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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.

1 vote
0 answers
66 views

Harmonic maps from $\mathbb{R}^2$ to $\mathbb{S}^n$ for $n>2.$

I have been recently reading the following paper about harmonic maps, where the authors prove the non-degeneracy of harmonic maps from $\mathbb{R}^2\to \mathbb{S}^2$. In their analysis they make use o …
1 vote
0 answers
92 views

Uniqueness of bubbling points in Struwe's global compactness theorem

I am reading the following paper of Struwe in which he proves the following result: Proposition 2.1: Let $n\geq 3$, $\lambda \in \mathbb{R}$ and $\Omega$ be a smoothly bounded domain in $\mathbb{R}^{n …
3 votes
0 answers
187 views

Parabolic smoothing for semilinear PDE

Consider the semilinear energy-critical parabolic PDE in $\mathbb{R}^3$ \begin{align} \partial_t u &= \Delta u + |u|^{4/(n-2)}u = \Delta u + u^5\\ u(0,x) &= u_0\in \smash{\dot{H}}^1(\mathbb{R}^n). \en …
4 votes
1 answer
135 views

Hölder Gradient Estimates for Linear Elliptic Equations in higher dimensions

I was looking at Theorem 12.4 of Gilbarg and Trudinger's Elliptic partial differential equations of second order (MR1814364, Zbl 1042.35002): Theorem 12.4. Let $u$ be a bounded $C^2(\Omega)$ solution …
0 votes
0 answers
27 views

Comparison Principle for Courant Nodal Domain Theorem

Recall that Courant's Nodal domain gives an upper bound on the number of nodal domains of the $k$-th eigenfunction. I was wondering if it is possible to deduce some relation between the number of noda …
5 votes
2 answers
449 views

Question about Neumann eigenvalues on manifolds

Question: Let $\Omega\subset \mathbb{S}^2_+$ be any geodesically convex subset of the hemisphere $\mathbb{S}^2_+$. Then is it true that $\mu_1(\Omega)\geq \mu_1(\mathbb{S}^2_+)$ where $\mu_1$ is the f …
0 votes
0 answers
79 views

Convergence of metric implies convergence of eigenvalues?

Consider the metric $g_\varepsilon=d\theta^2 + \varepsilon^2 \sin(\theta)^2 \, d\varphi^2$ on the hemisphere $S^2_+.$ I had two naive questions: Does $g_\varepsilon$ converge to the flat metric on th …
2 votes
0 answers
243 views

Convergence of metric and eigenvalues on a tubular neighbourhood

Background: Consider the sphere $S^2$ with the round metric $g$ and let $\gamma$ be one half of a great circle of length $\pi.$ Let $T_\epsilon$ denote a geodesically convex tube around $\gamma$ of t …
5 votes
1 answer
333 views

Does the Poincaré inequality hold on annular domains?

Does the following Poincaré inequality hold $$\int_{B_{r_2}\setminus B_{r_1}} |f-\bar{f}|^2 dx \leq C(r_2-r_1)^2 \int_{B_{r_2}\setminus B_{r_1}} |\nabla f|^2 dx,$$ where $B_r$ denotes a ball of radius …
2 votes
0 answers
108 views

Poincare inequality on the hemisphere

Background: Let $\mathbb{S}^2_+$ be the hemisphere. Then we know that for $f:\mathbb{S}^2_+\to \mathbb{R}$ satisfying (when written in coordinates) $\int_{0}^{2\pi}\int_{0}^{\pi/2}f(r,\theta)\sin(r)dr …
1 vote
0 answers
60 views

$L^p$ estimates for critical heat equation on $\mathbb{R}^n$

Background: Let $u:\mathbb{R}^n\times [0,T_+)\to \mathbb{R}$ be a solution to $$\partial_t u = \Delta u + |u|^{p-1}u$$ where $p=2^{*}-1=\frac{2n}{n-2}-1=\frac{n+2}{n-2}$, $n\geq 3$ and $T_+>0$ denotes …
0 votes
1 answer
388 views

What functions are equal to their symmetric decreasing rearrangement?

I am trying to understand the set $$\mathcal{A}=\{f:f\text{ non-negative, measurable}, \text{vanishes at infinity and } f(x)=f^*(x)\}$$ where $f:\mathbb{R}^n\to \mathbb{R}$ is vanishing at infinity if …
5 votes
1 answer
198 views

Convergence of heat flow on non-compact manifolds?

Consider the heat equation $\partial_t u= \Delta u+\lambda_1 u$ on a non-compact complete manifold $M$ (with nonpositive curvature) where $\lambda_1$ is the first eigenvalue and we start with some smo …
5 votes
0 answers
147 views

weak maximum principle for weighted laplacian

Consider a radial weight $w=|x|^2 \geq 0$ for all $x\in \mathbb{R}^n$ and consider the operator $$Lu= \frac{1}{w}\operatorname{div}(w\nabla u).$$ Then if $-Lu\leq 0$ on a smooth bounded domain $\Omega …
1 vote
1 answer
143 views

comparison principle for the minimal surface equation

Consider the (inhomogeneous) minimal surface equation for functions $u,f:D\to \mathbb{R}$ for some smooth domain $D\subset \mathbb{R}^n$ $$Lu:=\operatorname{div} \frac{\nabla u}{\sqrt{1+|\nabla u|^2}} …

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