# Tagged Questions

**2**

votes

**0**answers

41 views

### Can we say translation/dilation of the $L^p-$multiplier is again a $L^{p}-$multiplier?

Suppose that $m:\mathbb R \to \mathbb C$ such that
$\| (m \hat{f})^{\vee} \|_{L^{p}} \leq C \|f\|_{L^{p}}$ (where $C$ is some constant, $f\in L^{p}$). (That is, $m$ is an $L^{p}-$ multiplier) ...

**2**

votes

**0**answers

42 views

### 1D inhomogeneous linear Schrodinger equation

I have the following problem:
$iu_t - u_{xx} = f$ on the interval $[0,L]$ with $u(0,t)=u(L,t)=0$ and $u(x,0)=0$. I can show that $\|u\|_{L^2(x,t)}$ (space-time) is controlled by the norm ...

**3**

votes

**1**answer

140 views

### Conserved quantities for the Cauchy momentum equation

I apologize if this question is too elementary for mathoverflow; I asked it (unsuccessfully) on MATH.SE first.
As a bit of background: one way to study the mechanics of deformation of a continuous ...

**7**

votes

**3**answers

329 views

### Connection between solution for Schrödinger equation and solution for heat equation

It's known, that if you write imaginary unit into a heat equation you'll get time-dependent Schrödinger equation. Recently one guy discovered a connection between solutions for these two equations ...

**1**

vote

**1**answer

55 views

### The dependence of constant in a trace theorem on the diameter of domain

The trace theorem says for a nice domain, say bounded Lipchitz domain, $\Omega$, we have
$$\left\Vert Tu \right\Vert_{H^{1/2}(\partial \Omega)} \leq C \left\Vert u \right\Vert_{H^1(\Omega)},$$
where ...

**1**

vote

**0**answers

66 views

### How is the duality pairing of $H^{1/2}$ and $H^{-1/2}$ defined on a subset of the boundary?

Let $\Omega\in \mathbb{R}^d$,for $d\in \{2,3\}$ be a bounded polyhedral set with $n$ boundary faces labeled $\{e_i\}_{i=1}^n$.
Let $\vec{q}\in H^{\mathrm{div}}(\Omega).$ Given $u\in ...

**1**

vote

**1**answer

75 views

### Estimating sums with restrictions to different Frequencies

I have problems understanding two (for my research important) details in the Proof of Theorem 4 (page 14) in this paper: http://arxiv.org/abs/1209.1518. Notation is very straightforward and is found ...

**1**

vote

**0**answers

68 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

**2**answers

81 views

### Nonlinear PDE for a 2D foliation

I am trying to solve a 4th order nonlinear PDE for a real function $u(x,y)$ of two variables. It is too complicated to reproduce here but it exhibits the following two very nice properties:
1) if ...

**1**

vote

**0**answers

55 views

### Characteristic field

On page 423 (page 9 of PDF) of the paper by Lions and Perthame, the authors write as equation (39) the Vlasov equation as
$$\frac{\partial f}{\partial t} + v \cdot \nabla_x f + F \cdot \nabla_v f = ...

**25**

votes

**1**answer

611 views

### A long-lasting conjecture of Pólya & Szegő

There is a conjecture by Pólya & Szegő (~1950) which is as follows:
"Of all $n$-gons of a fixed area, the regular $n$-gon minimizes the first Dirichlet eigenvalue."
Surprisingly, this is still ...

**4**

votes

**0**answers

91 views

### Well-definedness on $C_{0}^{\infty}(\mathbb{R}^{n})$

Let $T$ be a Calderon-Zygmund operator associated to a Calderon-Zygmund kernel $K\in CZK_{\alpha}$ of order $\alpha>0$ and $b\in BMO(\mathbb{R}^{n})$. Then for $f\in C_{0}^{\infty}(\mathbb{R}^{n})$ ...

**2**

votes

**0**answers

72 views

### Getting an a priori energy estimate from PDE weak formulation

On a bounded domain $\Omega$, I have two functions $u$ and $v$ in $L^2(0,T;H^1(\Omega))\cap H^1(0,T;(H^1(\Omega))^*)$ satisfying
$$\frac{d}{dt}\int u^2 + c_1\int |\nabla u|^2 + n\int u^2 \leq n\int ...

**1**

vote

**0**answers

33 views

### About the “method of lines”: when are such solutions good approximations for **all** future time?

This question is about approximate solutions to some classes of PDEs obtained using the "method of lines".
For example, for an initial-value problem given by a PDE on a circle, one can choose $n$ ...

**2**

votes

**4**answers

170 views

### Ill-posedness of a generalized heat equation

Suppose we have the following one-dimensional generalized heat equation:
$$u_t(x,t)=g(x,t)\Delta u(x,t), \quad x\in \mathbb{R},t\in(0,\infty).$$
I need to prove that this equation is ill-posed, for ...

**2**

votes

**1**answer

101 views

### Does $u_{t}=g(t)u_{x}^{2}$ blow-up for bounded positive g? What about $u_{t}=u_{xx}+g(t)u_{x}^{2}$?

My original problem is to see if the following pde develops blow-ups in $(-L,L)$
$$u_{t}=u_{xx}+g(t)(u_{x})^{2}$$
for periodic boundary $u_{0}(-L)=u_{0}(L)$, where $0<g(t)<1$; specifically ...

**0**

votes

**0**answers

33 views

### Stability of a nonlinear heat PDE

I have the following PDE:
$$u_t = u_{xx} , u(x,0)=1 , u_x(0,t)=0 , u_x(1,t)=-hu^4(1,t) , h>0$$
And I want to check is it stable, i.e do we have constants $\alpha , K$ independent of $u(x,0)$ s.t ...

**1**

vote

**1**answer

64 views

### Different Besov-Norm Definitions

first some notation: $\langle x\rangle=\sqrt{1+x^2}$, $P_{j}$ is the Littlewood Paley Projector and $P_{\leq0}$ corresponds to the small frequencies.
I have a the following definition of the Besov ...

**1**

vote

**0**answers

143 views

### How to solve Schrödinger equations with potential $|x|^{2}$ [closed]

We consider the initial value problem (IVP):
$i \frac{\partial}{\partial } u(x,t) \pm (\Delta \pm |x|^2) u(x,t)=0$
and
$u(x,0)= u_0(x)$, where $x,t \in \mathbb R, \Delta$ is the Laplacian.
My ...

**2**

votes

**1**answer

77 views

### Strongly continuous semigroups and symbols of pseudo differential operators

I am considering the Cauchy IVP for the evolution equation
$$u_t + \Psi u =0$$
where $\Psi$ is a linear pseudo differential operator with symbol $\widehat{\Psi}\left(\underline{\xi}\right)$.
The ...

**1**

vote

**1**answer

119 views

### Harmonic/Subharmonic lifting of functions on an annulus

Suppose $\Omega_1, \Omega_2 \subset R^2$ are bounded open regions with $\Omega_1 \Subset \Omega_2$. Let $f_1\in C(\partial \Omega_1)$ and $f_2\in C(\partial \Omega_2)$. Is there a function $h\in ...

**3**

votes

**1**answer

149 views

### How to use Gronwall's inequality? [closed]

I am just trying to understand the role of Grownwall's Lemma to show global wellposedness results, in the paper I have been reading. And So I hope this is OK for MO.
Let $u\in C(\mathbb R, ...

**2**

votes

**1**answer

79 views

### How to find the eigenvalues equation of this PDE problem

Given the problem:
$$(\kappa(x)X^{'})^{'}+\lambda\rho(x)X=0$$
for $0<x<l$ with $X(0)=X(l)=0$
where $\kappa(x)=\kappa_{1}^{2}$ for $x<a$, $\kappa(x)=\kappa_{2}^{2}$ for $\kappa>a$. ...

**0**

votes

**0**answers

59 views

### Sobolev function and capacity

It seems that there is a theorem which claims that
If $u\in L_1^2(B_1\setminus T)$ and $T\subset B_1$ is of capacity zero (in some sense, since the author says $(1,2)$-capacity, of which I don't ...

**1**

vote

**0**answers

112 views

### Equation of the form $\overline{\partial}_{J}f=g$ made holomorphic

In section 2.6 of the book "Holomorphic curves in symplectic geometry" by Audin and Lafontaine there is explained when one can transform a perturbed holomorphic curve in a holomorphic curve. I tried ...

**0**

votes

**0**answers

69 views

### Hausdorff dimension and Hausdorff measure

Let us consider a curve $E\subset \mathbb{R}^2$ which is a $(\delta,R)$- Reifenberg flat domain and suppose also that it holds the following estimate on the Hausdorff dimension:$\mbox{ ...

**1**

vote

**0**answers

28 views

### Vector fields for volumetric-deviatoric decomposition

The strain tensor $\epsilon(u) = \frac12 (\nabla u + (\nabla u)^T)$ in linear elasticity can be decomposed additively into volumetric and deviatoric strains
\begin{gather*}
\epsilon_D(u) &= ...

**8**

votes

**2**answers

398 views

### $L^\infty-L^2$ smoothing for heat equation on manifold using Nash-Moser-De-Giorgi technique

Let $M$ be a compact and closed smooth Riemannian manifold, and consider weak solution $u$ of the equation
$$u_t - \Delta u = f$$
given $f \in L^2(Q)$ and $u(0)=u_0 \in L^\infty(M)$.
I'm looking for ...

**3**

votes

**0**answers

112 views

### Distance to the level sets of an almost linear function

While reading about the almost-splitting theorem of Cheeger and Colding, I thought about the following question, which asks about wether the level sets of a function which is close to being a linear ...

**1**

vote

**0**answers

61 views

### Properties of a Sobolev bound

I am interested in computing
$$
A:=\inf_{f\in L^{2}(\mathbb{R}^3)}\frac{||K^{\frac{1}{4}}f||_2^2}{||f||_{\frac{5}{2}}^2}
$$
where $K:=-\Delta+1$. We call $f_c$ the function that saturates the bound.
...

**2**

votes

**1**answer

107 views

### Strichartz Estimates for radial Klein-Gordon equation

I'm trying to prove global wellposedness for the Klein-Gordon-Equation with radial initial data. I'm therefore searching for/trying to prove strichartz estimates of the form: $$ ||e^{it\langle ...

**0**

votes

**1**answer

81 views

### Explicit solution for one-dimensional Gelfand problem

I wonder if the ODE
$y''+e^{y}=a$
can be solved explicitly. For $a=0$, it is well-known that there is a two-parameter family of explicit solutions
$y=\ln(2)-2\ln(\cosh(cx+d))+2\ln(c)$, $c,d \in ...

**3**

votes

**0**answers

44 views

### Prescribed curvature problem of a connection beyond the real analytic category for $SL(3,R)$ bundles?

With reference to the questions Does the Riemann-Christoffel curvature determine the connection? and When is a given matrix of two forms a curvature form?, and recalling the following important result ...

**2**

votes

**1**answer

79 views

### Estimates for Klein-Gordon-Equation follow directly from Wave equation Estimates

in this paper http://arxiv.org/pdf/1412.1626.pdf it says that Lemma 3.1/(3.1) follows from Theorem 1.3 in http://arxiv.org/pdf/math/0402192.pdf without extra details. Can somebody please explain that?
...

**0**

votes

**0**answers

40 views

### trace sobolecv inequality for $q=2(n-1)/(n-2)-\varepsilon$ in half space

Can I do the following inequality, for $ u\in D^{1,2}(R^n_+)$, we have
$(\int _{R^{n-1}} |u|^{2(n-1)/(n-2)-\varepsilon}dx')^{\frac{ 1}{2(n-1)/(n-2)-\varepsilon } }\leq C ( \int _{ R^n_+}| \nabla ...

**1**

vote

**1**answer

93 views

### Integral representation of the Cauchy problem solution for the heat equation

Consider the Cauchy problem for the heat equation
$u_t=\Delta u$, $u|_{t=0}=\varphi$.
S. Täcklind showed its solution $u$ is unique in the class $|u|\le e^{|x|h(|x|)}$, $|x|>1$, iff $\int_1^\infty ...

**2**

votes

**1**answer

112 views

### $L^p$-bounding inequality [closed]

Do we have that$$\|Du\|_{L^{2p}} \le C\|u\|_{L^\infty}^{1\over2} \|D^2u\|_{L^p}^{1\over2}$$for $1 \le p < \infty$ and all $u \in C_c^\infty(U)$? Here, $U$ denotes an open subset of $\mathbb{R}^n$.

**1**

vote

**1**answer

94 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 $1\leq p<q<+\infty$, ...

**0**

votes

**0**answers

52 views

### Trace Theorem for $q< 2(n-1)/(n-2)$

Can I get a trace theorem inequalite for $R^n_+$: For $q\in [2,2(n-1)/(n-2]$,
we have
$(\int_ {R^{n-1} } |u|^q dx) ^{2/q}\leq C(\int_{R^n_+} |\nabla u |^2dx)^{1/2}.$

**3**

votes

**0**answers

261 views

### Funk-Hecke theorem on the complex sphere

I am interested in paper " Sharp constants in several inequalities on the Heisenberg group " of Rupert L.Frank and Elliott H.Lieb " http://arxiv.org/pdf/1009.1410v2.pdf. In this paper ( page 17 ), ...

**0**

votes

**1**answer

69 views

### Proving short time existence for semi-linear parabolic PDE

I am following up on the answer of Denis Serre to this same question here Short time existence on nonlinear parabolic PDE
I have tried to generalise the proof of the Picard-Lindelof theorem, as ...

**2**

votes

**0**answers

50 views

### Constant in a trace Sobolev theorem for concave domains

I wonder is the following inequality is true/known:
Let $\Omega\subset \mathbb{R}^n$ be a (locally) Lipschitz domain which is the complement of a convex set, then
$$
\int_{\partial\Omega} |u|^2 ds
...

**1**

vote

**0**answers

58 views

### Perturbation in Besov space

$\|f\|_{B^{0}_{p,p}}=(\sum_{j\geq -1} \|\Delta_j f\|_p^p)^{1/p}$ is the Besov norm of $f$.
Here the Fourier transform of $\Delta_jf~(j\geq 0)$ is $\psi(2^{-j}\xi)\hat{f}(\xi)$ and $\psi$ is a smooth ...

**1**

vote

**1**answer

65 views

### Davey-Stewartson Lagrangian formulation

The system is
$i u_t + c_0 u_{xx} + u_{yy} = c_1 |u|^2 u + c_2 u \phi_x,\,$
$\phi_{xx} + c_3 \phi_{yy} = ( |u|^2 )_x.\,$
This is like the NLS but with the extra y-dimension. The NLS has the ...

**3**

votes

**1**answer

166 views

### How many second-order PDEs can be obtained from a contact EDS?

Let $(M,[\theta])$ be a contact manifold, $\dim M=2n+1$, and denote by $\mathcal{I}^\theta$ the differential ideal generated by the contact form $\theta$.
An exterior differential system on $M$ of ...

**0**

votes

**0**answers

111 views

### Extension of harmonic function with bounded $L^{2}$ norm

Let $h:D\setminus \{0\}\rightarrow \mathbb{R}$ be a harmonic function, where $D$ is the unit disc in $\mathbb{R}^{2}$, with bounded $L^{2}$ norm, i.e. $||h||_{L^{2}(D)}^{2}=\int_{D}|h|^{2}(x,y)dxdy ...

**0**

votes

**0**answers

52 views

### Lp space and zero order Besov space

I want to ask a basic question(may stupid), does the following relation holds:
$$\|f\|_{B^0_{p,p}}\approx\|f\|_{L^p}$$
where, $\|f\|_{B^{0}_{p,p}}=(\sum_{j\geq -1} \|\Delta_j f\|_p^p)^{1/p}$ is the ...

**6**

votes

**1**answer

191 views

### Simplicity of eigenvalues

Consider the Sturm-Liouville operator$$Au = -(pu')' + qu \text{ on }I = (0, 1),$$where $p \in C([0, 1])$, $p \ge \alpha > 0$ on $I$, and $q \in C([0, 1])$. No further assumptions are made; in ...

**0**

votes

**2**answers

98 views

### $L^p$ estimates for elliptic equation of divergence form

Consider the scalar elliptic equation of divergence form
$$div((1+a)\nabla\pi)=div F\ \ in\ \ R^3,$$
where $a$ is a Schwartz function with $1+a\geq c=const>0$, $F=(F_1,F_2,F_3)$ is a vector-valued ...

**11**

votes

**2**answers

234 views

### Asymptotic behavior of Sturm-Liouville eigenvalues

I have two questions.
Consider the operator $Av = -v'' + a(x)v$ on $I = (0, L)$, with zero Dirichlet condition and $a \in C([0, L])$.
Let $(\lambda_n)$ denote the sequence of eigenvalues of ...