** Moebius structures on boundaries, I **

This is an informal series of 3 lectures. I start with boundaries of hyperbolic groups. I will continue with Furstenberg boundaries of higher rank symmetric spaces (joint work with my student Beyrer).

**1. Moebius structures **

4 distinct points in a set can be split into pairs in 3 ways, whence an epimorphism , with kernel

Say a quadruple is *regular* if all points are distinct, and *admissible* if no 3 of them coincide.

** 1.1. Semi-metrics **

A semi-metric on a set is a function which is

- symmetric,
- positive on distinct pairs,
- at most one point can have infinite distance to an other point. For this point, distance to all points is infinity.

** 1.2. Cross-ratios **

The Moebius structure of a semi-metric can be defined in 3 equivalent ways. Given a quadruple , let and define cross-ratio

The resulting 6 numbers (after permutations) can be organized in different ways.

- View
This extends to union 3 points. Under permutation, changes via as above.

- View
where is the 3-point set of splittings in pairs. The product of the 3 functions equals 1. Under permutation, changes via taking inverse and acting on .

- Alternatively, one may replace values by their logarithms.

** 1.3. Sub-Moebius structures **

Definition 1A sub-Moebius structure on a set is a function satisfying

- Normalization. .
- Symmetry. .

** 1.4. The cocycle condition **

Not all sub-Moebius structures arise from semi-metrics. Those arising from semi-metrics satisfy an extra equation, the cocycle condition:

Theorem 2 (Buyalo)A sub-Moebius structure arises from a semi-metric if and only if it satisfies the cocycle condition.

Indeed, set

This is a semi-metric. Different choices of define the same sub-Moebius structure.

** 1.5. Moebius equivalent semi-metrics **

Say two semi-metrics are Moebius equivalent if they define the same sub-Moebius structure. Here are constructions of semi-metrics Moebius equivalent to a given one .

- Multiplication with a constant . .
- Involution. Given ,
- Multiplication with a positive function .

**2. Boundaries **

Let be a metric space. Fix origin . Then (Bourdon) the semi-metric

is a metric. Changing gives a Moebius equivalent metric.

** 2.1. The Ptolemaic inequality **

Moebius structures on boundaries of spaces satisfy an extra inequality, which we call Ptolemaic inequality,

This means that takes its values in the triangle with vertices at the extra points.

metrics are Ptolemaic. The sphere with the chordal metric (i.e. isometric to a subset of Euclidean space) is Ptolemaic. The sphere in its Riemannian metric is not Ptolemaic.

Thus triangle inequality does not imply Ptolemaic. Conversely, Ptolemaic inequality does not imply triangle inequality for all metrics in the class. However, if a sub-metric Ptolemaic Moebius structure has a point at infinity, then it satisfies triangle inequality. Also the Moebius class contains bounded sub-metrics which are metrics.

** 2.2. Hyperbolic groups **

Mineyev has constructed metrics on hyperbolic groups whose visual distances define a Ptolemaic Moebius structure. More on this next time.