# Projective plane

Last updated

In mathematics, a projective plane is a geometric structure that extends the concept of a plane. In the ordinary Euclidean plane, two lines typically intersect in a single point, but there are some pairs of lines (namely, parallel lines) that do not intersect. A projective plane can be thought of as an ordinary plane equipped with additional "points at infinity" where parallel lines intersect. Thus any two distinct lines in a projective plane intersect in one and only one point.

## Contents

Renaissance artists, in developing the techniques of drawing in perspective, laid the groundwork for this mathematical topic. The archetypical example is the real projective plane, also known as the extended Euclidean plane. [1] This example, in slightly different guises, is important in algebraic geometry, topology and projective geometry where it may be denoted variously by PG(2, R), RP2, or P2(R), among other notations. There are many other projective planes, both infinite, such as the complex projective plane, and finite, such as the Fano plane.

A projective plane is a 2-dimensional projective space, but not all projective planes can be embedded in 3-dimensional projective spaces. Such embeddability is a consequence of a property known as Desargues' theorem, not shared by all projective planes.

## Definition

A projective plane consists of a set of lines, a set of points, and a relation between points and lines called incidence, having the following properties: [2]

1. Given any two distinct points, there is exactly one line incident with both of them.
2. Given any two distinct lines, there is exactly one point incident with both of them.
3. There are four points such that no line is incident with more than two of them.

The second condition means that there are no parallel lines. The last condition excludes the so-called degenerate cases (see below). The term "incidence" is used to emphasize the symmetric nature of the relationship between points and lines. Thus the expression "point P is incident with line " is used instead of either "P is on " or " passes through P ".

## Examples

### The extended Euclidean plane

To turn the ordinary Euclidean plane into a projective plane proceed as follows:

1. To each parallel class of lines (a maximum set of mutually parallel lines) associate a single new point. That point is to be considered incident with each line in its class. The new points added are distinct from each other. These new points are called points at infinity .
2. Add a new line, which is considered incident with all the points at infinity (and no other points). This line is called the line at infinity.

The extended structure is a projective plane and is called the extended Euclidean plane or the real projective plane. The process outlined above, used to obtain it, is called "projective completion" or projectivization. This plane can also be constructed by starting from R3 viewed as a vector space, see § Vector space construction below.

### Projective Moulton plane

The points of the Moulton plane are the points of the Euclidean plane, with coordinates in the usual way. To create the Moulton plane from the Euclidean plane some of the lines are redefined. That is, some of their point sets will be changed, but other lines will remain unchanged. Redefine all the lines with negative slopes so that they look like "bent" lines, meaning that these lines keep their points with negative x-coordinates, but the rest of their points are replaced with the points of the line with the same y-intercept but twice the slope wherever their x-coordinate is positive.

The Moulton plane has parallel classes of lines and is an affine plane. It can be projectivized, as in the previous example, to obtain the projective Moulton plane. Desargues' theorem is not a valid theorem in either the Moulton plane or the projective Moulton plane.

### A finite example

This example has just thirteen points and thirteen lines. We label the points P1,...,P13 and the lines m1,...,m13. The incidence relation (which points are on which lines) can be given by the following incidence matrix. The rows are labelled by the points and the columns are labelled by the lines. A 1 in row i and column j means that the point Pi is on the line mj, while a 0 (which we represent here by a blank cell for ease of reading) means that they are not incident. The matrix is in Paige-Wexler normal form.

Lines
Points
m1m2m3m4m5m6m7m8m9m10m11m12m13
P11111
P21   111
P31      111
P41         111
P5 1  1  1  1
P6 1   1  1  1
P71111
P81111
P91111
P101111
P111111
P121111
P131111

To verify the conditions that make this a projective plane, observe that every two rows have exactly one common column in which 1's appear (every pair of distinct points are on exactly one common line) and that every two columns have exactly one common row in which 1's appear (every pair of distinct lines meet at exactly one point). Among many possibilities, the points P1,P4,P5,and P8, for example, will satisfy the third condition. This example is known as the projective plane of order three.

## Vector space construction

Though the line at infinity of the extended real plane may appear to have a different nature than the other lines of that projective plane, this is not the case. Another construction of the same projective plane shows that no line can be distinguished (on geometrical grounds) from any other. In this construction, each "point" of the real projective plane is the one-dimensional subspace (a geometric line) through the origin in a 3-dimensional vector space, and a "line" in the projective plane arises from a (geometric) plane through the origin in the 3-space. This idea can be generalized and made more precise as follows. [3]

Let K be any division ring (skewfield). Let K3 denote the set of all triples x = (x0, x1, x2) of elements of K (a Cartesian product viewed as a vector space). For any nonzero x in K3, the minimal subspace of K3 containing x (which may be visualized as all the vectors in a line through the origin) is the subset

${\displaystyle \{kx:k\in K\}}$

of K3. Similarly, let x and y be linearly independent elements of K3, meaning that kx + my = 0 implies that k = m = 0. The minimal subspace of K3 containing x and y (which may be visualized as all the vectors in a plane through the origin) is the subset

${\displaystyle \{kx+my:k,m\in K\}}$

of K3. This 2-dimensional subspace contains various 1-dimensional subspaces through the origin that may be obtained by fixing k and m and taking the multiples of the resulting vector. Different choices of k and m that are in the same ratio will give the same line.

The projective plane over K, denoted PG(2,K) or KP2, has a set of points consisting of all the 1-dimensional subspaces in K3. A subset L of the points of PG(2,K) is a line in PG(2,K) if there exists a 2-dimensional subspace of K3 whose set of 1-dimensional subspaces is exactly L.

Verifying that this construction produces a projective plane is usually left as a linear algebra exercise.

An alternate (algebraic) view of this construction is as follows. The points of this projective plane are the equivalence classes of the set K3 ∖ {(0, 0, 0)} modulo the equivalence relation

x ~ kx, for all k in K×.

Lines in the projective plane are defined exactly as above.

The coordinates (x0, x1, x2) of a point in PG(2,K) are called homogeneous coordinates. Each triple (x0, x1, x2) represents a well-defined point in PG(2,K), except for the triple (0, 0, 0), which represents no point. Each point in PG(2,K), however, is represented by many triples.

If K is a topological space, then KP2, inherits a topology via the product, subspace, and quotient topologies.

### Classical examples

The real projective plane RP2, arises when K is taken to be the real numbers, R. As a closed, non-orientable real 2-manifold, it serves as a fundamental example in topology. [4]

In this construction consider the unit sphere centered at the origin in R3. Each of the R3 lines in this construction intersects the sphere at two antipodal points. Since the R3 line represents a point of RP2, we will obtain the same model of RP2 by identifying the antipodal points of the sphere. The lines of RP2 will be the great circles of the sphere after this identification of antipodal points. This description gives the standard model of elliptic geometry.

The complex projective plane CP2, arises when K is taken to be the complex numbers, C. It is a closed complex 2-manifold, and hence a closed, orientable real 4-manifold. It and projective planes over other fields (known as pappian planes ) serve as fundamental examples in algebraic geometry. [5]

The quaternionic projective plane HP2 is also of independent interest.[ citation needed ]

### Finite field planes

By Wedderburn's Theorem, a finite division ring must be commutative and so be a field. Thus, the finite examples of this construction are known as "field planes". Taking K to be the finite field of q = pn elements with prime p produces a projective plane of q2 + q + 1 points. The field planes are usually denoted by PG(2,q) where PG stands for projective geometry, the "2" is the dimension and q is called the order of the plane (it is one less than the number of points on any line). The Fano plane, discussed below, is denoted by PG(2,2). The third example above is the projective plane PG(2,3).

The Fano plane is the projective plane arising from the field of two elements. It is the smallest projective plane, with only seven points and seven lines. In the figure at right, the seven points are shown as small black balls, and the seven lines are shown as six line segments and a circle. However, one could equivalently consider the balls to be the "lines" and the line segments and circle to be the "points" – this is an example of duality in the projective plane: if the lines and points are interchanged, the result is still a projective plane (see below). A permutation of the seven points that carries collinear points (points on the same line) to collinear points is called a collineation or symmetry of the plane. The collineations of a geometry form a group under composition, and for the Fano plane this group (PΓL(3,2) = PGL(3,2)) has 168 elements.

### Desargues' theorem and Desarguesian planes

The theorem of Desargues is universally valid in a projective plane if and only if the plane can be constructed from a three-dimensional vector space over a skewfield as above. [6] These planes are called Desarguesian planes, named after Girard Desargues. The real (or complex) projective plane and the projective plane of order 3 given above are examples of Desarguesian projective planes. The projective planes that can not be constructed in this manner are called non-Desarguesian planes, and the Moulton plane given above is an example of one. The PG(2,K) notation is reserved for the Desarguesian planes. When K is a field, a very common case, they are also known as field planes and if the field is a finite field they can be called Galois planes.

## Subplanes

A subplane of a projective plane is a subset of the points of the plane which themselves form a projective plane with the same incidence relations.

( Bruck 1955 ) proves the following theorem. Let Π be a finite projective plane of order N with a proper subplane Π0 of order M. Then either N = M2 or NM2 + M.

When N is a square, subplanes of order N are called Baer subplanes. Every point of the plane lies on a line of a Baer subplane and every line of the plane contains a point of the Baer subplane.

In the finite Desarguesian planes PG(2,pn), the subplanes have orders which are the orders of the subfields of the finite field GF(pn), that is, pi where i is a divisor of n. In non-Desarguesian planes however, Bruck's theorem gives the only information about subplane orders. The case of equality in the inequality of this theorem is not known to occur. Whether or not there exists a subplane of order M in a plane of order N with M2 + M = N is an open question. If such subplanes existed there would be projective planes of composite (non-prime power) order.

### Fano subplanes

A Fano subplane is a subplane isomorphic to PG(2,2), the unique projective plane of order 2.

If you consider a quadrangle (a set of 4 points no three collinear) in this plane, the points determine six of the lines of the plane. The remaining three points (called the diagonal points of the quadrangle) are the points where the lines that do not intersect at a point of the quadrangle meet. The seventh line consists of all the diagonal points (usually drawn as a circle or semicircle).

In finite desarguesian planes, PG(2,q), Fano subplanes exist if and only if q is even (that is, a power of 2). The situation in non-desarguesian planes is unsettled. They could exist in any non-desarguesian plane of order greater than 6, and indeed, they have been found in all non-desarguesian planes in which they have been looked for (in both odd and even orders).

An open question is: Does every non-desarguesian plane contain a Fano subplane?

A theorem concerning Fano subplanes due to ( Gleason 1956 ) is:

If every quadrangle in a finite projective plane has collinear diagonal points, then the plane is desarguesian (of even order).

## Affine planes

Projectivization of the Euclidean plane produced the real projective plane. The inverse operation starting with a projective plane, remove one line and all the points incident with that line produces an affine plane.

### Definition

More formally an affine plane consists of a set of lines and a set of points, and a relation between points and lines called incidence, having the following properties:

1. Given any two distinct points, there is exactly one line incident with both of them.
2. Given any line l and any point P not incident with l, there is exactly one line incident with P that does not meet l.
3. There are four points such that no line is incident with more than two of them.

The second condition means that there are parallel lines and is known as Playfair's axiom. The expression "does not meet" in this condition is shorthand for "there does not exist a point incident with both lines."

The Euclidean plane and the Moulton plane are examples of infinite affine planes. A finite projective plane will produce a finite affine plane when one of its lines and the points on it are removed. The order of a finite affine plane is the number of points on any of its lines (this will be the same number as the order of the projective plane from which it comes). The affine planes which arise from the projective planes PG(2,q) are denoted by AG(2,q).

There is a projective plane of order N if and only if there is an affine plane of order N. When there is only one affine plane of order N there is only one projective plane of order N, but the converse is not true. The affine planes formed by the removal of different lines of the projective plane will be isomorphic if and only if the removed lines are in the same orbit of the collineation group of the projective plane. These statements hold for infinite projective planes as well.

### Construction of projective planes from affine planes

The affine plane K2 over K embeds into KP2 via the map which sends affine (non-homogeneous) coordinates to homogeneous coordinates,

${\displaystyle (x_{1},x_{2})\mapsto (1,x_{1},x_{2}).}$

The complement of the image is the set of points of the form (0, x1, x2). From the point of view of the embedding just given, these points are the points at infinity. They constitute a line in KP2 namely, the line arising from the plane

${\displaystyle \{k(0,0,1)+m(0,1,0):k,m\in K\}}$

in K3 called the line at infinity. The points at infinity are the "extra" points where parallel lines intersect in the construction of the extended real plane; the point (0, x1, x2) is where all lines of slope x2 / x1 intersect. Consider for example the two lines

${\displaystyle u=\{(x,0):x\in K\}}$
${\displaystyle y=\{(x,1):x\in K\}}$

in the affine plane K2. These lines have slope 0 and do not intersect. They can be regarded as subsets of KP2 via the embedding above, but these subsets are not lines in KP2. Add the point (0, 1, 0) to each subset; that is, let

${\displaystyle {\bar {u}}=\{(1,x,0):x\in K\}\cup \{(0,1,0)\}}$
${\displaystyle {\bar {y}}=\{(1,x,1):x\in K\}\cup \{(0,1,0)\}}$

These are lines in KP2; ū arises from the plane

${\displaystyle \{k(1,0,0)+m(0,1,0):k,m\in K\}}$

in K3, while ȳ arises from the plane

${\displaystyle {k(1,0,1)+m(0,1,0):k,m\in K}.}$

The projective lines ū and ȳ intersect at (0, 1, 0). In fact, all lines in K2 of slope 0, when projectivized in this manner, intersect at (0, 1, 0) in KP2.

The embedding of K2 into KP2 given above is not unique. Each embedding produces its own notion of points at infinity. For example, the embedding

${\displaystyle (x_{1},x_{2})\to (x_{2},1,x_{1}),}$

has as its complement those points of the form (x0, 0, x2), which are then regarded as points at infinity.

When an affine plane does not have the form of K2 with K a division ring, it can still be embedded in a projective plane, but the construction used above does not work. A commonly used method for carrying out the embedding in this case involves expanding the set of affine coordinates and working in a more general "algebra".

### Generalized coordinates

One can construct a coordinate "ring"a so-called planar ternary ring (not a genuine ring)corresponding to any projective plane. A planar ternary ring need not be a field or division ring, and there are many projective planes that are not constructed from a division ring. They are called non-Desarguesian projective planes and are an active area of research. The Cayley plane (OP2), a projective plane over the octonions, is one of these because the octonions do not form a division ring. [3]

Conversely, given a planar ternary ring (R,T), a projective plane can be constructed (see below). The relationship is not one to one. A projective plane may be associated with several non-isomorphic planar ternary rings. The ternary operator T can be used to produce two binary operators on the set R, by:

a + b = T(a,1,b), and
a • b = T(a,b,0).

The ternary operator is linear if T(x,m,k) = x•m + k. When the set of coordinates of a projective plane actually form a ring, a linear ternary operator may be defined in this way, using the ring operations on the right, to produce a planar ternary ring.

Algebraic properties of this planar ternary coordinate ring turn out to correspond to geometric incidence properties of the plane. For example, Desargues' theorem corresponds to the coordinate ring being obtained from a division ring, while Pappus's theorem corresponds to this ring being obtained from a commutative field. A projective plane satisfying Pappus's theorem universally is called a Pappian plane. Alternative, not necessarily associative, division algebras like the octonions correspond to Moufang planes.

There is no known purely geometric proof of the purely geometric statement that Desargues' theorem implies Pappus' theorem in a finite projective plane (finite Desarguesian planes are Pappian). (The converse is true in any projective plane and is provable geometrically, but finiteness is essential in this statement as there are infinite Desarguesian planes which are not Pappian.) The most common proof uses coordinates in a division ring and Wedderburn's theorem that finite division rings must be commutative; Bamberg & Penttila (2015) give a proof that uses only more "elementary" algebraic facts about division rings.

To describe a finite projective plane of order N(≥ 2) using non-homogeneous coordinates and a planar ternary ring:

Let one point be labelled ().
Label N points, (r) where r = 0, ..., (N  1).
Label N2 points, (r, c) where r, c = 0, ..., (N  1).

On these points, construct the following lines:

One line [] = { (), (0), ..., (N  1)}
N lines [c] = {(), (c,0), ..., (c, N  1)}, where c = 0, ..., (N  1)
N2 lines [r, c] = {(r) and the points (x, T(x,r,c)) }, where x, r, c = 0, ..., (N  1) and T is the ternary operator of the planar ternary ring.

For example, for N=2 we can use the symbols {0,1} associated with the finite field of order 2. The ternary operation defined by T(x,m,k) = xm + k with the operations on the right being the multiplication and addition in the field yields the following:

One line [] = { (), (0), (1)},
2 lines [c] = {(), (c,0), (c,1) : c = 0, 1},
[0] = {(), (0,0), (0,1) }
[1] = {(), (1,0), (1,1) }
4 lines [r, c]: (r) and the points (i,ir + c), where i = 0, 1 : r, c = 0, 1.
[0,0]: {(0), (0,0), (1,0) }
[0,1]: {(0), (0,1), (1,1) }
[1,0]: {(1), (0,0), (1,1) }
[1,1]: {(1), (0,1), (1,0) }

## Degenerate planes

Degenerate planes do not fulfill the third condition in the definition of a projective plane. They are not structurally complex enough to be interesting in their own right, but from time to time they arise as special cases in general arguments. There are seven kinds of degenerate plane according to ( Albert & Sandler 1968 ). They are:

1. the empty set;
2. a single point, no lines;
3. a single line, no points;
4. a single point, a collection of lines, the point is incident with all of the lines;
5. a single line, a collection of points, the points are all incident with the line;
6. a point P incident with a line m, an arbitrary collection of lines all incident with P and an arbitrary collection of points all incident with m;
7. a point P not incident with a line m, an arbitrary (can be empty) collection of lines all incident with P and all the points of intersection of these lines with m.

These seven cases are not independent, the fourth and fifth can be considered as special cases of the sixth, while the second and third are special cases of the fourth and fifth respectively. The special case of the seventh plane with no additional lines can be seen as an eighth plane. All the cases can therefore be organized into two families of degenerate planes as follows (this representation is for finite degenerate planes, but may be extended to infinite ones in a natural way):

1) For any number of points P1, ..., Pn, and lines L1, ..., Lm,

L1 = { P1, P2, ..., Pn}
L2 = { P1 }
L3 = { P1 }
...
Lm = { P1 }

2) For any number of points P1, ..., Pn, and lines L1, ..., Ln, (same number of points as lines)

L1 = { P2, P3, ..., Pn }
L2 = { P1, P2 }
L3 = { P1, P3 }
...
Ln = { P1, Pn }

## Collineations

A collineation of a projective plane is a bijective map of the plane to itself which maps points to points and lines to lines that preserves incidence, meaning that if σ is a bijection and point P is on line m, then Pσ is on mσ. [7]

If σ is a collineation of a projective plane, a point P with P = Pσ is called a fixed point of σ, and a line m with m = mσ is called a fixed line of σ. The points on a fixed line need not be fixed points, their images under σ are just constrained to lie on this line. The collection of fixed points and fixed lines of a collineation form a closed configuration, which is a system of points and lines that satisfy the first two but not necessarily the third condition in the definition of a projective plane. Thus, the fixed point and fixed line structure for any collineation either form a projective plane by themselves, or a degenerate plane. Collineations whose fixed structure forms a plane are called planar collineations.

### Homography

A homography (or projective transformation) of PG(2,K) is a collineation of this type of projective plane which is a linear transformation of the underlying vector space. Using homogeneous coordinates they can be represented by invertible 3 × 3 matrices over K which act on the points of PG(2,K) by y = MxT, where x and y are points in K3 (vectors) and M is an invertible 3 × 3 matrix over K. [8] Two matrices represent the same projective transformation if one is a constant multiple of the other. Thus the group of projective transformations is the quotient of the general linear group by the scalar matrices called the projective linear group.

Another type of collineation of PG(2,K) is induced by any automorphism of K, these are called automorphic collineations. If α is an automorphism of K, then the collineation given by (x0,x1,x2) → (x0α,x1α,x2α) is an automorphic collineation. The fundamental theorem of projective geometry says that all the collineations of PG(2,K) are compositions of homographies and automorphic collineations. Automorphic collineations are planar collineations.

## Plane duality

A projective plane is defined axiomatically as an incidence structure, in terms of a set P of points, a set L of lines, and an incidence relation I that determines which points lie on which lines. As P and L are only sets one can interchange their roles and define a plane dual structure.

By interchanging the role of "points" and "lines" in

C = (P,L,I)

we obtain the dual structure

C* = (L,P,I*),

where I* is the converse relation of I.

In a projective plane a statement involving points, lines and incidence between them that is obtained from another such statement by interchanging the words "point" and "line" and making whatever grammatical adjustments that are necessary, is called the plane dual statement of the first. The plane dual statement of "Two points are on a unique line." is "Two lines meet at a unique point." Forming the plane dual of a statement is known as dualizing the statement.

If a statement is true in a projective plane C, then the plane dual of that statement must be true in the dual plane C*. This follows since dualizing each statement in the proof "in C" gives a statement of the proof "in C*."

In the projective plane C, it can be shown that there exist four lines, no three of which are concurrent. Dualizing this theorem and the first two axioms in the definition of a projective plane shows that the plane dual structure C* is also a projective plane, called the dual plane of C.

If C and C* are isomorphic, then C is called self-dual. The projective planes PG(2,K) for any division ring K are self-dual. However, there are non-Desarguesian planes which are not self-dual, such as the Hall planes and some that are, such as the Hughes planes.

The Principle of Plane Duality says that dualizing any theorem in a self-dual projective plane C produces another theorem valid in C.

## Correlations

A duality is a map from a projective plane C = (P, L, I) to its dual plane C* = (L, P, I*) (see above) which preserves incidence. That is, a duality σ will map points to lines and lines to points (Pσ = L and Lσ = P) in such a way that if a point Q is on a line m (denoted by Q I m) then Qσ I* mσmσ I Qσ. A duality which is an isomorphism is called a correlation. [9] If a correlation exists then the projective plane C is self-dual.

In the special case that the projective plane is of the PG(2,K) type, with K a division ring, a duality is called a reciprocity. [10] These planes are always self-dual. By the fundamental theorem of projective geometry a reciprocity is the composition of an automorphic function of K and a homography. If the automorphism involved is the identity, then the reciprocity is called a projective correlation.

A correlation of order two (an involution) is called a polarity. If a correlation φ is not a polarity then φ2 is a nontrivial collineation.

## Finite projective planes

It can be shown that a projective plane has the same number of lines as it has points (infinite or finite). Thus, for every finite projective plane there is an integer N ≥ 2 such that the plane has

N2 + N + 1 points,
N2 + N + 1 lines,
N + 1 points on each line, and
N + 1 lines through each point.

The number N is called the order of the projective plane.

The projective plane of order 2 is called the Fano plane. See also the article on finite geometry.

Using the vector space construction with finite fields there exists a projective plane of order N = pn, for each prime power pn. In fact, for all known finite projective planes, the order N is a prime power.

The existence of finite projective planes of other orders is an open question. The only general restriction known on the order is the Bruck-Ryser-Chowla theorem that if the order N is congruent to 1 or 2 mod 4, it must be the sum of two squares. This rules out N = 6. The next case N = 10 has been ruled out by massive computer calculations. Nothing more is known; in particular, the question of whether there exists a finite projective plane of order N = 12 is still open.

Another longstanding open problem is whether there exist finite projective planes of prime order which are not finite field planes (equivalently, whether there exists a non-Desarguesian projective plane of prime order).

A projective plane of order N is a Steiner S(2, N + 1, N2 + N + 1) system (see Steiner system). Conversely, one can prove that all Steiner systems of this form (λ = 2) are projective planes.

The number of mutually orthogonal Latin squares of order N is at most N 1. N 1 exist if and only if there is a projective plane of order N.

While the classification of all projective planes is far from complete, results are known for small orders:

• 2 : all isomorphic to PG(2,2)
• 3 : all isomorphic to PG(2,3)
• 4 : all isomorphic to PG(2,4)
• 5 : all isomorphic to PG(2,5)
• 6 : impossible as the order of a projective plane, proved by Tarry who showed that Euler's thirty-six officers problem has no solution. However, the connection between these problems was not known until Bose proved it in 1938. [11]
• 7 : all isomorphic to PG(2,7)
• 8 : all isomorphic to PG(2,8)
• 9 : PG(2,9), and three more different (non-isomorphic) non-Desarguesian planes: a Hughes plane, a Hall plane, and the dual of this Hall plane. All are described in ( Room & Kirkpatrick 1971 ).
• 10 : impossible as an order of a projective plane, proved by heavy computer calculation. [12]
• 11 : at least PG(2,11), others are not known but possible.
• 12 : it is conjectured to be impossible as an order of a projective plane.

## Projective planes in higher-dimensional projective spaces

Projective planes may be thought of as projective geometries of "geometric" dimension two. [13] Higher-dimensional projective geometries can be defined in terms of incidence relations in a manner analogous to the definition of a projective plane. These turn out to be "tamer" than the projective planes since the extra degrees of freedom permit Desargues' theorem to be proved geometrically in the higher-dimensional geometry. This means that the coordinate "ring" associated to the geometry must be a division ring (skewfield) K, and the projective geometry is isomorphic to the one constructed from the vector space Kd+1, i.e. PG(d,K). As in the construction given earlier, the points of the d-dimensional projective space PG(d,K) are the lines through the origin in Kd + 1 and a line in PG(d,K) corresponds to a plane through the origin in Kd + 1. In fact, each i-dimensional object in PG(d,K), with i<d, is an (i + 1)-dimensional (algebraic) vector subspace of Kd + 1 ("goes through the origin"). The projective spaces in turn generalize to the Grassmannian spaces.

It can be shown that if Desargues' theorem holds in a projective space of dimension greater than two, then it must also hold in all planes that are contained in that space. Since there are projective planes in which Desargues' theorem fails (non-Desarguesian planes), these planes can not be embedded in a higher-dimensional projective space. Only the planes from the vector space construction PG(2,K) can appear in projective spaces of higher dimension. Some disciplines in mathematics restrict the meaning of projective plane to only this type of projective plane since otherwise general statements about projective spaces would always have to mention the exceptions when the geometric dimension is two. [14]

## Notes

1. The phrases "projective plane", "extended affine plane" and "extended Euclidean plane" may be distinguished according to whether the line at infinity is regarded as special (in the so-called "projective" plane it isn't, in the "extended" planes it is) and to whether Euclidean metric is regarded as meaningful (in the projective and affine planes it isn't). Similarly for projective or extended spaces of other dimensions.
2. In a more formal version of the definition it is pointed out that the terms point, line and incidence are primitive notions (undefined terms). This formal viewpoint is needed to understand the concept of duality when applied to projective planes.
3. Baez (2002).
4. The real projective plane appears 37 times in the index of Bredon (1993), for example.
5. The projective planes over fields are used throughout Shafarevich (1994), for example.
6. David Hilbert proved the more difficult "only if" part of this result.
7. Geometers tend to like writing mappings in an exponential notation, so Pσ means σ(P) in a more conventional notation.
8. The points are viewed as row vectors, so to make the matrix multiplication work in this expression, the point x must be written as a column vector.
9. ( Dembowski 1968 ) pg.151.
10. ( Casse 2006 ) pg.94.
11. Lam, Clement W.H. (1991). "The Search for a Finite Projective Plane of Order 10". The American Mathematical Monthly . 98 (4): 305–318. doi:10.1080/00029890.1991.12000759. In 1938, Bose explained why there is no projective plane of order 6. He related the existence of a finite projective plane of order n to the existence of a hyper-Graeco-Latin square. (p. 306)
12. ( Lam 1991 ) harv error: multiple targets (2×): CITEREFLam1991 (help)
13. There are competing notions of dimension in geometry and algebra (vector spaces). In geometry, lines are 1 dimensional, planes are 2 dimensional, solids are 3 dimensional, etc. In a vector space however, the dimension is the number of vectors in a basis. When geometries are constructed from vector spaces, these two notions of dimension can lead to confusion, so it is often the case that the geometric concept is called geometric or projective dimension and the other is algebraic or vector space dimension. The two concepts are numerically related by: algebraic dimension = geometric dimension + 1.
14. "One might say, with some justice, that projective geometry, in so far as present day research is concerned, has split into two quite separate fields. On the one hand, the researcher into the foundations of geometry tends to regard Desarguesian spaces as completely known. Since the only possible non-Desarguesian spaces are planes, his attention is restricted to the theory of projective planes, especially the non- Desarguesian planes. On the other hand stand all those researchers - and especially, the algebraic geometers - who are unwilling to be bound to two-dimensional space and uninterested in permitting non-Desarguesian planes to assume an exceptional role in their theorems. For the latter group of researchers, there are no projective spaces except the Desarguesian spaces." ( Bruck & Bose 1964 , Introduction)

## Related Research Articles

In mathematics, the concept of a projective space originated from the visual effect of perspective, where parallel lines seem to meet at infinity. A projective space may thus be viewed as the extension of a Euclidean space, or, more generally, an affine space with points at infinity, in such a way that there is one point at infinity of each direction of parallel lines.

In mathematics, projective geometry is the study of geometric properties that are invariant with respect to projective transformations. This means that, compared to elementary Euclidean geometry, projective geometry has a different setting, projective space, and a selective set of basic geometric concepts. The basic intuitions are that projective space has more points than Euclidean space, for a given dimension, and that geometric transformations are permitted that transform the extra points to Euclidean points, and vice-versa.

In mathematics, affine geometry is what remains of Euclidean geometry when not using the metric notions of distance and angle.

A finite geometry is any geometric system that has only a finite number of points. The familiar Euclidean geometry is not finite, because a Euclidean line contains infinitely many points. A geometry based on the graphics displayed on a computer screen, where the pixels are considered to be the points, would be a finite geometry. While there are many systems that could be called finite geometries, attention is mostly paid to the finite projective and affine spaces because of their regularity and simplicity. Other significant types of finite geometry are finite Möbius or inversive planes and Laguerre planes, which are examples of a general type called Benz planes, and their higher-dimensional analogs such as higher finite inversive geometries.

In projective geometry, Desargues's theorem, named after Girard Desargues, states:

In finite geometry, the Fano plane is the finite projective plane of order 2. It is the finite projective plane with the smallest possible number of points and lines: 7 points and 7 lines, with 3 points on every line and 3 lines through every point. The standard notation for this plane, as a member of a family of projective spaces, is PG(2, 2) where PG stands for "projective geometry", the first parameter is the geometric dimension and the second parameter is the order.

In geometry, a striking feature of projective planes is the symmetry of the roles played by points and lines in the definitions and theorems, and (plane) duality is the formalization of this concept. There are two approaches to the subject of duality, one through language and the other a more functional approach through special mappings. These are completely equivalent and either treatment has as its starting point the axiomatic version of the geometries under consideration. In the functional approach there is a map between related geometries that is called a duality. Such a map can be constructed in many ways. The concept of plane duality readily extends to space duality and beyond that to duality in any finite-dimensional projective geometry.

In mathematics, a duality translates concepts, theorems or mathematical structures into other concepts, theorems or structures, in a one-to-one fashion, often by means of an involution operation: if the dual of A is B, then the dual of B is A. Such involutions sometimes have fixed points, so that the dual of A is A itself. For example, Desargues' theorem is self-dual in this sense under the standard duality in projective geometry.

In geometry, an affine plane is a system of points and lines that satisfy the following axioms:

In mathematics, incidence geometry is the study of incidence structures. A geometric structure such as the Euclidean plane is a complicated object that involves concepts such as length, angles, continuity, betweenness, and incidence. An incidence structure is what is obtained when all other concepts are removed and all that remains is the data about which points lie on which lines. Even with this severe limitation, theorems can be proved and interesting facts emerge concerning this structure. Such fundamental results remain valid when additional concepts are added to form a richer geometry. It sometimes happens that authors blur the distinction between a study and the objects of that study, so it is not surprising to find that some authors refer to incidence structures as incidence geometries.

In mathematics, a translation plane is a projective plane which admits a certain group of symmetries. Along with the Hughes planes and the Figueroa planes, translation planes are among the most well-studied of the known non-Desarguesian planes, and the vast majority of known non-Desarguesian planes are either translation planes, or can be obtained from a translation plane via successive iterations of dualization and/or derivation.

In projective geometry, a homography is an isomorphism of projective spaces, induced by an isomorphism of the vector spaces from which the projective spaces derive. It is a bijection that maps lines to lines, and thus a collineation. In general, some collineations are not homographies, but the fundamental theorem of projective geometry asserts that is not so in the case of real projective spaces of dimension at least two. Synonyms include projectivity, projective transformation, and projective collineation.

In projective geometry, a collineation is a one-to-one and onto map from one projective space to another, or from a projective space to itself, such that the images of collinear points are themselves collinear. A collineation is thus an isomorphism between projective spaces, or an automorphism from a projective space to itself. Some authors restrict the definition of collineation to the case where it is an automorphism. The set of all collineations of a space to itself form a group, called the collineation group.

In projective geometry an oval is a circle-like pointset (curve) in a plane that is defined by incidence properties. The standard examples are the nondegenerate conics. However, a conic is only defined in a pappian plane, whereas an oval may exist in any type of projective plane. In the literature, there are many criteria which imply that an oval is a conic, but there are many examples, both infinite and finite, of ovals in pappian planes which are not conics.

Affine geometry, broadly speaking, is the study of the geometrical properties of lines, planes, and their higher dimensional analogs, in which a notion of "parallel" is retained, but no metrical notions of distance or angle are. Affine spaces differ from linear spaces in that they do not have a distinguished choice of origin. So, in the words of Marcel Berger, "An affine space is nothing more than a vector space whose origin we try to forget about, by adding translations to the linear maps." Accordingly, a complex affine space, that is an affine space over the complex numbers, is like a complex vector space, but without a distinguished point to serve as the origin.

In mathematics, a non-Desarguesian plane is a projective plane that does not satisfy Desargues' theorem, or in other words a plane that is not a Desarguesian plane. The theorem of Desargues is true in all projective spaces of dimension not 2; in other words, the only projective spaces of dimension not equal to 2 are the classical projective geometries over a field. However, David Hilbert found that some projective planes do not satisfy it. The current state of knowledge of these examples is not complete.

In algebraic geometry, a smooth scheme over a field is a scheme which is well approximated by affine space near any point. Smoothness is one way of making precise the notion of a scheme with no singular points. A special case is the notion of a smooth variety over a field. Smooth schemes play the role in algebraic geometry of manifolds in topology.

In mathematics, a Hall plane is a non-Desarguesian projective plane constructed by Marshall Hall Jr. (1943). There are examples of order for every prime p and every positive integer n provided .

In geometry, specifically projective geometry, a blocking set is a set of points in a projective plane that every line intersects and that does not contain an entire line. The concept can be generalized in several ways. Instead of talking about points and lines, one could deal with n-dimensional subspaces and m-dimensional subspaces, or even more generally, objects of type 1 and objects of type 2 when some concept of intersection makes sense for these objects. A second way to generalize would be to move into more abstract settings than projective geometry. One can define a blocking set of a hypergraph as a set that meets all edges of the hypergraph.

In geometry, a unital is a set of n3 + 1 points arranged into subsets of size n + 1 so that every pair of distinct points of the set are contained in exactly one subset. n ≥ 3 is required by some authors to avoid small exceptional cases. This is equivalent to saying that a unital is a 2-(n3 + 1, n + 1, 1) block design. Some unitals may be embedded in a projective plane of order n2. In this case of embedded unitals, every line of the plane intersects the unital in either 1 or n + 1 points. In the Desarguesian planes, PG(2,q2), the classical examples of unitals are given by nondegenerate Hermitian curves. There are also many non-classical examples. The first and the only known unital with non prime power parameters, n=6, was constructed by Bhaskar Bagchi and Sunanda Bagchi. It is still unknown if this unital can be embedded in a projective plane of order 36, if such a plane exists.