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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

Equivalence of statements about level sets: $u|_{S \times [\tau, \infty)}$ depends only on $...

If I am not missing something then the corrected statements look equivalent to me on the level of sets, no need for smoothness. Fix $\tau > 0$ and assume that 1. holds. For any $t\geq \tau$ and $y \in …
mlk's user avatar
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4 votes
Accepted

Regular Lagrangian flow for "square root example": $\frac{d}{dt} X(t,x) = \sqrt{X(t,x)}$

Your intuition is right. The key is in the paper you cite, in that they consider uniqueness in the class $L^1_{\text{loc}}$, which does not allow for concentrations. If you add to this, that the Lagra …
mlk's user avatar
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6 votes

What is Young measure?

Related to Nate River's answer, I personally prefer to think of Young measures as single measures $\nu$ on $U \times \mathbb{R}^m$, which have the condition that their projection on the first componen …
mlk's user avatar
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2 votes

Existence of first variation

In general, a first variation is just the collection of all directional derivatives $\frac{d}{d\epsilon} \mathcal{F}(\rho+\epsilon\chi)|_{\epsilon = 0}$. For fixed $\rho$ one can treat them as a funct …
mlk's user avatar
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4 votes
Accepted

Averaging the mass of a Sobolev function $f\in W^{1,p}(\Omega)$ near $\partial\Omega$

In Evans & Gariepy's "Measure theory and fine properties of functions", Sec. 5.3., they construct the trace operator on a bounded Lipschitz domain $\Omega$ for BV-functions (and thus by inclusion for …
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3 votes
Accepted

Must a Schauder basis for $W^{1,p}_0(\Omega)$ be oscillatory?

Even the modified question does not hold. Let $u_n$ be a basis such that $\mathcal{L}^d(\operatorname{spt} u_n) \to 0$, e.g. a wavelet basis and let $\phi \in C_0^\infty(\Omega)$ a function such that …
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0 votes

Derivative of Wasserstein distance $W^p_p$ along solutions of the continuity equation (contr...

Without having read the sources in detail, I would notice the following: In (4) the second measure is time independent. To satisfy (1) this then corresponds to $(\rho^{(2)}_t,v_t^{(2)}) = (\nu,0)$. Bu …
mlk's user avatar
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3 votes
Accepted

How to interpret the vector fields $F_p(x,u,Du)$ in a Lagrangian optimization problem

There is the following interpretation coming from physics and continuum mechanics, which is a bit too long for a comment but might be helpful: If you think of $\mathcal{F}$ as an energy that you want …
mlk's user avatar
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5 votes
Accepted

Existence of directional heat equation without uniform ellipticity

As you do not have any sort of coupling in any spatial direction other than $x_1$, what you have here is not actually a time-dependent PDE in $d$-dimensions but a $(d-1)$-parameter family of time-depe …
mlk's user avatar
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