In
geometryGeometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of premodern mathematics, the other being the study of numbers ....
, the
crossratio, also called
double ratio and
anharmonic ratio, is a special number associated with an ordered quadruple of collinear points, particularly points on a
projective lineIn mathematics, a projective line is a onedimensional projective space. The projective line over a field K, denoted P1, may be defined as the set of onedimensional subspaces of the twodimensional vector space K2 .For the generalisation to the projective line over an associative ring, see...
. Variants of this concept exist for a quadruple of concurrent lines on the projective plane and a quadruple of points on the
Riemann sphereIn mathematics, the Riemann sphere , named after the 19th century mathematician Bernhard Riemann, is the sphere obtained from the complex plane by adding a point at infinity...
.
The crossratio is preserved by the fractional linear transformations and
it is essentially the only projective
invariantIn mathematics, an invariant is a property of a class of mathematical objects that remains unchanged when transformations of a certain type are applied to the objects. The particular class of objects and type of transformations are usually indicated by the context in which the term is used...
of a quadruple of points, which underlies its importance for
projective geometryIn mathematics, projective geometry is the study of geometric properties that are invariant under projective transformations. This means that, compared to elementary geometry, projective geometry has a different setting, projective space, and a selective set of basic geometric concepts...
. In the Cayley–Klein model of
hyperbolic geometryIn mathematics, hyperbolic geometry is a nonEuclidean geometry, meaning that the parallel postulate of Euclidean geometry is replaced...
, the distance between points is expressed in terms of a certain crossratio.
Crossratio had been defined in deep antiquity, possibly already by
EuclidEuclid , fl. 300 BC, also known as Euclid of Alexandria, was a Greek mathematician, often referred to as the "Father of Geometry". He was active in Alexandria during the reign of Ptolemy I...
, and was considered by
PappusPappus of Alexandria was one of the last great Greek mathematicians of Antiquity, known for his Synagoge or Collection , and for Pappus's Theorem in projective geometry...
, who noted its key invariance property. It was extensively studied in the 19th century.
Definition
The crossratio of a 4tuple of distinct points on the
real lineIn mathematics, the real line, or real number line is the line whose points are the real numbers. That is, the real line is the set of all real numbers, viewed as a geometric space, namely the Euclidean space of dimension one...
with coordinates
z_{1},
z_{2},
z_{3},
z_{4} is given by
It can also be written as a "double ratio" of two division ratios of triples of points:
The same formulas can be applied to four different
complex numberA complex number is a number consisting of a real part and an imaginary part. Complex numbers extend the idea of the onedimensional number line to the twodimensional complex plane by using the number line for the real part and adding a vertical axis to plot the imaginary part...
s or, more generally, to elements of any
fieldIn abstract algebra, a field is a commutative ring whose nonzero elements form a group under multiplication. As such it is an algebraic structure with notions of addition, subtraction, multiplication, and division, satisfying certain axioms...
and can also be extended to the case when one of them is the symbol ∞, by removing the corresponding two differences from the formula.
The formula shows that crossratio is a
functionIn mathematics, a function associates one quantity, the argument of the function, also known as the input, with another quantity, the value of the function, also known as the output. A function assigns exactly one output to each input. The argument and the value may be real numbers, but they can...
of four points, generally four numbers
taken from a field. The generalization using elements of a mathematical
ringIn mathematics, a ring is an algebraic structure consisting of a set together with two binary operations usually called addition and multiplication, where the set is an abelian group under addition and a semigroup under multiplication such that multiplication distributes over addition...
requires methods of
inversive ring geometryIn mathematics, inversive ring geometry is the extension of the concepts of projective line, homogeneous coordinates, projective transformations, and crossratio to the context of associative rings, concepts usually built upon rings that happen to be fields....
.
In geometry, if A, B, C and D are collinear points, then the cross ratio is defined similarly as
where each of the distances is signed according to a fixed orientation of the line.
Terminology and history
Pappus of AlexandriaPappus of Alexandria was one of the last great Greek mathematicians of Antiquity, known for his Synagoge or Collection , and for Pappus's Theorem in projective geometry...
made implicit use of concepts equivalent to the crossratio in his
Collection: Book VII. Early users of Pappus included
Isaac NewtonSir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
,
Michel ChaslesMichel Floréal Chasles was a French mathematician.He was born at Épernon in France and studied at the École Polytechnique in Paris under Siméon Denis Poisson. In the War of the Sixth Coalition he was drafted to fight in the defence of Paris in 1814...
, and
Robert SimsonRobert Simson was a Scottish mathematician and professor of mathematics at the University of Glasgow. The pedal line of a triangle is sometimes called the "Simson line" after him.Life:...
. In 1986 Alexander Jones made a translation of the original by Pappus, then wrote a commentary on how the lemmas of Pappus relate to modern terminology.
Modern use of the cross ratio in projective geometry began with
Lazare CarnotLazare Nicolas Marguerite, Comte Carnot , the Organizer of Victory in the French Revolutionary Wars, was a French politician, engineer, and mathematician.Education and early life:...
in 1803 with his book
Géométrie de Position. The term used was
le rapport anharmonique (Fr: anharmonic ratio). German geometers call it
das Doppelverhältnis (Ger: double ratio). However, in 1847 Karl von Staudt introduced the term
Throw (
Wurf) to avoid the metrical implication of a ratio. His construction of the Algebra of Throws provides an approach to numerical propositions, usually taken as axioms, but proven in projective geometry.
The English term "crossratio" was introduced in 1878 by
William Kingdon CliffordWilliam Kingdon Clifford FRS was an English mathematician and philosopher. Building on the work of Hermann Grassmann, he introduced what is now termed geometric algebra, a special case of the Clifford algebra named in his honour, with interesting applications in contemporary mathematical physics...
.
Projective geometry
Crossratio is a
projective invariant in the sense that it is preserved by the projective transformations of a projective line. In particular, if four points lie on a straight line
L in
R^{2} then their crossratio is a welldefined quantity, because any choice of the origin and even of the scale on the line will yield the same value of the crossratio. Furthermore, let {
L_{i}, 1 ≤
i ≤ 4}, be four distinct lines in the plane passing through the same point
Q. Then any line
L not passing through
Q intersects these lines in four distinct points
P_{i} (if
L is
parallelParallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...
to
L_{i} then the corresponding intersection point is "at infinity"). It turns out that the crossratio of these points (taken in a fixed order) does not depend on the choice of a line
L, and hence it is an invariant of the 4tuple of lines {
L_{i}}. This can be understood as follows: if
L and
L′ are two lines not passing through
Q then the perspective transformation from
L to
L′ with the center
Q is a projective transformation that takes the quadruple {
P_{i}} of points on
L into the quadruple {
P_{i}′} of points on
L′. Therefore, the
invariance of the crossratio under projective automorphisms of the line implies (in fact, is equivalent to) the independence of the crossratio of the four collinear points {
P_{i}} on the lines {
L_{i}} from the choice of the line that contains them.
Definition in homogeneous coordinates
If the four points are represented in homogeneous coordinates by vectors a,b,c,d such that c=a+b and d=ka+b, then their crossratio is k.
Role in nonEuclidean geometry
Arthur CayleyArthur Cayley F.R.S. was a British mathematician. He helped found the modern British school of pure mathematics....
and
Felix KleinChristian Felix Klein was a German mathematician, known for his work in group theory, function theory, nonEuclidean geometry, and on the connections between geometry and group theory...
found an application of the crossratio to
nonEuclidean geometryNonEuclidean geometry is the term used to refer to two specific geometries which are, loosely speaking, obtained by negating the Euclidean parallel postulate, namely hyperbolic and elliptic geometry. This is one term which, for historical reasons, has a meaning in mathematics which is much...
. Given a nonsingular
conicIn mathematics, a conic section is a curve obtained by intersecting a cone with a plane. In analytic geometry, a conic may be defined as a plane algebraic curve of degree 2...
C in the real
projective planeIn 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 that do not intersect...
, its stabilizer
G_{C} in the projective group
actsIn algebra and geometry, a group action is a way of describing symmetries of objects using groups. The essential elements of the object are described by a set, and the symmetries of the object are described by the symmetry group of this set, which consists of bijective transformations of the set...
transitively on the points in the interior of
C. However, there is an invariant for the action of
G_{C} on
pairs of points. In fact, every such invariant is expressible as a function of the appropriate cross ratio.
Explicitly, let the conic be the
unit circleIn mathematics, a unit circle is a circle with a radius of one. Frequently, especially in trigonometry, "the" unit circle is the circle of radius one centered at the origin in the Cartesian coordinate system in the Euclidean plane...
. For any two points in the unit disk,
p,
q, the line connecting them intersects the circle in two points,
a and
b. The points are, in order, . Then the distance between
p and
q in the Cayley–Klein model of the plane
hyperbolic geometryIn mathematics, hyperbolic geometry is a nonEuclidean geometry, meaning that the parallel postulate of Euclidean geometry is replaced...
can be expressed as
(the factor one half is needed to make the
curvatureIn differential geometry, the Gaussian curvature or Gauss curvature of a point on a surface is the product of the principal curvatures, κ1 and κ2, of the given point. It is an intrinsic measure of curvature, i.e., its value depends only on how distances are measured on the surface, not on the way...
−1). Since the crossratio is invariant under projective transformations, it follows that the hyperbolic distance is invariant under the projective transformations that preserve the conic
C. Conversely, the group
G acts transitively on the set of pairs of points (
p,
q) in the unit disk at a fixed hyperbolic distance.
Six crossratios
There is a number of definitions of the crossratio. However, they all differ from each other by a suitable
permutationIn mathematics, the notion of permutation is used with several slightly different meanings, all related to the act of permuting objects or values. Informally, a permutation of a set of objects is an arrangement of those objects into a particular order...
of the coordinates. In general, there are six possible different values the crossratio
can take depending on the order in which the points
z_{i} are given.
Action of symmetric group
Since there are 24 possible permutations of the four coordinates, some permutations must leave the crossratio unaltered. In fact, exchanging any two pairs of coordinates preserves the crossratio:
Using these symmetries, there can then be 6 possible values of the crossratio, depending on the order in which the points are given. These are:
Six crossratios as Möbius transformations
Viewed as
Möbius transformations, the six crossratios listed above represent torsion elements of
PGLIn mathematics, especially in the group theoretic area of algebra, the projective linear group is the induced action of the general linear group of a vector space V on the associated projective space P...
(2,Z). Namely,
,
, and
are of order 2 in PGL(2,Z), with fixed points, respectively, −1, 1/2, and 2 (namely, the orbit of the harmonic crossratio). Meanwhile, elements
and
are of order 3 in PGL(2,Z) – in fact in PSL(2,Z). Each of them fixes both values
of the "most symmetric" crossratio.
Role of Klein fourgroup
In the language of
group theoryIn mathematics and abstract algebra, group theory studies the algebraic structures known as groups.The concept of a group is central to abstract algebra: other wellknown algebraic structures, such as rings, fields, and vector spaces can all be seen as groups endowed with additional operations and...
, the
symmetric groupIn mathematics, the symmetric group Sn on a finite set of n symbols is the group whose elements are all the permutations of the n symbols, and whose group operation is the composition of such permutations, which are treated as bijective functions from the set of symbols to itself...
S_{4} acts on the crossratio by permuting coordinates. The kernel of this action is isomorphic to the
Klein fourgroupIn mathematics, the Klein fourgroup is the group Z2 × Z2, the direct product of two copies of the cyclic group of order 2...
K. This group consists of 2cycle permutations of type
(in addition to the identity), which preserve the crossratio. The effective symmetry group is then the
quotient groupIn mathematics, specifically group theory, a quotient group is a group obtained by identifying together elements of a larger group using an equivalence relation...
, which is isomorphic to
S_{3}.
Exceptional orbits
For certain values of λ there will be an enhanced symmetry and therefore fewer than six possible values for the crossratio. These values of λ correspond to
fixed pointsIn mathematics, a fixed point of a function is a point that is mapped to itself by the function. A set of fixed points is sometimes called a fixed set...
of the action of
S_{3} on the Riemann sphere (given by the above six functions); or, equivalently, those points with a nontrivial stabilizer in this permutation group.
The first set of fixed points is {0, 1, ∞}. However, the crossratio can never take on these values if the points {
z_{i}} are all distinct. These values are limit values as one pair of coordinates approach each other:
The second set of fixed points is {−1, 1/2, 2}. This situation is what is classically called the
harmonic crossratio, and arises in
projective harmonic conjugatesIn projective geometry, the harmonic conjugate point of a triple of points on the real projective line is defined by the following construction due to Karl von Staudt:...
. In the real case, there are no other exceptional orbits.
The most symmetric crossratio occurs when
. These are then the only two possible values of the crossratio.
Transformational approach
The crossratio is invariant under the projective transformations of the line. In the case of a
complexA complex number is a number consisting of a real part and an imaginary part. Complex numbers extend the idea of the onedimensional number line to the twodimensional complex plane by using the number line for the real part and adding a vertical axis to plot the imaginary part...
projective line, or the
Riemann sphereIn mathematics, the Riemann sphere , named after the 19th century mathematician Bernhard Riemann, is the sphere obtained from the complex plane by adding a point at infinity...
, these transformation are known as
Möbius transformations. A general Möbius transformation has the form
These transformations form a
groupIn mathematics, a group is an algebraic structure consisting of a set together with an operation that combines any two of its elements to form a third element. To qualify as a group, the set and the operation must satisfy a few conditions called group axioms, namely closure, associativity, identity...
actingIn algebra and geometry, a group action is a way of describing symmetries of objects using groups. The essential elements of the object are described by a set, and the symmetries of the object are described by the symmetry group of this set, which consists of bijective transformations of the set...
on the
Riemann sphereIn mathematics, the Riemann sphere , named after the 19th century mathematician Bernhard Riemann, is the sphere obtained from the complex plane by adding a point at infinity...
, the Möbius group.
The projective invariance of the crossratio means that
The crossratio is
realIn mathematics, a real number is a value that represents a quantity along a continuum, such as 5 , 4/3 , 8.6 , √2 and π...
if and only if the four points are either collinear or concyclic, reflecting the fact that every Möbius transformation maps generalized circles to generalized circles.
The action of the Möbius group is simply transitive on the set of triples of distinct points of the Riemann sphere: given any ordered triple of distinct points, (
z_{2},
z_{3},
z_{4}), there is a unique Möbius transformation
f(
z) that maps it to the triple (1,0,∞). This transformation can be conveniently described using the crossratio: since (
z,
z_{2},
z_{3},
z_{4}) must equal (
f(
z),1;0,∞) which in turn equals
f(
z), we obtain
An alternative explanation for the invariance of the crossratio is based on the fact that the group of projective transformations of a line is generated by the translations, the homotheties, and the multiplicative inversion. The differences
z_{j} 
z_{k} are invariant under the translations

where
a is a
constantIn mathematics, a constant is a nonvarying value, i.e. completely fixed or fixed in the context of use. The term usually occurs in opposition to variable In mathematics, a constant is a nonvarying value, i.e. completely fixed or fixed in the context of use. The term usually occurs in opposition...
in the ground field
F. Furthermore, the division ratios are invariant under a homothety
for a nonzero constant
b in
F. Therefore, the crossratio is invariant under the
affine transformationIn geometry, an affine transformation or affine map or an affinity is a transformation which preserves straight lines. It is the most general class of transformations with this property...
s.
In order to obtain a welldefined
inversion mappingIn mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...
the affine line needs to be augmented by the
point at infinity, denoted ∞, forming the projective line
P^{1}(
F). Each affine mapping
f:
F →
F can be uniquely extended to a mapping of
P^{1}(
F) into itself that fixes the point at infinity. The map
T swaps 0 and ∞. The projective group is
generated byIn abstract algebra, a generating set of a group is a subset that is not contained in any proper subgroup of the group. Equivalently, a generating set of a group is a subset such that every element of the group can be expressed as the combination of finitely many elements of the subset and their...
T and the affine mappings extended to
P^{1}(
F). In the case
F =
C, the
complex planeIn mathematics, the complex plane or zplane is a geometric representation of the complex numbers established by the real axis and the orthogonal imaginary axis...
, this results in the Möbius group. Since the crossratio is also invariant under
T, it is invariant under any projective mapping of
P^{1}(
F) into itself.
Differentialgeometric point of view
The theory takes on a differential calculus aspect as the four points are brought into proximity. This leads to the theory of the
Schwarzian derivativeIn mathematics, the Schwarzian derivative, named after the German mathematician Hermann Schwarz, is a certain operator that is invariant under all linear fractional transformations. Thus, it occurs in the theory of the complex projective line, and in particular, in the theory of modular forms and...
, and more generally of
projective connectionIn differential geometry, a projective connection is a type of Cartan connection on a differentiable manifold.The structure of a projective connection is modeled on the geometry of projective space, rather than the affine space corresponding to an affine connection. Much like affine connections,...
s.
Higherdimensional generalizations
The crossratio does not generalize in a simple manner to higher dimensions, due to other geometric properties of configurations of points, notably collinearity –
configuration space Configuration space in physics :In classical mechanics, the configuration space is the space of possible positions that a physical system may attain, possibly subject to external constraints...
s are more complicated, and distinct
ktuples of points are not in
general positionIn algebraic geometry, general position is a notion of genericity for a set of points, or other geometric objects. It means the general case situation, as opposed to some more special or coincidental cases that are possible...
.
While the projective linear group of the plane is 3transitive (any three distinct points can be mapped to any other three points), and indeed simply 3transitive (there is a
unique projective map taking any triple to another triple), with the cross ratio thus being the unique projective invariant of a set of four points, there are basic geometric invariants in higher dimension. The projective linear group of
nspace
has (
n + 1)
^{2} − 1 dimensions (because it is
projectivization removing one dimension), but in other dimensions the projective linear group is only 2transitive – because three collinear points must be mapped to three collinear points (which is not a restriction in the projective line) – and thus there is not a "generalized cross ratio" providing the unique invariant of
n^{2} points.
Collinearity is not the only geometric property of configurations of points that must be maintained – for example,
five points determine a conicIn geometry, just as two points determine a line , five points determine a conic . There are additional subtleties for conics that do not exist for lines, and thus the statement and its proof for conics are both more technical than for lines.Formally, given any five points in the plane in general...
, but six general points do not lie on a conic, so whether any 6tuple of points lies on a conic is also a projective invariant. One can study orbits of points in
general positionIn algebraic geometry, general position is a notion of genericity for a set of points, or other geometric objects. It means the general case situation, as opposed to some more special or coincidental cases that are possible...
– in the line "general position" is equivalent to being distinct, while in higher dimensions it requires geometric considerations, as discussed – but, as the above indicates, this is more complicated and less informative.
External links