Lipschitz ___domain

In mathematics, a Lipschitz ___domain (or ___domain with Lipschitz boundary) is a ___domain in Euclidean space whose boundary is "sufficiently regular" in the sense that it can be thought of as locally being the graph of a Lipschitz continuous function. The term is named after the German mathematician Rudolf Lipschitz.

Definition

Let ${\displaystyle n\in \mathbb {N} }$ . Let ${\displaystyle \Omega }$  be a ___domain of ${\displaystyle \mathbb {R} ^{n}}$  and let ${\displaystyle \partial \Omega }$  denote the boundary of ${\displaystyle \Omega }$ . Then ${\displaystyle \Omega }$  is called a Lipschitz ___domain if for every point ${\displaystyle p\in \partial \Omega }$  there exists a hyperplane ${\displaystyle H}$  of dimension ${\displaystyle n-1}$  through ${\displaystyle p}$ , a Lipschitz-continuous function ${\displaystyle g:H\rightarrow \mathbb {R} }$  over that hyperplane, and reals ${\displaystyle r>0}$  and ${\displaystyle h>0}$  such that

• ${\displaystyle \Omega \cap C=\left\{x+y{\vec {n}}\mid x\in B_{r}(p)\cap H,\ -h<y<g(x)\right\}}$ 
• ${\displaystyle (\partial \Omega )\cap C=\left\{x+y{\vec {n}}\mid x\in B_{r}(p)\cap H,\ g(x)=y\right\}}$ 

where

${\displaystyle {\vec {n}}}$  is one of the two unit vectors that are normal to ${\displaystyle H,}$ 

${\displaystyle B_{r}(p):=\{x\in \mathbb {R} ^{n}\mid \|x-p\|<r\}}$  is the open ball of radius ${\displaystyle r}$ ,

${\displaystyle C:=\left\{x+y{\vec {n}}\mid x\in B_{r}(p)\cap H,\ {-h}<y<h\right\}.}$ 

In other words, at each point of its boundary, ${\displaystyle \Omega }$  is locally the set of points located above the graph of some Lipschitz function.

Generalization

A more general notion is that of weakly Lipschitz domains, which are domains whose boundary is locally flattable by a bilipschitz mapping. Lipschitz domains in the sense above are sometimes called strongly Lipschitz by contrast with weakly Lipschitz domains.

A ___domain ${\displaystyle \Omega }$  is weakly Lipschitz if for every point ${\displaystyle p\in \partial \Omega ,}$  there exists a radius ${\displaystyle r>0}$  and a map ${\displaystyle \ell _{p}:B_{r}(p)\rightarrow Q}$  such that

• ${\displaystyle \ell _{p}}$  is a bijection;
• ${\displaystyle \ell _{p}}$  and ${\displaystyle \ell _{p}^{-1}}$  are both Lipschitz continuous functions;
• ${\displaystyle \ell _{p}\left(\partial \Omega \cap B_{r}(p)\right)=Q_{0};}$ 
• ${\displaystyle \ell _{p}\left(\Omega \cap B_{r}(p)\right)=Q_{+};}$ 

where ${\displaystyle Q}$  denotes the unit ball ${\displaystyle B_{1}(0)}$  in ${\displaystyle \mathbb {R} ^{n}}$  and

${\displaystyle Q_{0}:=\{(x_{1},\ldots ,x_{n})\in Q\mid x_{n}=0\};}$ 

${\displaystyle Q_{+}:=\{(x_{1},\ldots ,x_{n})\in Q\mid x_{n}>0\}.}$ 

A (strongly) Lipschitz ___domain is always a weakly Lipschitz ___domain but the converse is not true. An example of weakly Lipschitz domains that fails to be a strongly Lipschitz ___domain is given by the two-bricks ___domain ^[1]

Applications of Lipschitz domains

Many of the Sobolev embedding theorems require that the ___domain of study be a Lipschitz ___domain. Consequently, many partial differential equations and variational problems are defined on Lipschitz domains.

References

1. Werner Licht, M. "Smoothed Projections over Weakly Lipschitz Domains", arXiv, 2016.

• Dacorogna, B. (2004). Introduction to the Calculus of Variations. Imperial College Press, London. ISBN 1-86094-508-2.