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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.

22 votes
5 answers
16k views

Physical interpretation of Robin boundary conditions

In a (bounded) domain $\Omega \subset \mathbb{R}^n$, if we're studying the Laplace equation or heat equation or such PDE's we can impose the Dirichlet $u|_{\partial\Omega} \equiv 0$, Neumann $D_{\nu} …
Otis Chodosh's user avatar
  • 7,197
15 votes
Accepted

A long-lasting conjecture of Pólya & Szegő

I'm pretty sure that this is still open for $n$-gons (with $n\geq 5$). As far as I know, basically no progress has been made since the original proofs for triangles/quadrilaterals. There have been som …
Otis Chodosh's user avatar
  • 7,197
10 votes
Accepted

Morse index in PDEs

In finite dimensional Morse theory, you study a function $f:M\to\mathbb{R}$ and look for it's critical points, i.e., where $(df)_p = 0$. Then, Morse theory says that a count of these critical points i …
Otis Chodosh's user avatar
  • 7,197
7 votes

fixed point arguments in PDE

The graphical minimal surface equation is a great example of a PDE where Leray-Schauder fixed point theory is applied: $$ \left(\delta_{ij} - \frac{D_i u D_j u}{1+|Du|^2}\right)D_{ij}u = 0 $$ This r …
Otis Chodosh's user avatar
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6 votes

metric has morse index 2

I assume you mean these notes: http://www.math.jhu.edu/~js/Math646/schoen.totalscalar.pdf? In general it does not make sense to say that a "metric has morse index 2" without any context. Before proce …
Otis Chodosh's user avatar
  • 7,197
5 votes
Accepted

Finding vector fields on $S^2$ with equal divergence

I think that this is not possible: Per my comment on Divergence of conformal Killing vector fields on $S^2$ and the spherical harmonics you want to solve $$ \textrm{div} (Y) = -2a\cdot x $$ for $Y$ or …
Otis Chodosh's user avatar
  • 7,197
5 votes
3 answers
2k views

Characterizing the harmonic oscillator creation and annihilation operators in a rotationally...

I am interested in a characterization of the creation and annihilation operators that is in some sense invariant under $O(n)$ rotations of $\mathbb{R}^n$: Background The Harmonic Oscillator on $\mat …
Otis Chodosh's user avatar
  • 7,197
5 votes
Accepted

What is the infinite Morse index solution?

NEW ANSWER: Yes there exist infinite Morse index solutions in all dimensions $N\geq 3$. For example you can take the solution in the Li Chen paper $$ \phi(x,y) = \frac{\ln(32)}{(4+|(x,y)|^2)^2} $$ and …
Otis Chodosh's user avatar
  • 7,197
4 votes

Progress on isospectral plane domains

The Dirichlet/Neumann spectrum determines the domain among the class of analytic domains with some discrete symmetries by work of Zelditch. The unit disk is determined by its Dirichlet spectrum (amon …
Otis Chodosh's user avatar
  • 7,197
4 votes
Accepted

Derivation of yamabe flow

The Yamabe flow is (up to a constant) the gradient flow of the Yamabe functional on the unit volume conformal class, as you expected. The comment by @Mark Peletier hints at your error: you aren't usin …
Otis Chodosh's user avatar
  • 7,197
4 votes
Accepted

On-diagonal to off-diagonal heat kernel lower bounds, Davies' argument

This is a classic trick for heat kernel proofs: The idea is that if $f(t,x)$ is a solution to the heat equation with initial data $f_0(x)$, then $g(t,x) := f(t+s,x)$ is a solution to the heat equatio …
Otis Chodosh's user avatar
  • 7,197
4 votes
Accepted

Analytic dependence on the metric

I find it strange that Besse does not discuss this! Here's my understanding of the issue: Sun and Wang care about analyticity because they want to apply the Łojasiewicz--Simon inequality. This inequa …
Otis Chodosh's user avatar
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2 votes

Replacing large-dimensional ODE systems with one PDE

This idea is studied in the field "kinetic PDE." See these lecture notes by Clément Mouhot: http://cmouhot.wordpress.com/1900/10/25/mathematical-topics-in-kinetic-theory-part-iii-course/ Ch 2 in par …
Otis Chodosh's user avatar
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