Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

The tag has no usage guidance.

0
votes
0answers
42 views

About well-posedness and regularity for a wave equation with nonhomogeneous Dirichlet boundary condition?

I want to get any result about well-posedness and regularity for the following wave equation with nonhomogeneous Dirichlet boundary condition described by Let $\Omega \subset %TCIMACRO{\U{211d} }% %...
3
votes
1answer
122 views

Global regularity for Neumann problem

Let $\Omega\subset \mathbb{R}^d $ be a bounded open subset ($d\in \mathbb{N}$) and denote $\partial\Omega$ its boundary which we assume to be Lipschitz. The classical inhomogeneous Neumann problem ...
1
vote
1answer
115 views

Elliptic regularity of harmonic forms in $L^1$

$\newcommand{\M}{M}$ This is a cross-post. I am looking for a reference for the regularity of harmonic forms which belong to $L^1(M)$. Explicitly, let $\M$ be a smooth oriented Riemannian manifold. ...
3
votes
2answers
127 views

Density on Hölder spaces whose elements vanish on the boundary

I would like to ask the following problem. Let $\Omega$ be a $C^{r+1,\alpha}$ domain, $r\in \mathbb{N}, 0<\alpha<1.$ We denote $$C^{r,\alpha}_{0}(\overline{\Omega})=\{f\in C^{r,\alpha}(\...
1
vote
0answers
82 views

2nd oder evolution equations and regularity results of their solution

I am interested in regularity results for solutions to 2nd order evolution equations in the shape of $$ u''(t) + A(t)u(t) = f(t) \text{ in } V^*\text{ a.e. in }S, \\ u(0) = u_0 \text{ in } H, u'(0)...
0
votes
0answers
31 views

What is the minimum setting in which regularity results are available for the solutions of Poisson's equation?

Let the generator $L$ of a diffusion process be given in Hörmander form, i.e. $$L=\frac{1}{2}\sum_{i=1}^k X_i^2+X_0,$$ where $k\leq n$ and $X_i$, $i=0,1,...,k$, are vector fields on $\mathbb{R}^n$. ...
3
votes
0answers
38 views

Is a Sobolev map with smooth minors smooth on the whole domain?

This question is related (but not identical) to this question. Let $d>2$ be an integer and let $2 \le k \le d-1$ be a fixed integer. Suppose that at least one of $k,d$ is not even. Let $\Omega$ be ...
0
votes
0answers
32 views

Higher regularity of weak solution of Laplace equation with Robin condition

From [Grisvard, Thm. 2.4.2.7, p. 126], the BVP \begin{align} -\Delta u &= 0 & \text{in}\ \Omega\\ -\frac{\partial u}{\partial n} + au &= g & \text{on}\ \Gamma\\ \end{align} where $a&...
0
votes
1answer
77 views

Regularity of Laplace equation with Dirichlet data on a part of the boundary

From the introductory part of Chapter 2 of Grisvard's book, we know that the PDE system \begin{align} -\Delta u &= 0 &\text{in}\ \Omega\subset \mathbb{R}^2\\ u &= g &\text{on}\ \...
0
votes
1answer
64 views

$H^2$ regularity for Laplace equation with Robin-Robin boundary condition

From [Grisvard, Thm. 2.4.2.7, p. 126], the BVP \begin{align} -\Delta u &= 0 & \text{in}\ \Omega\\ -\frac{\partial u}{\partial n} + au &= g & \text{on}\ \Gamma\\ \end{align} where $a&...
2
votes
1answer
97 views

Higher regularity of solutions for Laplace equation with mixed boundary condition

Let $\Omega \subset \mathbb{R}^2$ be an open bounded Lipschitz domain of class $C^{1,1}$ with boundary $\partial \Omega = \Gamma_i \cup \Gamma_o$, $\Gamma_i \cap \Gamma_o = \emptyset$ and dist$(\...
5
votes
0answers
199 views

Is a Sobolev map with smooth minors smooth?

$\newcommand{\Cof}{\text{cof}}$ Let $d>2$ be an integer and let $2 \le k \le d-1$ be a fixed integer. Suppose that $k,d$ are both even. Let $\Omega$ be an open subset of $\mathbb{R}^d$, and let $f \...
2
votes
2answers
302 views

Reference for De Giorgi-Nash-Moser theory

I am interested in Holder regularity for equations of the form $$u_t - div A(x,t) \nabla u = 0$$ where $A(x,t)$ is bounded, measurable and elliptic. This was proved in the seminal paper of John Nash ...
2
votes
1answer
189 views

Is this approach for establishing regularity of harmonic maps between manifolds valid?

$\newcommand{\M}{\mathcal{M}}$ $\newcommand{\N}{\mathcal{N}}$ While trying to understand some regularity results, I thought about the following "naive" approach for establishing regularity of weakly ...
1
vote
0answers
22 views

Hidden regularity for the coupled wave equation with dynamaic boundary condition

We have the equation \begin{equation} \left\{ \begin{array}{rrrr} u_{tt}-\Delta u=0,&\text{in} & \Omega \times ]0,T[ & \left( 1.1\right) \\ u=0, & \text{on } & \Gamma _{0}\...
0
votes
0answers
38 views

Looser condition for regularity for Neumann problems

If $u(x) = g(|x|)$ is a rotationally symmetric function in $\mathbb{R}^{n+1}$ then $$\Delta u = g''(|x|) + n |x|^{-1} g'(|x|).$$ Let's say we are studying rotationally symmetric solutions to ...
1
vote
0answers
49 views

Dependence of the Hölder exponent in De Giorgi-Nash-Moser

I am curious about the Hölder exponent obtained by the De Giorgi-Nash-Moser theory, as a function of the ellipticity. More precisely: suppose $u$ satisfies weakly $$ D_i(a^{ij}D_ju)=f $$ on the $d$-...
1
vote
0answers
34 views

Harnack type Estimates for a p-Poisson equation with constant source term

Let $B=B_1(0)\subset \mathbb R^N$ and let $u\geq 0$ solve the PDE $$ -\Delta_p u=1\,\,\mbox{in $B$} $$ Let another function $f$ be such that $$ \begin{cases} -\Delta_p f =1 \;\;\mbox{in $B$}\\ f=0 \...
3
votes
0answers
85 views

Is there a weak isometric completion to a $W^{2,2}$ isometric immersion?

Let $g$ be a smooth Riemannian metric on $\mathbb{R}^d$. Let $D=D^k \subseteq \mathbb{R}^d$ be the $k$-dimensional closed unit disk. ($k<d$). Suppose we are given a $W^{2,2}$ isometric immersion $...
1
vote
1answer
83 views

Is there any “extra regularity” to the solution to Poisson's equation posed on a 3-dimensional polyhedron?

I am trying to write a proof and I am out of my depth. I need an elliptic regularity result of the form $$ \|u\|_{H^{1+\epsilon}(\Omega)} \le C \|f\|_{L^2(\Omega)} $$ for some $\epsilon >0 $ ...
8
votes
2answers
280 views

Is there a $W^{2,2}$ isometric embedding of the flat torus into $\mathbb{R}^3$?

It is well known that there exists a $C^1$ isometric embedding of flat torus into $\mathbb{R}^3$, and that this embedding cannot be $C^2$. Is there a $W^{2,2}$ isometric embedding? (i.e an isometric ...
1
vote
1answer
68 views

Advection equation regularity (2D and time independent)

I have been studying the 2D time-independent advection equation on the unit square $[0,1] \times [0,1]$. One such example is: $$ \frac{\partial}{\partial x} u(x,y) + \frac{\partial}{\partial y} u(x,y) ...
0
votes
0answers
69 views

Consequence of John and Nirenberg's lemma?

The lemma I'm referring to in the title is the following: John and Nirenberg's lemma: Let $C_0 \subset \mathbf{R}^n$ a finite cube. Let $u \in L^1(C_0)$ and assume there exists a constant $k$ such ...
5
votes
0answers
207 views

Eisenbud-Goto conjecture in Positive Characteristic

Eisenbud-Goto conjecture predicted that the Castelnuovo-Mumford regularity ${\rm reg}(X)$ of a non-degenerate projective variety $X\subset \mathbb{P}^N$ is bounded by the $\deg(X)-{\rm codim}(X,\...
3
votes
0answers
148 views

Elliptic regularity for Robin boundary conditions

Suppose I have a (non-smooth) domain $\Omega$ on which I have a $H^1$ solution $u$ of a constant coefficient elliptic PDE $L$. Suppose also that $\Gamma$ is a smooth portion of the boundary $\partial\...
9
votes
1answer
312 views

Existence and uniqueness of geodesics in low regularity

Consider a Riemannian manifold $(M,g)$. How much regularity is required of $g$ so that for any $x\in M$ and $v\in T_xM$ with $|v|=1$ there exists a unique geodesic $\gamma\colon(-\epsilon,\epsilon)\to ...
3
votes
0answers
70 views

Partial regularity for transmission problem in corner domains

Let $n=2$ or $3$ and $\Omega \subset \mathbb{R}^n$ be an open bounded domain. Let suppose that $\Omega$ is divided in two subdomains $\Omega_1$ and $\Omega_2$ and we define $\Gamma = \partial \Omega_1 ...
5
votes
3answers
269 views

Request for reference for some proofs about Gowers' norm

For any map $f : \mathbb{F}_2^n \rightarrow \mathbb{C}$ we define its $d^{th}-$Gowers' Norm (for $1 \leq d \leq n$) as, $\|f\|_{U^d(\mathbb{F}_2^n)}^{2^d} = \mathbb{E}_{L : \mathbb{F}_2^d \rightarrow \...
1
vote
0answers
43 views

Entire solutions of first order linear homogeneous evolution PDEs

I've posted this question on MSE, and haven't got any feedback yet, so I will try again here: I'll start off with the following example for $u(x,t):\mathbb{C}^2 \to \mathbb{C}$: $$\begin{align} u_t&...
13
votes
1answer
451 views

Is this expression for the Laplacian of conformal maps between Riemannian manifolds known?

$\newcommand{\M}{\mathcal{M}}$ $\newcommand{\N}{\mathcal{N}}$ $\newcommand{\Hom}{\operatorname{Hom}}$ $\newcommand{\tr}{\operatorname{tr}}$ $\newcommand{\TM}{\operatorname{T\M}}$ $\newcommand{\TN}{\...
14
votes
2answers
396 views

Do curvature differences obstruct a.e orientation-preserving isometries?

Is there an example of a pair $M,N$ of connected, oriented equidimensional Riemannian manifolds with the following properties: $M$ is everywhere non-flat, $N$ is flat. There exist a map $f:M \to N$ ...
2
votes
2answers
513 views

Simultaneous extensions of strictly convex functions

21/03/2017: I have decided to accept Denis Serre's answer, even though it does not exactly answer my question, however I like its simplicity and I'd say it is close enough to the desired claim. Of ...
3
votes
1answer
137 views

Uniqueness conditions for linear transport equation with nonconstant velocity

Considering the following equation, $$ u_t + \operatorname{div} \, (u \, \mathbf{b}(\mathbf{x},t)) = 0 $$ in a cylinder $K = \{(\mathbf{x},t) \in \Omega \times (0,T) \}$ where $\Omega \subset \mathbb{...
20
votes
1answer
567 views

A differentiable isometry is smooth?

I posted this question in MSE but got no response (even after giving a bounty), so I am trying here. Let $M,N$ be smooth $d$-dimensional Riemannian manifolds. Suppose $f:M \to N$ is a differentiable ...
0
votes
0answers
104 views

Boundary regularity of the solution of a Poisson equation in a polyhedron

Let $d\in\mathbb N$ $\Lambda\subseteq\mathbb R^d$ be bounded and open $f\in L^2(\Lambda,\mathbb R^d)$ $u\in H_0^1(\Lambda,\mathbb R^d)$ with $$-\langle\nabla\phi,\nabla u\rangle_{L^2(\Lambda,\:\...
0
votes
0answers
183 views

Implicit Function Theorem

Let $f$ be a $C^2$ function defined on a neighborhood of $0$ in $\mathbb R^n$ such that $f(0)=0, df(0)\not=0$. By the Implicit Function Theorem, it is easy to get (after a rotation) that near 0 $$ f(x)...
1
vote
0answers
103 views

Does $u\in H^{3/2}(\Omega)$ imply continuity of $\nabla u\cdot\overrightarrow{n}$ across an interior interface?

When investigaing the regularity of certain functions, I encountered this problem: if $u\in H^{3/2}([0,1]\times [0,1])$, what can we say about the continuity of $\nabla u\cdot\overrightarrow{n}$ ...
0
votes
2answers
293 views

Local L^p regularity theory for elliptic operators

I've been studying the elliptic regularity theory using $H^s$ spaces as done in Folland's "Introduction to Partial Differential Equations". At the end of section C, chapter 6, Folland affirms that we ...
1
vote
1answer
139 views

Moser/Schauder estimates for coercive boundary conditions

Consider the uniformly elliptic equation $(\partial_t^2 + L)u = 0$ on $(0, \infty) \times \Omega$, where $\Omega \subset \mathbb{R}^n$ is an open bounded domain with smooth boundary, and $L$ is a ...
1
vote
0answers
89 views

$L^\infty(\Omega)$-regularity for strongly damped wave equation

I am interested in the following IBVP for the strongly damped wave equation: \begin{equation} u_{tt}-c^2\Delta u-b\Delta u_t+eu_t=f(x,t) \quad \text{in} \ \Omega \times (0,T), \\ u=0 \quad \text{on} \ ...
0
votes
0answers
273 views

Regularity for a div-curl system

Let $Q = [0,1]^3$ be the unit cube in $ \mathbb{R}^3$, and let $U \subset Q$ be a simply-connected subdomain with smooth boundary. Suppose $g \: \colon Q \to \mathbb{R}^3$ is a non-negative smooth ...
12
votes
2answers
587 views

Unexpected regularity of the distance from a $C^2$ submanifold

Let $\Gamma$ be a $C^2$ compact submanifold of $\mathbb{R}^n$. Consider the distance function $\delta$ from $\Gamma$. It is well known that, for sufficiently small $\varepsilon>0$, $\delta$ is $C^2$...
3
votes
0answers
139 views

Interior regularity for elliptic operators with non smooth coefficients

I need a pretty standard interior regularity result for a second order elliptic operator of the form $$ -\nabla^b \cdot (A(x) \nabla^b v)+c v=f, \qquad \nabla^b=\nabla+ib(x) $$ where $A(x)$ is a ...
1
vote
0answers
139 views

An H2 estimate for Helmholtz equation

How to show the following statement? Let $\Omega$ be a bounded Lipschitz convex domain. If $u$ satisfies the following equation, $$ -\Delta u - k^2 u = f \quad\mbox{ in }\Omega \\ \nabla u \...
1
vote
1answer
78 views

Fractional sobolev regularity of a truncated function

I want to generalize the following result to fractional derivatives, specifically the fractional Laplacian. Consider a function f which belongs to L2, and all its first order distributional ...
1
vote
1answer
319 views

$L^p-L^q$ estimates for heat equation - regularizing effect

Where can I find a proof of the following estimate $$\|S(t)v\|_{L^p(\Omega)}\leq C_{N,p,q} t^{-\frac{N}{2}\left(\frac{1}{q}-\frac{1}{p}\right)}\|v\|_{L^q(\Omega)}, $$ where $1\leq p<q<+\infty$, $...
2
votes
1answer
151 views

Regularity - mean curvature equation

In my research I arrived at the following equation: $$ \int_B \frac{\nabla u \cdot \nabla \varphi}{ \sqrt{1+|\nabla u|^2}}=\int_B f \varphi, (*)$$ for every $\varphi \in C^1(B)$, which is a weak form ...
3
votes
1answer
157 views

What is the function space $H^1_{m, \sigma}$?

I am reading Hildebrandt's and Widman's 1975 paper on "Some regularity results of quasilinear elliptic systems of second order". Theorem 3.1 is the first time in their paper that the function space $...
1
vote
0answers
139 views

Boundary regularity of solution to partial differential equation

I am conducting research on partial differential equations and I need a short-time existence result from the literature which I can not find at the moment. More precisely I would like to know the ...
5
votes
1answer
182 views

How often can subsets of a universe intersect exactly once?

My question is inspired by the following observation: Claim: It is not possible to choose $n$ subsets of the universe $[n]$, each of size $\Omega(n)$, such that for each subset $S$ and each element $...