Symmetrization methods

In mathematics the symmetrization methods are algorithms of transforming a set $A\subset \mathbb {R} ^{n}$ to a ball $B\subset \mathbb {R} ^{n}$ with $vol(B)=vol(A)$ and centered at the origin. We call B the symmetrized version of A, usually denoted $A^{*}$. These algorithms show up in solving the following types of problems: The classical isoperimetric inequality problem asks: Given all 2 dimensional shapes of a given area, which of them has the minimal perimeter (for details see Isoperimetric inequality). The conjectured answer was the disk and Steiner in 1838 showed this to be true using the Steiner symmetrization method (described below). From this many other isoperimetric problems sprung and other symmetrization algorithms. For example, Rayleigh's conjecture is that the first eigenvalue of the Dirichlet problem is minimized for the ball (see Rayleigh–Faber–Krahn inequality for details). Another problem is that the Newtonian capacity of a set A is minimized by $A^{*}$ and this was proved by Polya and G. Szego (1951) using circular symmetrization (described below).

Symmetrization

Let $\Omega \subset \mathbb {R} ^{n}$ be measurable, then we denote by $\Omega ^{*}$ the symmetrized version of $\Omega$ i.e. a ball $\Omega ^{*}:=B_{r}(0)\subset \mathbb {R} ^{n}$ such that $vol(\Omega ^{*})=vol(\Omega )$. We denote by $f^{*}$ the symmetric decreasing rearrangement of nonnegative measurable function f and define it as $f^{*}(x):=\int _{0}^{\infty }1_{\{f(x)>t\}^{*}}(t)dt$. The following methods have been proved to transform $\Omega$ to $\Omega ^{*}$ i.e. given a sequence of symmetrization transformations $\{T_{k}\}$ we have $\lim \limits _{k\to \infty }d_{Ha}(\Omega ^{*},T_{k}(K))=0$, where $d_{Ha}$ is the Hausdorff distance (for discussion see Burchard (2009) )

Steiner Symmetrization

Steiner Symmetrization of set Omega

Steiner symmetrization was introduced by Steiner (1838) to solve the isoperimetric theorem stated above. Let $H^{n-1}\subset \mathbb {R} ^{n}$ be a hyperplane through the origin. Rotate space so that $H^{n-1}$ is the $x_{n}=0$ hyperplane. For each $x\in H$ let the perpendicular line through $x\in H$ be $L_{x}=\{x+ye_{n}:y\in \mathbb {R} \}$. Then by replacing each $\Omega \cap L_{x}$ by a line centered at H and with length $|\Omega \cap L_{x}|$ we obtain the Steiner symmetrized version.

$St(\Omega ):=\{x+ye_{n}:x+ze_{n}\in \Omega ~{\text{for some z and}}~|y|\leq {\frac {1}{2}}|\Omega \cap L_{x}|\}.$

We denote by $St(f)$ the Steiner symmetrization wrt to $x_{n}=0$ hyperplane of nonnegative measurable function $f:\mathbb {R} ^{d}\to \mathbb {R}$ and for fixed $x_{1},...,x_{n-1}$ define it as

$St:f(x_{1},...,x_{n-1},\cdot )\mapsto (f(x_{1},...,x_{n-1},\cdot ))^{*}.$

Circular Symmetrization

Circular symmetrization of set Omega

A popular method for symmetrization in the plane is Polya's circular symmetrization. After we will describe its generalization to higher dimensions. Let $\Omega \subset \mathbb {C}$ be a domain then we define its circular symmetrization $Circ(\Omega )$ wrt the positive real axis as follows: Let

$\Omega _{t}:=\{\theta \in [0,2\pi ]:te^{i\theta }\in \Omega \}$

i.e. contain the arcs of radius t contained in $\Omega$. So we define

• If $\Omega _{t}$ is the full circle, then $Circ(\Omega )\cap \{|z|=t\}:=\{|z|=t\}$.
• If the length is $m(\Omega _{t})=\alpha$, then $Circ(\Omega )\cap \{|z|=t\}:=\{te^{i\theta }:|\theta |<{\frac {\alpha }{2}}\}$.
• $0,\infty \in Circ(\Omega )$ iff $0,\infty \in \Omega$.

In higher dimensions $\Omega \subset \mathbb {R} ^{n}$, we define its spherical symmetrization $Sp^{n}(\Omega )$ wrt to positive axis of $x_{1}$ as follows: Let $\Omega _{r}:=\{x\in \mathbb {S} ^{n-1}:rx\in \Omega \}$ i.e. contain the caps of radius r contained in $\Omega$. Also, for the first coordinate let $angle(x_{1}):=\theta$ if $x_{1}=rcos\theta$. So as above

• If $\Omega _{r}$ is the full cap, then $Sp^{n}(\Omega )\cap \{|z|=r\}:=\{|z|=r\}$.
• If the surface area is $m_{s}(\Omega _{t})=\alpha$, then $Sp^{n}(\Omega )\cap \{|z|=r\}:=\{x:|x|=r$ and $0\leq angle(x_{1})\leq \theta _{\alpha }\}=:C(\theta _{\alpha })$ where $\theta _{\alpha }$ is picked so that its surface area is $m_{s}(C(\theta _{\alpha })=\alpha$. In words, $C(\theta _{\alpha })$ is a cap symmetric around the positive axis $x_{1}$ with the same area as the intersection $\Omega \cap \{|z|=r\}$.
• $0,\infty \in Sp^{n}(\Omega )$ iff $0,\infty \in \Omega$.

Polarization

Polarization of set Omega

Let $\Omega \subset \mathbb {R} ^{n}$ be a domain and $H^{n-1}\subset \mathbb {R} ^{n}$ be a hyperplane through the origin. Denote the reflection across that plane to the positive halfspace $\mathbb {H} ^{+}$ as $\sigma _{H}$ or just $\sigma$ when it is clear from the context. Also, we denote the reflected $\Omega$ across hyperplane H as $\sigma \Omega$. Then, we denote the polarized $\Omega$ as $\Omega ^{\sigma }$ and define it as follows

• If $x\in \Omega \cap \mathbb {H} ^{+}$, then $x\in \Omega ^{\sigma }$.
• If $x\in \Omega \cap \sigma (\Omega )\cap \mathbb {H} ^{-}$, then $x\in \Omega ^{\sigma }$.
• If $x\in (\Omega \setminus \sigma (\Omega ))\cap \mathbb {H} ^{-}$, then $\sigma x\in \Omega ^{\sigma }$.

In words, we simply reflect $(\Omega \setminus \sigma (\Omega ))\cap \mathbb {H} ^{-}$ to the halfspace $\mathbb {H} ^{+}$.

References

• Morgan, Frank (2009). "Symmetrization". Retrieved November 2015. {{cite web}}: Check date values in: |accessdate= (help)
• Burchard, Almut (2009). "A Short Course on Rearrangement Inequalities" (PDF). Retrieved November 2015. {{cite web}}: Check date values in: |accessdate= (help)

• Kojar, Tomas (2015). "Brownian Motion and Symmetrization". Retrieved November 2015. {{cite web}}: Check date values in: |accessdate= (help)