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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.

6 votes
Accepted

Is the derivative of a Lipschitz function better than L^\infty

Lipschitz functions are exactly $W^{1,\infty}$ (See 'Sobolev space' on wikipedia - under other examples and perhaps the bit about absolute continuity on lines). This means the short answer to your que …
Spencer's user avatar
  • 1,771
17 votes
3 answers
3k views

Why is the harmonic oscillator so important? (pure viewpoint sought). How to motivate its ro...

I'm in the process of understanding the heat equation proof of the Atiyah-Singer Index Theorem for Dirac Operators on a spin manifold using Getzler scaling. I'm attending a masters-level course on it …
Spencer's user avatar
  • 1,771
4 votes
2 answers
2k views

Inclusions of $C^{k,\alpha}$ spaces

When is $C^{k,\alpha}(\bar{\Omega})$ a subset of $C^{k',\alpha'}(\bar{\Omega})$? Gilbarg and Trudinger says that "for the domains of interest in this work the inclusion will hold whenever $k …
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  • 1,771
6 votes
3 answers
848 views

Divergence form Elliptic PDE Removable Singularity/Regularity Question

Idea Given a $W^{1,2}$ solution to a linear divergence form uniformly elliptic pde with bounded coefficients, standard De Giorgi-Nash-Moser theory tells us that the solution is infact (Holder) contin …
Spencer's user avatar
  • 1,771
7 votes
4 answers
6k views

The characteristic (indicator) function of a set is not in the Sobolev space H¹

Is it true that the characteristic (indicator) function of a subset of Euclidean space with finite positive measure is never in the Sobolev space $H^1 = W^{1,2}$? And if so, what is the best/easiest/ …
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  • 1,771
3 votes
0 answers
109 views

What dimension bound is known on the singular set of a linear combination of eigenfunctions ...

Let $(M,g)$ be a smooth, closed Riemannian manifold and suppose that $\phi_1,\dots,\phi_m$ are eigenfunctions of the Laplacian on $M$. Write $f = \phi_1 + \dots + \phi_m$. How big can the set $\math …
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  • 1,771