## Section3.2Möbius geometry

Möbius geometry provides a unifying framework for studying planar geometries. In particular, the transformation groups of hyperbolic and elliptic geometries in the sections that follow are subgroups of the group of Möbius transformations.

### Subsection3.2.1Möbius transformations

A Möbius transformation (also called a linear fractional transformation) of the extended complex plane $\extC$ is a function of the form

\begin{equation} f(z)= \frac{az+b}{cz+d}\label{stdlinfractransf}\tag{3.2.1} \end{equation}

where $z$ is a complex variable, $a,b,c,d$ are complex constants, and $ad-bc\neq 0\text{.}$ To complete the definition, we make the assignments $f(-d/c)=\infty$ and $f(\infty)=a/c$ if $c\neq 0\text{.}$ If $c=0\text{,}$ we assign $f(\infty)=\infty\text{.}$

Note on notational convention: It is customary to use capital letters such as $S,T,U$ to denote Möbius transformations. It is also customary to omit the parentheses, and to write $Tz$ instead of $T(z)$ to denote the value of a Möbius transformation.

Let $f(x)=(ax+b)/(cx+d)$ be a function of a real variable $x$ with real constants $a,b,c,d$ with $ad-bc\neq 0$ and $c\neq 0\text{.}$

1. Show that $\displaystyle \lim_{x\to -d/c}|f(x)|=\infty\text{.}$
2. Show that $\displaystyle \lim_{x\to \infty}f(x)=a/c\text{.}$

Show that the condition $ad-bc\neq 0$ is necessary and sufficient for invertibility. Find a formula for the inverse of $z\to (az+b)/(cz+d)\text{.}$

Show that the composition of two Möbius transformations is a Möbius transformation. Suggestion: First show that the composition has the form $z\to \frac{rz+s}{tz+u}\text{.}$ Next, instead of a brute force calculation to check that $ru-ts\neq 0\text{,}$ use Checkpoint 3.2.2.

###### Definition3.2.4.

The set of all Möbius transformations forms a group $\M\text{,}$ called the Möbius group , under the operation of function composition. Möbius geometry is the pair $(\extC,\M)\text{.}$

There is a natural relationship between Möbius group operations and matrix group operations. The map ${\mathcal T}\colon GL(2,\C)\to \M$ be given by

\begin{equation} \left[ \begin{array}{cc}a\amp b\\ c\amp d\end{array}\right]\to \left[z\to \frac{az+b}{cz+d}\right]\label{gl2tomobgp}\tag{3.2.2} \end{equation}

is a group homomorphism. The kernel of ${\mathcal T}$ is the group of nonzero scalar matrices.

\begin{equation*} \ker{\mathcal T}=\left\{\left[ \begin{array}{cc}k\amp 0\\ 0\amp k\end{array}\right], k\neq 0\right\}\text{.} \end{equation*}

The quotient group $GL(2,\C)/\ker{\mathcal T}$ is called the projective linear group $PGL(2,\C)\text{.}$ Thus we have a group isomorphism

\begin{equation*} PGL(2,\C)\approx \M\text{.} \end{equation*}

Show that the map ${\mathcal T}$ is a group homomorphism. Show that the kernel of the ${\mathcal T}$ is

\begin{equation*} \ker{\mathcal T}=\left\{\left[ \begin{array}{cc}k\amp 0\\ 0\amp k\end{array}\right], k\neq 0\right\}\text{.} \end{equation*}

Homotheties, rotations, translations, and inversions (see Table 3.1.4 in Section 3.1) are special cases of Möbius transformations. These basic transformations can be viewed as building blocks for general Möbius transformations, as follows.

1. Identify the values of the coefficients $a,b,c,d$ in a Möbius transformation $z\to \frac{az+b}{cz+d}$ that is a homothety, rotation, translation, and inversion, respectively.
2. Write a Möbius transformation that does "clockwise rotation by one-quarter rotation about the point $2-i$".

Next, a simple observation, in the form of the following Lemma, leads to a result called the Fundamental Theorem of Möbius Geometry.

Hint: just solve $z=\frac{az+b}{cz+d}\text{.}$ You will need to consider cases.

Outline: Suppose there are two such transformations, $S$ and $T\text{.}$ Show that $S\circ T^{-1}$ fixes three points. Now apply the previous Lemma.

### Subsection3.2.2Cross ratio

In what follows, we consider the special case of the output triple $w_1=1\text{,}$ $w_2=0\text{,}$ $w_3=\infty\text{.}$ Given three distinct points $z_1,z_2,z_3$ in $\extC\text{,}$ we write $(\cdot,z_1,z_2,z_3)$ to denote the unique Möbius transformation that satisfies $z_1\to 1\text{,}$ $z_2\to 0\text{,}$ and $z_3\to \infty\text{.}$ We write $(z_0,z_1,z_2,z_3)$ to denote the image of $z_0$ under $(\cdot,z_1,z_2,z_3)\text{.}$ The expression $(z_0,z_1,z_2,z_3)$ is called the cross ratio of the 4-tuple $z_0,z_1,z_2,z_3\text{.}$ The next two propositions give important properties of cross ratio.

The transformations $(\cdot,z_1,z_2,z_3)$ and $(\cdot,Tz_1,Tz_2,Tz_3)\circ T$ both send $z_1\to 1\text{,}$ $z_2\to 0\text{,}$ and $z_3\to \infty\text{,}$ so they must be equal, by the Fundamental Theorem. Now apply both transformations to $z_0\text{.}$

A Euclidean circle or straight line is called a cline (pronounced "kline") or generalized circle. The propositions and corollaries above show that the set of all clines is a single congruence class of figures in Möbius geometry.

### Subsection3.2.3Symmetry with respect to a cline

Geometrically, the conjugation map $z\to z^{\ast}$ in the complex plane is reflection across the real line. This "mirror" symmetry generalizes to symmetry with respect to any cline, as follows. Given a cline $C$ that contains $z_1,z_2,z_3$ in $\extC\text{,}$ let $T=(\cdot,z_1,z_2,z_3)\text{.}$ Given any point $z\text{,}$ the symmetric point with respect to $C$ is

\begin{equation} z^{\ast C}= (T^{-1}\circ \conj \circ T)(z)\label{symmptdef}\tag{3.2.3} \end{equation}

where $\conj\colon \extC\to \extC$ is the extension of the conjugation map to the extended complex plane that sends $\infty\to \infty^\ast=\infty\text{.}$ The idea is to map $C$ to the real line via $T\text{,}$ then conjugate, then map the real line back to $C\text{.}$ See Figure 3.2.15. Figure 3.2.15. Symmetric points $z,z^{\ast C}$ with respect to the circle $C$

The definition of symmetric point depends only on the circle $C\text{,}$ and not on the three points $z_1,z_2,z_3\text{.}$ This fact is a corollary of the following Proposition.

Let $z_1,z_2,z_3$ be three points on $C\text{,}$ so that $Sz_1,Sz_2,Sz_3$ are three points on $S(C)\text{.}$ Let $T=(\cdot,z_1,z_2,z_3)$ and let $U=(\cdot,Sz_1,Sz_2,Sz_3)\text{.}$ By invariance of the cross ratio, we have
\begin{equation*} (U\circ S)z=Tz\text{.} \end{equation*}
Thus we have
\begin{align*} (Sz)^{\ast S(C)} \amp = (U^{-1}\circ \conj\circ U)(Sz) \;\; \text{(by definition)}\\ \amp = (S\circ S^{-1}\circ U^{-1}\circ \conj\circ U\circ S)(z)\\ \amp = S (S^{-1}\circ U^{-1}\circ \conj\circ U\circ S)(z)\\ \amp = S(T^{-1}\circ \conj \circ T)(z)\\ \amp = S(z^{\ast C}) \end{align*}
as desired.

### Subsection3.2.4Normal forms

We conclude this section on Möbius geometry with a discussion of the normal form of a Möbius transformation. We begin with a Lemma.

Now suppose that a Möbius transformation $T$ has two fixed points, $p$ and $q\text{.}$ Let $S$ be given by $Sz = \frac{z-p}{z-q}\text{.}$ Let $w=Sz$ and let $U=S\circ T\circ S^{-1}$ be the transformation of the $w$-plane that is conjugate to $T$ via $S$ (see Exercise Group 1.3.3.3–6). It is easy to check that $U$ has exactly two fixed points $0$ and $\infty\text{.}$ Applying the previous Lemma, we have $Uw=\alpha w$ for some nonzero $\alpha\in \C\text{.}$ Applying both sides of $S\circ T=U\circ S$ to $z\text{,}$ we have the following normal form for $T\text{.}$

\begin{equation} \frac{Tz-p}{Tz-q}= \alpha \frac{z-p}{z-q}\label{normalform2fixedpts}\tag{3.2.4} \end{equation}

The transformation $T$ is called elliptic, hyperbolic, or loxodromic if $U$ is a rotation ($|\alpha|=1$), a homothety ($\alpha \gt 0$), or neither, respectively.

Finally, suppose that a Möbius transformation $T$ has exactly one fixed point at $p\text{.}$ Let $S$ be given by $Sz = \frac{1}{z-p}\text{.}$ Again, let $w=Sz$ and let $U=S\circ T\circ S^{-1}\text{.}$ This time, $U$ has exactly one fixed point at $\infty\text{.}$ Applying the Lemma, we have $Uw=w+\beta$ for some nonzero $\beta\in \C\text{.}$ Applying both sides of $S\circ T=U\circ S$ to $z\text{,}$ we have the following normal form for $T\text{.}$

\begin{equation} \frac{1}{Tz-p}= \frac{1}{z-p}+\beta\label{normalformfixedpt}\tag{3.2.5} \end{equation}

A Möbius transformations of this type is called parabolic. Here is a summary of the classification terminology associated with normal forms.

### Subsection3.2.5Steiner circles

The discussion of normal forms show that any non-identity Möbius transformation is conjugate to one of two basic forms, $w\to \alpha w$ or $w\to w+\beta\text{.}$ The natural coordinate system for depicting the action of $w\to \alpha w$ is standard polar coordinates. See Figure 3.2.20. A homothety is a flow along radial lines and a rotation is a flow around polar circles. The natural "degenerate" coordinate system for depicting a translation $w\to w+\beta$ is a family of lines parallel to the line that contains the origin and $\beta\text{.}$ A translation by $\beta$ is a flow along these parallel lines. See Figure 3.2.21.

Pulling the polar and degenerate coordinate grids back to the $z$-plane by $S^{-1}$ leads to coordinate grids called Steiner circles. 1  In the case where $T$ has two fixed points $p,q\text{,}$ the conjugating map $Sz=\frac{z-p}{z-q}$ takes $p\to 0\text{,}$ $q\to \infty\text{.}$ Therefore $S^{-1}$ maps $0\to p$ and $\infty \to q\text{.}$ The transformation $S^{-1}$ maps radial lines in the $w$-plane to clines in the $z$-plane that contain $p$ and $q$ called Steiner circles of the first kind. and $S^{-1}$ maps polar circles in the $w$-plane to clines in the $z$-plane called Steiner circles of the second kind or circles of Apollonius. See Figure 3.2.20.

The convention for which Steiner circles are considered "first" or "second" kinds is not universal. Here we follow the convention used by Ahlfors  and Henle .

In the case where $T$ has one fixed point $p\text{,}$ the conjugating map $Sz=\frac{1}{z-p}$ sends $p\to \infty\text{,}$ so $S^{-1}$ maps $\infty \to p\text{,}$ and $S^{-1}$ maps lines in the $w$-plane that are parallel to the line through $0$ and $\beta$ to clines in the $z$-plane that contain $p\text{.}$ Every cline in this family is tangent to every other cline in this family at exactly the one point $p\text{.}$ Clines in this family are called degenerate Steiner circles. See Figure 3.2.21. Table 3.2.22 summarizes the graphical depiction of Möbius transformations.

### Exercises3.2.6Exercises

###### 1.Decomposition of Möbius transformations into four basic types..
1. Explain why a transformation of the form $z\to az\text{,}$ with $a$ any nonzero complex constant, is a composition of a homothety and a rotation.
2. Explicitly identify each homothety, rotation, translation, and inversion in (3.2.6) to (3.2.9) in the derivation below for the case $c\neq 0\text{.}$
\begin{align} z \amp \to cz+d\label{mobiusdecompfirst}\tag{3.2.6}\\ \amp\to \frac{1}{cz+d}\tag{3.2.7}\\ \amp\to \frac{bc-ad}{cz+d} + a\tag{3.2.8}\\ \amp\to \frac{1}{c}\left(\frac{bc-ad}{cz+d} + a\right)\label{mobiusdecomplast}\tag{3.2.9}\\ \amp = \frac{az+b}{cz+d}\tag{3.2.10} \end{align}
3. Write your own decomposition for the case $c=0\text{.}$
###### 2.Explicit form for the transformation $(\cdot,z_1,z_2,z_3)$.
1. Show that, for the special case when $z_1,z_2,z_3$ are complex (that is, none of the three points is $\infty$), we have the following.
\begin{align} (\cdot,z_1,z_2,z_3) \amp= \left[z\to \frac{z-z_2}{z-z_3}\frac{z_1-z_3}{z_1-z_2}\right]\label{mob10inftyform}\tag{3.2.11}\\ (z_0,z_1,z_2,z_3) \amp= \frac{z_0-z_2}{z_0-z_3}\frac{z_1-z_3}{z_1-z_2}\tag{3.2.12} \end{align}
2. Find explicit formulas for $(\cdot,z_1,z_2,z_3)$ when $z_1=\infty\text{,}$ then do the same for $z_2=\infty$ and $z_3=\infty\text{.}$
###### 3.

Find Möbius transformations that make the following assignments.

1. $\displaystyle 1\to a, 0\to b, \infty\to c$
2. $\displaystyle a\to d, b\to e, c\to f$
###### 4.

Prove Proposition 3.2.12. Suggestion: Let $Tz=\frac{az+b}{cz+d}\text{,}$ then manipulate $Tz=(Tz)^\ast$ to an equation with $|z|^2,z,z^\ast$ terms and coefficients involving $a,b,c,d$ and their conjugates. Then use "complex completing the square" (see (1.1.24)). The hint below shows a version of a circle equation; peek if you need to, and use it to work partially forwards from $Tz=(Tz)^\ast\text{,}$ and partially backwards from the equation in the hint.

Hint
$\left| z-\left(\frac{a^\ast d-bc^\ast}{ac^\ast -a^\ast c}\right)\right|^2 = \left|\frac{ad-bc}{ac^\ast -a^\ast c}\right|^2$
###### 6.Symmetry with respect to a cline.
1. Prove Corollary 3.2.17.
2. Let $C$ be the unit circle. Show that $z^{\ast C} = 1/z^\ast\text{.}$ Suggestion: This is just a computation, but the choice of $z_1,z_2,z_3$ might make it more or less tedious. You might try $\omega,\omega^2,\omega^3=1\text{,}$ where $\omega=e^{2\pi i/3}\text{.}$
3. Now let $C$ be a circle with center $a$ and radius $r\gt 0\text{.}$ Use Proposition 3.2.16 to show that $z^{\ast C} = \frac{r^2}{(z-a)^\ast}+a\text{.}$
4. Let $C$ be a straight line. Show that $z^{\ast C}$ is the reflection of $z$ across $C\text{.}$
###### 7.

Find the normal form and sketch a graph using Steiner circles for the following transformations.

1. $\displaystyle z\to \frac{1}{z}$
2. $\displaystyle z\to \frac{3z-1}{z+1}$
###### 8.

Let $p$ be the single fixed point of a Möbius transformation that is conjugate to $w\to w+\beta$ via $Sz=\frac{1}{z-p}\text{.}$ Show that the single line in the degenerate Steiner clines through $p$ is parallel to the direction given by $\beta^\ast\text{.}$

Hint

Show that $S^{-1}w=\frac{pw+1}{w}\text{,}$ so $S^{-1}$ takes $0,\beta,\infty$ to $\infty,\frac{p\beta+1}{\beta},p\text{.}$ Thus the single degenerate Steiner straight line through $p$ is in the direction given by $\frac{p\beta+1}{\beta}-p =\frac{1}{\beta}\propto \beta^\ast\text{.}$

###### 9.

Show that a (generalized) circle of Apollonius (a Steiner circle of the second kind) is characterized as the set of points of the form

\begin{equation*} C=\left\{P\in \C\colon \frac{d(P,A)}{d(P,B)}=k\right\} \end{equation*}

for some $A,B\in \C$ and some real constant $k\gt 0\text{.}$