## Finite projective planes
First, before any work can be done, it is necessary to create a list of axioms. These axioms are assumed in the context of the geometry. Changing the axioms will give you different types of geometries. Before even that, some definitions are required. Since everything must only be defined in terms of other objects in the geometry, it is necessary to leave some things undefined. For example, we don't care about what a point really is, only that it obeys all axioms for points. Thus points will be left undefined. It is possible and convenient in many cases to consider a line as a set of all points on it. This leads to three definitions: D Now we have some basic definitions, it is necessary to decide on axioms. Firstly, every two points should be joined by a line. Secondly, every two lines should meet at a point. This axiom is not true in Euclidean geometry as parallel lines do not meet, but in projective geometry there are no parallel lines. If there were parallel lines, the plane would be known as an affine plane. The Euclidean plane is an example of an affine plane but not a projective plane. Thirdly, we want to exclude certain trivial geometries which are not of interest to us. An example of a trivial geometry is a single line with arbitrarily many points on it. These trivial geometries lack many of the structures that projective geometries have, and many theorems fail to hold for them. There are three commonly accepted axioms: A A Let A, B, C, D be four distinct points such that no three of them lie
on the same line. Their existence is implied by A A more complex theorem shows that there exists a number n such that each point lies on exactly n+1 lines and each line contains exactly n+1 points. In this case n is known as the order of the projective plane. It is possible to optionally define a fourth axiom that determines the order of the plane: A This axiom is used if you want to only consider planes of a certain order.
It can be omitted if you want to consider all projective planes. Firstly,
no plane has order 1. This can be shown as in the previous example there
were lines containing 3 points and points on 3 lines, which is not allowed
in a plane of order 1. However, consider A This plane of order 2 is known as the Fano plane after Gino Fano who first described it in 1892. He also described a 3-dimensional projective geometry called the Fano 3-space where every plane was a Fano plane. The Fano plane is the smallest projective geometry and has many applications. For example, it is used in the construction of the Hamming code in information theory. It also represents the 7 basic imaginary octonions, where two octonions multiply to make the third in the same line. Using a directed graph can determine whether the product is positive or negative. Despite the abstract nature of these constructed geometries, it is possible to draw a graph representing them. However, such a graph may have counterintuitive properties. For example, many apparent "points" actually do not exist and lines may not look like straight lines. Here is a diagram of the Fano plane: It is also possible to define higher geometries such as projective 3-spaces, the simplest being the Fano 3-space. Slightly different axioms apply here but many principles are similar. Here an advantage of the axiomatic method shows. It is possible to define spaces in 4 or more dimensions, even though they do not appear in our physical world and are difficult to grasp intuitively. It is also possible to classify all finite projective planes of small orders. The Fano plane is the unique projective plane of order 2. There is exactly one projective plane of orders 3, 4, 5, 7, and 8. It was proved in 1901 by Gaston Tarry that there is no projective plane of order 6. There are exactly 4 projective planes of order 9. After intensive computer calculation, it was proved that no projective planes of order 10 exist. There is only one known projective plane of order 11 but others may exist. It is conjectured that there are no projective planes of order 12.
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