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Register a new userBi-Lipschitz invariance of planar $BV$- and $W^{1,1}$-extension domains
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We prove that a bi-Lipschitz image of a planar $BV$-extension domain is also a $BV$-extension domain, and that a bi-Lipschitz image of a planar $W^{1,1}$-extension domain is again a $W^{1,1}$-extension domain.
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Fix integers $m geq 2$, $n geq 1$. Let $C^{m-1,1}(mathbb{R}^n)$ be the space of $(m-1)$-times differentiable functions $F : mathbb{R}^n rightarrow mathbb{R}$ whose $(m-1)$st order partial derivatives are Lipschitz continuous, equipped with a standard seminorm. Given $E subseteq mathbb{R}^n$, let $C^{m-1,1}(E)$ be the trace space of all restrictions $F|_E$ of functions $F$ in $C^{m-1,1}(mathbb{R}^n)$, equipped with the standard quotient (trace) seminorm. We prove that there exists a bounded linear operator $T : C^{m-1,1}(E) rightarrow C^{m-1,1}(mathbb{R}^n)$ satisfying $Tf|_E = f$ for all $f in C^{m-1,1}(E)$, with operator norm at most $exp( gamma D^k)$, where $D := binom{m+n-1}{n}$ is the number of multiindices of length $n$ and order at most $m-1$, and $gamma,k > 0$ are absolute constants (independent of $m,n,E$). Our results improve on the previous construction of linear extension operators with norm at most $ exp(gamma D^k 2^D)$.
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Let $nge2$ and $phi : [0,fz) to [0,infty)$ be a Youngs function satisfying $sup_{x>0} int_0^1frac{phi( t x)}{ phi(x)}frac{dt}{t^{n+1} }<infty. $ We show that Ahlfors $n$-regular domains are Besov-Orlicz ${dot {bf B}}^{phi}$ extension domains, which is necessary to guarantee the nontrivially of ${dot {bf B}}^{phi}$. On the other hand, assume that $phi$ grows sub-exponentially at $fz$ additionally. If $Omega$ is a Besov-Orlicz ${dot {bf B}}^{phi}$ extension domain, then it must be Ahlfors $n$-regular.
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Given a Sobolev homeomorphism $fin W^{2,1}$ in the plane we find a piecewise quadratic homeomorphism that approximates it up to a set of $epsilon$ measure. We show that this piecewise quadratic map can be approximated by diffeomorphisms in the $W^{2,1}$ norm on this set.
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