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A Sobolev space is a vector space of functions equipped with a norm that is a combination of Lp-norms of the function itself and its derivatives up to a given order.

4 votes
Accepted

Is the product of $u \in W^{\sigma,1}(\Omega)$ and $v \in C^{0,\sigma}(\Omega)$ again in $W^...

It is not true that $W^{\sigma,1} \times C^{0,\sigma} \hookrightarrow W^{\sigma,1}$ with $\Omega = \mathbb{R}^n$. My reference for such questions is Thomas Runst, and Winfried Sickel. Sobolev spaces …
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  • 981
7 votes

Can we interpret fractional Sobolev spaces in terms of fractional derivatives?

Yes, such an interpretation exists, at least in the following case. Take $p = 2$, $n = 1$ and $\Omega = (0,1)$. Then, for $0 < s < 1$, $$ \| f \|_{H^s(0,1)} \approx \| \partial_t^s f \|_{L^2(0,1)} $$ …
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  • 981
3 votes

Sobolev inequality with holes

It looks like you are looking for the extreme case of a Gagliardo-Nirenberg interpolation inequality, in the case of an exterior domain (that is, an unbounded domain with compact boundary). Such inequ …
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  • 981
3 votes

The compact embedding of $H^{1/2}_{2\pi}$ in $L^s(0, 2\pi)$

I would say that the book Linear and Quasilinear Parabolic Problems, Volume II: Function Spaces by Herbert Amann qualifies as a modern reference (published in 2019). You can find a version of a Rellic …
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  • 981
5 votes
1 answer
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Hardy-Littlewood-Sobolev inequality using fractional sobolev norm on the RHS

Using Hardy-Littlewood-Sobolev inequality, we can prove that: $$\left| \int_0^1\int_0^1 |x-y|^{-\frac{1}{2}} f(x)f(y) \mathrm{d}x \mathrm{d}y \right| \leq C \left\| f \right\|_{L^{4/3}(0,1)}^2 \leq C …
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  • 981
2 votes

Bound for the product of Sobolev functions in $W^{s,1}$

I believe that $W^{s,1}(\mathbb{R}^d)$ is a Banach algebra when $s > d$. This is a particular case of Theorem 7.3 of the paper Multiplication in Sobolev spaces by Ali Behzadan and Michael Holst.
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