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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.

2 votes
1 answer
234 views

A bound in Sobolev spaces of negative order

Let's consider the domain $U=[-\pi,\pi]\times[-1,1]$. Assume that we have two functions $f\in H^2$ and $g\in H^{1/2}$. I wonder if the following bound is true: $$ \|f g_{x_1}\|_{H^{-0.5}(U)}\leq C( …
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2 votes

If $f \in H^{\frac 12}$ and $\varphi$ is Lipschitz, is $f\varphi \in H^{\frac 12}$ (on a Lip...

We have the bound $$ |fg|_{H^{0.5}}\leq C |f|_{H^{0.5+\delta}}|g|_{H^{0.5}}. $$ So, if you have a Lipschitz function, due to the compactness of the domain, the function is $H^1$.
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2 votes

References for well-posedness of weak solutions to Stefan problem

There is a huge literature in free boundaries. Here I collected some papers (in no particular order) addressing different physical systems (coming mainly from fluid dynamics) and questions (so, not on …
guacho's user avatar
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2 votes
1 answer
774 views

Interpolation in Sobolev spaces

Let $H^s$, $0\leq s<\infty$ be the $L^2$ based Sobolev spaces such that $$ \hat{f}(\xi)(1+|\xi|^2)^{s/2} \in L^2. $$ Let $r_1,r_2,p_1,p_2>0$ be given parameters. Assume that a linear operator $T$ sa …
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