatorname{div}(\star \circ \bigw...

$\newcommand{\id}{\operatorname{Id}} \newcommand{\R}{\mathbb{R}} \newcommand{\TM}{\operatorname{TM}} \newcommand{\Hom}{\operatorname{Hom}} \newcommand{\Cof}{\operatorname{Cof}} \newcommand{\Det}{\oper …
2 votes
0 answers
101 views

Is there a non-degenerate solution for this PDE on $\mathbb{R}^3$?

$\newcommand{\tr}{\operatorname{tr}}$ $\newcommand{\R}{\mathbb{R}}$ Does there exist a smooth map $f:\mathbb{R}^3 \to \mathbb{R}^3$, which satisfies $$\tr \big( df \otimes \delta(df \wedge df) \big)=0 …
1 vote
0 answers
102 views

A PDE involving a diffeomorphism of $\mathbb{S}^1$

This question is a special case of this one. Let $s(\theta)>0, b(\theta)$ be two smooth non-constant real-valued functions on $\mathbb{S}^1$. Do there exist a diffeomorphism $\phi:\mathbb{S}^1 \to \ma …
2 votes
0 answers
143 views

Does this geometric PDE have a solution?

Let $s(\theta), b(\theta)$ be two smooth non-constant real-valued functions on $\mathbb{S}^1$, and assume that $s$ never vanishes. Does there exist a map $h:(0,1) \times \mathbb{S}^1 \to \mathbb{S}^1$ …
5 votes
1 answer
595 views

A vector field whose flow has constant singular values

$\newcommand{\tr}{\operatorname{tr}}$ $\renewcommand{\div}{\operatorname{div}}$ Let $D \subseteq \mathbb{R}^2$ be the closed unit disk. Given a vector field $X$ on $D$, let $\psi_t$ be its flow. Does …
2 votes
1 answer
164 views

The only rotation fields satisfying this PDE are constant

$\newcommand{\div}{\operatorname{div}}$$\newcommand{\SO}{\operatorname{SO(2)}}$$\newcommand{\R}{\operatorname{\mathbb{R}}}$$\newcommand{\bdx}{\partial_x}$$\newcommand{\bdy}{\partial_y}$$\newcommand{\t …
1 vote
2 answers
309 views

Is there an area-preserving concentric diffeomorphism of the ellipse?

$\DeclareMathOperator\Vol{Vol}$This is a cross-post. Let $0<b<1$ be a fixed parameter, and let $(R(\theta),\theta)$ be the polar coordinates of the ellipse $$E=\{(x,y) \in \mathbb R^2 \, | \, \Bigl(\ …
1 vote
0 answers
108 views

Is there a concentric map from the disk onto the ellipse with constant sum of singular values?

$\newcommand{Vol}{\text{Vol}}$ Let $c > 2$, and let $0<b<1$ be fixed parameters. Does there exist a $C^1$ monotone bijection $\psi:(0,1] \to (0,1]$, and a $C^1$ function $h:(0,1] \to \mathbb{R}$ that …
3 votes
0 answers
112 views

Are continuous harmonic maps between Riemannian manifolds smooth up to the boundary?

Let $M,N$ be smooth, connected, compact, oriented, two-dimensional Riemannian manifolds, with $C^k$ boundaries. Let $f:M \to N$ be a Lipschitz continuous weakly harmonic map**, and assume that $f(\par …
1 vote
1 answer
248 views

Local obstructions for maps with constant singular values

$\newcommand{\M}{\mathcal{M}}$ $\newcommand{\N}{\mathcal{N}}$ Let $\M, \N$ be smooth two-dimensional Riemannian manifolds. Are there any local obstructions for the existence of a smooth map $f:\M \to …
0 votes
0 answers
152 views

Metric obstructions for area-preserving diffeomorphisms with constant singular values

Let $\mathbb{T}^2$ be the topological $2$-dimensional torus, and let $0<\sigma_1 < \sigma_2$ satisfy $\sigma_1 \sigma_2=1$. Let $g$ be an arbitrary smooth Riemannian metric on $\mathbb{T}^2$. Does the …
7 votes
1 answer
403 views

A diffeomorphism of the torus with constant singular values

Let $\mathbb{T}^2=\mathbb{S}^1 \times \mathbb{S}^1$ be the flat $2$-dimensional torus, and let $0<\sigma_1 < \sigma_2$ satisfy $\sigma_1 \sigma_2=1$. Does there exist an area-preserving diffeomorphism …
2 votes
3 answers
258 views

How to show continuity and monotonicity of solutions to this parametrized equation?

Let $1 \le p <2$ be a parameter. Consider the equation $$ \frac{2^{p/2} (1-\sqrt{s})^p-1}{\sqrt{s}}=-2^{p/2-1}p(1-\sqrt{s})^{p-1}. \tag{1} $$ I am rather certain that for each $1 \le p <2$, there is u …
2 votes
1 answer
372 views

Is there a diffeomorphism of the disk with constant sum of singular values?

This question is a relaxed version of this question. Let $D \subseteq \mathbb{R}^2$ be the closed unit disk, and let $c \ge 2$. Does there exist a diffeomorphism $f:D \to D$ with constant sum of …

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