5. Foundations of geometry

5.1. Introduction to the foundations of geometry
What is geometry
The use of real numbers
Dimensions and semantic completeness
5.2. First-order invariants in concrete categories
Multiple automorphisms and models
The Galois connection (Aut, sInv)
The double meaning of "invariance"
Basis and algebraic structures
5.3. Second-order invariants
Endomorphisms
Parametered subspaces
Dual types
Subspaces, embeddings and sections
Quotients
5.4. Affine spaces
Intuitive descriptions of affine maps
Affine subspaces
Straight lines and algebraic structure
Directions
Duality structure
Other affine structures
5.5. Duality
Duality systems
Coordinate systems
...

The below is under construction

(For a short simplified version without reference to the prerequisites, just read the text in green)



Beyond affine geometry

Some geometric spaces, such as vector spaces, Euclidean spaces, and both space-times without gravitation: the classical one (the Galilean space-time), and the one of Special Relativity (the Minkowski space), are "richer" than affine geometry, i.e. "contain an affine structure", in the following (roughly) equivalent senses:

But other geometries do not contain an affine structure, either because they have no notion of straight line, or because these lines do not satisfy the axioms of affine geometry. For example

Let us mention some more.

Projective geometry

Like affine spaces, projective spaces are also based on the structure of alignment, but have no other affine structures from the above list. Parallels cannot be defined there as any pair of straight lines in a plane has one intersection point. Such spaces can be described as affine spaces together with extra points "at infinity" in the role of intersection points of parallel lines, while ignoring which points are at infinity and which are not. These points "at infinity" form the line of "horizon" that is a straight line like others. Projective transformations of the plane (automorphisms of the projective plane) are those involved for perspective representation. Among them, affine transformations are those keeping the horizon to itself, so that affine geometry is equivalent to projective geometry with a constant symbol named "horizon" with type "straight line".

Inversive geometry

It has a notion of circle (or sphere) but cannot distinguish straight lines among them.
Follow the link for details.

Differential geometry

Still, affine geometry is not far away from the above, as small regions of these spaces are approximately affine too: the smaller a region, the better its description can be approximated by affine geometry (with possibly more structures). Usual geometric spaces will have this property of being approximately affine in small regions : we will say they are smooth.

Differential geometry is the "geometry" whose only structure is the notion of smoothness, and smooth curves.
In particular, smooth spaces have an approximation for ratios of small volumes as they become smaller and closer to each other.

The smoothness structure cannot be restricted to a structure of "being approximately projective in small regions" because the "approximately affine structure" of small regions would anyway be definable from it, as the horizon relatively to a small region, has its needed approximative definition as "what is not near this small region".

Topology

This is even weaker than differential geometry, as it has a notion of curve requesting their continuity, that is less restrictive than smoothness. For example the Koch snowflake is continuous but not smooth, so that it is distinguished from curves by differential geometry, but not by topology.

Introduction to topology


Euclidean geometry

Let us first only present the structures, assumed to obey the properties of the intended spaces. The axiomatic specification of these properties will only be completed later.

Euclidean geometry (with any dimension >1) consists in affine geometry together with one structure, thus named the Euclidean structure, that can be expressed (represented) in the following equivalent forms (any one suffices to define the others) :

Optionally, Euclidean geometry may also admit one more structure from the previous list : the unit of area (or unit of volume, in the case of higher dimension), which finally (thanks to the above Euclidean structure) can be called a choice of unit of distance. The choice to include or not this structure in the definition of "Euclidean geometry" is debatable, with motivations for or against it, as follows

For any dimension, the operation of distance d(A,B) between any two points A and B (with values either among real numbers or in a set of quantities, depending on the above choice) can be seen as the fundamental structure of Euclidean geometry, as it is completely sufficient to define all other structures of this geometry in a rather natural way.

Indeed, distance suffices to define the affine structures: the betweenness relation is defined as

B∈[AC] ⇔ d(A,C) = d(A,B) + d(B,C)

and others can be defined from it. Therefore, the transformations preserving distance (even if its values are mere quantities), called isometries, preserve all other structures of Euclidean geometry as well.

Other presentations of the Euclidean structure, assume a priori structures:

As for the notions of circle (or sphere) and intersection angles, they may suffice to define affine geometry but only

The details of these correspondences between different formulations of the Euclidean structure, will be explained in the introduction to inversive geometry.

The isometries of an Euclidean plane or space are called Euclidean moves (to be distinguished from the isometries of other spaces such as a sphere, with also an operation of distance but that does not satisfy all the same axioms). The space of isometries of the Euclidean plane is 3-dimensional, and split in 2 pieces (each of which is also 3-dimensional):



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