# Projective space

The collection of all subspaces of an incidence system , where the elements of the set are called points, the elements of the set are called lines and I is the incidence relation. A subspace of is defined to be a subset of for which the following condition holds: If and , then the set of points of the line passing through and also belongs to . The incidence system satisfies the following requirements:

1) for any two different points and there exists a unique line such that and ;

2) every line is incident to at least three points;

3) if two different lines and intersect at a point and if the following four relations hold: , , , , then the straight lines passing through the pairs of points and intersect.

A subspace is generated by a set of points in (written ) if is the intersection of all subspaces containing . A set of points is said to be independent if for any one has . An ordered maximal and independent set of points of a subspace is called a basis of , and the number of its elements is called the dimension of the subspace . A subspace of dimension is a point, a subspace of dimension is a projective straight line, a subspace of dimension is called a projective plane.

In a projective space the operations of addition and intersection of spaces are defined. The sum of two subspaces and is defined to be the smallest of the subspaces containing both and . The intersection of two subspaces and is defined to be the largest of the subspaces contained in both and . The dimensions of the subspaces , , of their sum, and of their intersection are connected by the relation

For any there is a such that and ( is a complement of in ), and if , then

for any (Dedekind's rule), that is, with respect to the operation just introduced the projective space is a complemented modular lattice.

A projective space of dimension exceeding two is Desarguesian (see Desargues assumption) and hence is isomorphic to a projective space (left or right) over a suitable skew-field . The (for example) left projective space of dimension over a skew-field is the collection of linear subspaces of an -dimensional left linear space over ; the points of are the lines of , i.e. the left equivalence classes of rows consisting of elements of which are not simultaneously equal to zero (two rows and are left equivalent if there is a such that , ); the subspaces , , are the -dimensional subspaces . It is possible to establish a correspondence between a left and a right projective space under which to a subspace corresponds (the subspaces and are called dual to one another), to an intersection of subspaces corresponds a sum, and to a sum corresponds an intersection. If an assertion based only on properties of linear subspaces, their intersections and sums is true for , then the corresponding assertion is true for . This correspondence between the properties of the spaces and is called the duality principle for projective spaces (see [2]).

A finite skew-field is necessarily commutative; consequently, a finite projective space of dimension exceeding two and of order is isomorphic to the projective space over the Galois field. The finite projective space contains points and subspaces of dimension (see [4]).

A collineation of a projective space is a permutation of its points that maps lines to lines so that subspaces are mapped to subspaces. A non-trivial collineation of the projective space has at most one centre and at most one axis. The group of collineations of a finite projective space has order

Every projective space admits a cyclic transitive group of collineations (see [3]).

A correlation of a projective space is a permutation of subspaces that reverses inclusions, that is, if , then . A projective space admits a correlation only if it is finite-dimensional. An important role in projective geometry is played by the correlations of order two, also called polarities (Polarity).

#### References

[1] | E. Artin, "Geometric algebra" , Interscience (1957) |

[2] | W.V.D. Hodge, D. Pedoe, "Methods of algebraic geometry" , 1 , Cambridge Univ. Press (1947) |

[3] | R. Dembowski, "Finite geometries" , Springer (1968) pp. 254 |

[4] | B. Segre, "Lectures on modern geometry" , Cremonese (1961) |

#### Comments

The real and complex projective spaces , respectively , of all real, respectively complex, lines through the origin in , respectively , are the Grassmann manifolds , (cf. Grassmann manifold).

has a CW-decomposition of exactly one cell in each even dimension. Consequently, its homology is for and for .

Real projective space has a CW-decomposition with exactly one cell in each dimension. For odd the homology groups are: , , ; ; for . For even the homology groups are: ; , ; , .

The real projective plane can be obtained by glueing a disc along its boundary to the boundary of a crosscap (i.e. a Möbius strip). An easy way to see this is to view as obtained from a disc by identifying diametrically-opposite boundary points. Now remove a central disc and cut and glue as indicated below.

Figure: p075350a

The real projective plane cannot be imbedded in , but can be imbedded in . Its Euler characteristic is 1.

#### References

[a1] | O. Veblen, J.W. Young, "Projective geometry" , 1–2 , Blaisdell (1938–1946) |

[a2] | R. Baer, "Linear algebra and projective geometry" , Acad. Press (1952) |

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Projective space.

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