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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.

2 votes
0 answers
68 views

Hess-Schrader-Uhlenbrock inequality for non-symmetric operators

Let $M$ be a (compact, let's say) Riemannian manifold, $\mathcal{V}$ a vector bundle over $M$ with covariant derivative $\nabla$ and a fiber metric. Let $L = - \mathrm{tr}(\nabla^2) + V$ with some pot …
Matthias Ludewig's user avatar
0 votes
1 answer
233 views

What is the Newtonian Capacity of a subset of $S^n$?

In their paper "Conformally flat Manifolds, Kleinian Groups and Scalar Curvature", Schoen and Yau repeatedly use the term "Newtonian capacity" for a subset of $S^n$. I know the following definition: …
Matthias Ludewig's user avatar
10 votes
1 answer
467 views

Special Second-Order PDE

Let $\Phi$ be a given smooth function on a neighborhood of zero in $\mathbb{R}^n$ with $$\Phi(0) = 0, ~~~~D \Phi(0) = 0, ~~~~ D^2\Phi(0) >0,$$ the latter meaning that the Hessian is positive definite. …
Matthias Ludewig's user avatar
2 votes
0 answers
77 views

Well-posedness of a certain linear Cauchy-problem

I am interested in solutions to the linear Cauchy problem $$\Bigl(\frac{\partial^2}{\partial t^2} + a(t, x)\frac{\partial}{\partial t} + \sum_{j=1}^n b_j(t, x) \frac{\partial}{\partial x_j}\Bigr)u(t, …
Matthias Ludewig's user avatar
8 votes
1 answer
2k views

Solutions to the eikonal equation

Theorem. Let V be a $C^\infty$ function on a riemannian manifold $M$ and $p$ be a nondegenerate local minimum with $V(p)=0$. Then there is a unique positive function $\varphi \in C^\infty(U)$ such tha …
Matthias Ludewig's user avatar
5 votes
1 answer
232 views

Zeta-Determinant Theorem

Recently, someone asked on MO about lecture notes from Graeme Segal's "Stanford lectures" on TQFT, and the answer was to check here. When scrolling over the notes, I stumpled of Prop. 2.8.2 in lectu …
Matthias Ludewig's user avatar
2 votes
0 answers
184 views

Heat kernel on manifold with boundary

Let $M$ be a Riemannian manifold with boundary, let $\mathcal{V}$ be a metric vector bundle over $M$ and let $L$ be a formally self-adjoint Laplace type operator, acting on sections of $\mathcal{V}$. …
Matthias Ludewig's user avatar
2 votes
0 answers
104 views

Existence of harmonic maps between loops

Given a Riemannian manifold $M$ and two smooth loops $\gamma_0, \gamma_1: S^1 \longrightarrow M$ in it, I am looking for maps $\phi: [0, T] \times S^1 \longrightarrow M$ which minimize the energy $$E[ …
Matthias Ludewig's user avatar
2 votes
0 answers
103 views

Inhomogeneous heat kernel estimates

I am looking for existence results on inhomogeneous linear heat equations. Concretely, I have the equation $$ \frac{\partial}{\partial t} u(t, x) = \Delta_t u(t, x), ~~~~~u(0, x) = u_0(x)$$ where $\De …
Matthias Ludewig's user avatar
7 votes
2 answers
1k views

First order Elliptic operator

Assume that there exists a first order elliptic operator $D$ acting on functions from $\mathbb{R}^n$ to some vector space $V$. What can we conclude about $V$? For example, is the dimension of $V$ alw …
Matthias Ludewig's user avatar
3 votes
1 answer
728 views

Decay of Solutions to the Heat equation

Consider the heat equation $$ (\partial_t + \Delta + V)u = 0$$ on a complete (open) Riemannian manifold with bounded geometry, where $V$ is a smooth and bounded potential. Consider the semigroup gene …
Matthias Ludewig's user avatar
6 votes
1 answer
215 views

Boundary values of boundary value problems

Let $M$ be a manifold with smooth boundary. We can consider the Dirichlet or the Neumann problem on $M$. Let $(\phi_k)$ be an orthonormal basis of eigenfunctions to the Dirichlet problem and let $(\ps …
Matthias Ludewig's user avatar
5 votes
0 answers
219 views

Parametrices for the wave equation on manifolds with boundary

I am trying to understand parametrices for the solution operator $G_t = \sin(t\sqrt{\Delta})/\sqrt{\Delta}$ to the wave equation $$(\partial_{tt} + \Delta)u=0, ~~~~~~~ u_0 =0, ~~~~~~\partial_tu_0 = f$ …
Matthias Ludewig's user avatar
-1 votes
2 answers
258 views

$L^{n/2}$ norm of scalar curvature

In the Wikipedia article on scalar curvature, it is noticed that the Hölder inequality implies $$Y(g) \geq - \left(\int_M |R(g)|^{n/2} \mathrm{d}V_g\right)^{n/2}$$ for the Yamabe functional $Y(g)$ and …
Matthias Ludewig's user avatar
11 votes

How to prove Liouville measure is invariant under geodesic flow?

A hands down proof not using the theory of Hamiltonian systems can be done by just proving that the Jacobian determinant of the transformation is zero. We have $$ TSM \cong \pi^* TM \oplus VSM,$$ wh …
Matthias Ludewig's user avatar

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