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2
votes
Accepted
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 $\partial \Omega$
$$
\int_{\Omega}div X\ dx=\int_{\part …
3
votes
Accepted
Fourier transform of a differential operator
Let me change slightly your notations and consider the quadratic form in $\mathbb R_{\xi,\eta}^2$
$$
Q(\xi,\eta)=\alpha \xi ^2+2\gamma \xi \eta+\beta \eta^2,
$$
where $\alpha, \beta$ are real paramete …
2
votes
Rate of change of mass of a parameterized region
With $H$ the Heaviside function (characteristic function of $\mathbb R_+$),
you have
$$
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) \ …
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 …
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 …