# 4: Geometry

- Page ID
- 23321

Recall the two paragraphs from Section 1.2 that we intended to spend time making sense of and working through:

Whereas Euclid's approach to geometry was additive (he started with basic definitions and axioms and proceeded to build a sequence of results depending on previous ones), Klein's approach was subtractive. He started with a space and a group of allowable transformations of that space. He then threw out all concepts that did not remain unchanged under these transformations. Geometry, to Klein, is the study of objects and functions that remain unchanged under allowable transformations.

Klein's approach to geometry, called the

after the university at which he worked at the time, has the benefit that all three geometries (Euclidean, hyperbolic and elliptic) emerge as special cases from a general space and a general set of transformations.Erlangen Program

We now have both the space (\(\mathbb{C}^+\)) and the transformations (Möbius transformations), and are just about ready to embark on non-Euclidean adventures. Before doing so, however, one more phrase needs defining: * group of transformations*. This phrase has a precise meaning. Not every collection of transformations is lucky enough to form a group.

- 4.1: The Basics
- The reader who has seen group theory will know that in addition to the three properties listed in our definition, the group operation must satisfy a property called associativity. In the context of transformations, the group operation is composition of transformations, and this operation is always associative. So, in the present context of transformations, we omit associativity as a property that needs checking.

- 4.2: Möbius Geometry
- We spent a fair amount of time studying Möbius transformations in Chapter 3, and this will pay dividends now. We emphasize that angles are preserved in Möbius geometry, which is a good thing. Also, rather than pursuing the very general Möbius geometry, we take the preceeding facts and apply them straight away to two of its special “subgeometries,” hyperbolic geometry and elliptic geometry.