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2 votes
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the relationship between integration by parts and surface integrals

This follows immediately from Green's formula, which says for $X$ vector field, $\Omega$ open set, $n$ the unit exterior normal to the boundary $\partial \Omega$ $$ \int_{\Omega}div X\ dx=\int_{\part …
Bazin's user avatar
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3 votes
Accepted

Fourier transform of a differential operator

Let me change slightly your notations and consider the quadratic form in $\mathbb R_{\xi,\eta}^2$ $$ Q(\xi,\eta)=\alpha \xi ^2+2\gamma \xi \eta+\beta \eta^2, $$ where $\alpha, \beta$ are real paramete …
Bazin's user avatar
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2 votes

Rate of change of mass of a parameterized region

With $H$ the Heaviside function (characteristic function of $\mathbb R_+$), you have $$ M(t)=\int_{\mathbb R^n} f(x) H(t-h(x)) dx $$ and thus, at least formally, $$ \dot M(t)=\int_{\mathbb R^n} f(x) \ …
Bazin's user avatar
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5 votes

Fourier transform of Analytic Functions

The following basic result needs to be quoted on these matters of analyticity: the Paley-Wiener-Schwartz theorem gives a characterization of distributions with compact support. Let $u$ be a tempered d …
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1 vote

Estimating the flow when we know the vector field

Let $X=\sum_{1\le j\le n}a_j(x)\partial_{x_j}$ be a Lipschitz-continuous vector field on some open subset of $\mathbb R^n$. The flow is then Lipschitz-continuous: it is a consequence of Gronwall's in …
Bazin's user avatar
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