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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
2 answers
763 views

Proof of the du Bois-Reymond lemma "by approximation" [closed]

I'm curious about the following argument in Morrey ("Multiple integrals in the calculus of variations", Lemma 2.3.1). Suppose $f\in L^1[0,1]$ and $$\int_0^1 fg\,dx=0$$ for every test function $g\in C^ …
Ryan Unger's user avatar
4 votes

Dirichlet problem for manifold, how to prove $W^{1,2}_0(\Omega)$ solution is $C^{2,\alpha}(\...

I will essentially explain the comment under Theorem 8.14 in Gilbarg-Trudinger. I will assume the result stated there: given smooth boundary data and RHS, the Poisson equation has a unique smooth so …
Ryan Unger's user avatar
4 votes
0 answers
182 views

Traces of manifold-valued Sobolev maps

Let $(M^m,g)$ be a compact Riemannian manifold with smooth nonempty boundary, and $N^n\subseteq \Bbb R^d$ a boundaryless isometrically embedded Riemannian manifold. For $1\le p<\infty$ we define as us …
Ryan Unger's user avatar
1 vote
0 answers
224 views

Weak elliptic maximum principle on manifolds without strict ellipticity

This question is not to be confused with the similarly titled question here. In the above lined question, I gave a complete answer, but noticed that things are apparently not so simple in the ellipt …
Ryan Unger's user avatar
2 votes
0 answers
155 views

Ricci flow with surgery without the "no locally separating $\Bbb RP^2$" assumption

In many places, Ricci flow with surgery is done with orientable manifolds. Morgan and Tian do not require orientability, but instead they impose the condition that $M^3$ have no embedded $\Bbb RP^2$ w …
Ryan Unger's user avatar
10 votes
1 answer
419 views

Does harmonic map heat flow of a curve always fully converge to a geodesic?

Consider a smooth closed curve $u_0$ in a compact Riemannian manifold $(M,g)$. Let $u_0$ evolve by harmonic map heat flow, $\partial_tu=\nabla_{\partial_su}\partial_su$, and call the result $u(t)$. …
Ryan Unger's user avatar
11 votes
0 answers
339 views

Elliptic regularity of perturbed scalar curvature in Kazdan & Warner

In their paper A Direct Approach to the Determination of Gaussian and Scalar Curvature Functions, Kazdan and Warner claim something along the lines of: if $g$ is a metric in $W^{2,p}$ ($p>n$) whose sc …
Ryan Unger's user avatar