In

mathematicsMathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...

,

**complex projective space** is the

projective spaceIn mathematics a projective space is a set of elements similar to the set P of lines through the origin of a vector space V. The cases when V=R2 or V=R3 are the projective line and the projective plane, respectively....

with respect to the field of

complex numberA complex number is a number consisting of a real part and an imaginary part. Complex numbers extend the idea of the one-dimensional number line to the two-dimensional complex plane by using the number line for the real part and adding a vertical axis to plot the imaginary part...

s. By analogy, whereas the points of a

real projective spaceIn mathematics, real projective space, or RPn, is the topological space of lines through 0 in Rn+1. It is a compact, smooth manifold of dimension n, and a special case of a Grassmannian.-Construction:...

label the lines through the origin of a real Euclidean space, the points of a complex projective space label the

*complex*In mathematics, the complex plane or z-plane is a geometric representation of the complex numbers established by the real axis and the orthogonal imaginary axis...

lines through the origin of a complex Euclidean space (see below for an intuitive account). Formally, a complex projective space is the space of complex lines through the origin of an (

*n*+1)-dimensional complex

vector spaceA vector space is a mathematical structure formed by a collection of vectors: objects that may be added together and multiplied by numbers, called scalars in this context. Scalars are often taken to be real numbers, but one may also consider vector spaces with scalar multiplication by complex...

. The space is denoted variously as

**P**(

**C**^{n+1}),

**P**_{n}(

**C**) or

**CP**^{n}. When , the complex projective space

**CP**^{1} is 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...

, and when ,

**CP**^{2} is the

complex projective plane (see there for a more elementary discussion).

Complex projective space was first introduced by as an instance of what was then known as the "geometry of position", a notion originally due to

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:...

, a kind of

synthetic geometrySynthetic or axiomatic geometry is the branch of geometry which makes use of axioms, theorems and logical arguments to draw conclusions, as opposed to analytic and algebraic geometries which use analysis and algebra to perform geometric computations and solve problems.-Logical synthesis:The process...

that included other projective geometries as well. Subsequently, near the turn of the 20th century it became clear to the

Italian school of algebraic geometryIn relation with the history of mathematics, the Italian school of algebraic geometry refers to the work over half a century or more done internationally in birational geometry, particularly on algebraic surfaces. There were in the region of 30 to 40 leading mathematicians who made major...

that the complex projective spaces were the most natural domains in which to consider the solutions of

polynomialIn mathematics, a polynomial is an expression of finite length constructed from variables and constants, using only the operations of addition, subtraction, multiplication, and non-negative integer exponents...

equations—

algebraic varietiesIn mathematics, an algebraic variety is the set of solutions of a system of polynomial equations. Algebraic varieties are one of the central objects of study in algebraic geometry...

. In modern times, both the

topologyTopology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...

and geometry of complex projective space are well-understood and closely related to that of the sphere. Indeed, in a certain sense the (2

*n*+1)-sphere can be regarded as a family of circles parametrized by

**CP**^{n}: this is the Hopf fibration. Complex projective space carries a (Kähler)

metricIn the mathematical field of differential geometry, a metric tensor is a type of function defined on a manifold which takes as input a pair of tangent vectors v and w and produces a real number g in a way that generalizes many of the familiar properties of the dot product of vectors in Euclidean...

, called the Fubini–Study metric, in terms of which it is a

Hermitian symmetric spaceIn mathematics, a Hermitian symmetric space is a Kähler manifold M which, as a Riemannian manifold, is a Riemannian symmetric space. Equivalently, M is a Riemannian symmetric space with a parallel complex structure with respect to which the Riemannian metric is Hermitian...

of rank 1.

Complex projective space has many applications in both mathematics and quantum physics. In

algebraic geometryAlgebraic geometry is a branch of mathematics which combines techniques of abstract algebra, especially commutative algebra, with the language and the problems of geometry. It occupies a central place in modern mathematics and has multiple conceptual connections with such diverse fields as complex...

, complex projective space is the home of projective varieties, a well-behaved class of

algebraic varietiesIn mathematics, an algebraic variety is the set of solutions of a system of polynomial equations. Algebraic varieties are one of the central objects of study in algebraic geometry...

. In topology, the complex projective space plays an important role as a

classifying spaceIn mathematics, specifically in homotopy theory, a classifying space BG of a topological group G is the quotient of a weakly contractible space EG by a free action of G...

for complex

line bundleIn mathematics, a line bundle expresses the concept of a line that varies from point to point of a space. For example a curve in the plane having a tangent line at each point determines a varying line: the tangent bundle is a way of organising these...

s: families of complex lines parametrized by another space. In this context, the infinite union of projective spaces (

direct limitIn mathematics, a direct limit is a colimit of a "directed family of objects". We will first give the definition for algebraic structures like groups and modules, and then the general definition which can be used in any category.- Algebraic objects :In this section objects are understood to be...

), denoted

**CP**^{∞}, is the classifying space K(Z,2). In quantum physics, the wave function associated to a pure states of a quantum mechanical system is a

probability amplitudeIn quantum mechanics, a probability amplitude is a complex number whose modulus squared represents a probability or probability density.For example, if the probability amplitude of a quantum state is \alpha, the probability of measuring that state is |\alpha|^2...

, meaning that it has unit norm, and has an inessential overall phase: that is, the wave function of a pure state is naturally a point in the

projective Hilbert spaceIn mathematics and the foundations of quantum mechanics, the projective Hilbert space P of a complex Hilbert space H is the set of equivalence classes of vectors v in H, with v ≠ 0, for the relation given by...

of the state space.

## Introduction

The notion of a projective plane arises out of the idea of perspective in geometry and art: that it is sometimes useful to include in the Euclidean plane an additional "imaginary" line that represents the horizon that an artist painting the plane might see. Following each direction from the origin, there is a different point on the horizon, so the horizon can be thought of as the set of all directions from the origin. The Euclidean plane, together with its horizon, is called the

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...

, and the horizon is sometimes called a line at infinity. By the same construction, projective spaces can be considered in higher dimensions. For instance, the projective 3-space is a Euclidean space together with a plane at infinity that represents the horizon that an artist (who must, necessarily, live in 4-dimensions) would see.

These are, properly speaking, the

real projective spaceIn mathematics, real projective space, or RPn, is the topological space of lines through 0 in Rn+1. It is a compact, smooth manifold of dimension n, and a special case of a Grassmannian.-Construction:...

s. They can be constructed in a slightly more rigorous way as follows. Here, let

**R**^{n+1} denote the real coordinate space of

*n*+1 dimensions, and regard the landscape to be painted as a

hyperplaneA hyperplane is a concept in geometry. It is a generalization of the plane into a different number of dimensions.A hyperplane of an n-dimensional space is a flat subset with dimension n − 1...

in this space. Suppose that the eye of the artist is the origin in

**R**^{n+1}. Then along each line through his eye, there is a point of the landscape or a point on its horizon. Thus the real projective space is the space of lines through the origin in

**R**^{n+1}. Without reference to coordinates, this is the space of lines through the origin in an (

*n*+1)-dimensional real

vector spaceA vector space is a mathematical structure formed by a collection of vectors: objects that may be added together and multiplied by numbers, called scalars in this context. Scalars are often taken to be real numbers, but one may also consider vector spaces with scalar multiplication by complex...

.

To describe the complex projective space in an analogous manner requires a generalization of the idea of vector, line, and direction. Imagine that instead of standing in a real Euclidean space, the artist is standing in a complex Euclidean space

**C**^{n+1} (which has real dimension 2

*n*+2) and the landscape is a

*complex* hyperplane (of real dimension 2

*n*). Unlike the case of real Euclidean space, in the complex case there are directions in which the artist can look which do not see the landscape (because it does not have high enough dimension). However, in a complex space, there is an additional "phase" associated with the directions through a point, and by adjusting this phase the artist can guarantee that he typically sees the landscape. The "horizon" is then the space of directions, but such that two directions are regarded as "the same" if they differ only by a phase. The complex projective space is then the landscape (

**C**^{n}) with the horizon attached "at infinity". Just like the real case, the complex projective space is the space of directions through the origin of

**C**^{n+1}, where two directions are regarded as the same if they differ by a phase.

## Construction

Complex projective space is a

complex manifoldIn differential geometry, a complex manifold is a manifold with an atlas of charts to the open unit disk in Cn, such that the transition maps are holomorphic....

that may be described by

*n* + 1 complex coordinates as

where the tuples differing by an overall rescaling are identified:

That is, these are

homogeneous coordinatesIn mathematics, homogeneous coordinates, introduced by August Ferdinand Möbius in his 1827 work Der barycentrische Calcül, are a system of coordinates used in projective geometry much as Cartesian coordinates are used in Euclidean geometry. They have the advantage that the coordinates of points,...

in the traditional sense of

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...

. The point set

**CP**^{n} is covered by the patches

. In

*U*_{i}, one can define a coordinate system by

The coordinate transitions between two different such charts

*U*_{i} and

*U*_{j} are

holomorphic functionIn mathematics, holomorphic functions are the central objects of study in complex analysis. A holomorphic function is a complex-valued function of one or more complex variables that is complex differentiable in a neighborhood of every point in its domain...

s (in fact they are fractional linear transformations). Thus

**CP**^{n} carries the structure of a

complex manifoldIn differential geometry, a complex manifold is a manifold with an atlas of charts to the open unit disk in Cn, such that the transition maps are holomorphic....

of complex dimension

*n*, and

*a fortiori* the structure of a real

differentiable manifoldA differentiable manifold is a type of manifold that is locally similar enough to a linear space to allow one to do calculus. Any manifold can be described by a collection of charts, also known as an atlas. One may then apply ideas from calculus while working within the individual charts, since...

of real dimension 2

*n*.

One may also regard

**CP**^{n} as a

quotientIn topology and related areas of mathematics, a quotient space is, intuitively speaking, the result of identifying or "gluing together" certain points of a given space. The points to be identified are specified by an equivalence relation...

of the unit 2

*n* + 1

sphereA sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

in

**C**^{n+1} under the action of

U(1)In mathematics, the unitary group of degree n, denoted U, is the group of n×n unitary matrices, with the group operation that of matrix multiplication. The unitary group is a subgroup of the general linear group GL...

:

**CP**^{n} = *S*^{2n+1}/U(1).

This is because every line in

**C**^{n+1} intersects the unit sphere in a

circleA circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

. By first projecting to the unit sphere and then identifying under the natural action of U(1) one obtains

**CP**^{n}. For

*n* = 1 this construction yields the classical

Hopf bundleIn the mathematical field of topology, the Hopf fibration describes a 3-sphere in terms of circles and an ordinary sphere. Discovered by Heinz Hopf in 1931, it is an influential early example of a fiber bundle...

. From this perspective, the differentiable structure on

**CP**^{n} is induced from that of

*S*^{2n+1}, being the quotient of the latter by a compact group that acts properly.

## Topology

The topology of

**CP**^{n} is determined inductively by the following

cell decompositionIn topology, a CW complex is a type of topological space introduced by J. H. C. Whitehead to meet the needs of homotopy theory. This class of spaces is broader and has some better categorical properties than simplicial complexes, but still retains a combinatorial naturethat allows for...

. Let

*H* be a fixed hyperplane through the origin in

**C**^{n+1}. Under the projection map ,

*H* goes into a subspace that is homeomorphic to

**CP**^{n−1}. The complement of

*H* in

**CP**^{n} is homeomorphic to

**C**^{n}. Thus

**CP**^{n} arises by attaching a 2

*n*-cell to

**CP**^{n−1}:

Alternatively, if the 2

*n*-cell is regarded instead as the open unit ball in

**C**^{n}, then the attaching map is the Hopf fibration of the boundary. An analogous inductive cell decomposition is true for all of the projective spaces; see .

### Point-set topology

Complex projective space is

compactIn mathematics, specifically general topology and metric topology, a compact space is an abstract mathematical space whose topology has the compactness property, which has many important implications not valid in general spaces...

and

connectedIn topology and related branches of mathematics, a connected space is a topological space that cannot be represented as the union of two or more disjoint nonempty open subsets. Connectedness is one of the principal topological properties that is used to distinguish topological spaces...

, being a quotient of a compact, connected space.

### Homotopy groups

From the fiber bundle

or more suggestively

**CP**^{n} is simply connected. Moreover, by the long exact homotopy sequence, the second homotopy group is , and all the higher homotopy groups agree with those of

*S*^{2n+1}: for all

*k* > 2.

### Homology

In general, the

algebraic topologyAlgebraic topology is a branch of mathematics which uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify up to homotopy equivalence.Although algebraic topology...

of

**CP**^{n} is based on the rank of the homology groups being zero in odd dimensions; also

*H*_{2i}(

**CP**^{n},

**Z**) is infinite cyclic for

*i* = 0 to

*n*. Therefore the

Betti numberIn algebraic topology, a mathematical discipline, the Betti numbers can be used to distinguish topological spaces. Intuitively, the first Betti number of a space counts the maximum number of cuts that can be made without dividing the space into two pieces....

s run

- 1, 0, 1, 0, ..., 0, 1, 0, 0, 0, ...

That is, 0 in odd dimensions, 1 in even dimensions up to 2n. The

Euler characteristicIn mathematics, and more specifically in algebraic topology and polyhedral combinatorics, the Euler characteristic is a topological invariant, a number that describes a topological space's shape or structure regardless of the way it is bent...

of

**CP**^{n} is therefore

*n* + 1. By

Poincaré dualityIn mathematics, the Poincaré duality theorem named after Henri Poincaré, is a basic result on the structure of the homology and cohomology groups of manifolds...

the same is true for the ranks of the cohomology groups. In the case of cohomology, one can go further, and identify the graded ring structure, for

cup productIn mathematics, specifically in algebraic topology, the cup product is a method of adjoining two cocycles of degree p and q to form a composite cocycle of degree p + q. This defines an associative graded commutative product operation in cohomology, turning the cohomology of a space X into a...

; the generator of

*H*^{2}(

**CP**^{n},

**Z**) is the class associated to a

hyperplaneA hyperplane is a concept in geometry. It is a generalization of the plane into a different number of dimensions.A hyperplane of an n-dimensional space is a flat subset with dimension n − 1...

, and this is a ring generator, so that the ring is isomorphic with

**Z**[*T*]/(*T*^{n+1}),

with

*T* a degree two generator. This implies also that the Hodge number

*h*^{i,i} = 1, and all the others are zero. See .

*K*-theory

It follows from induction and Bott periodicity that

The

tangent bundleIn differential geometry, the tangent bundle of a differentiable manifold M is the disjoint unionThe disjoint union assures that for any two points x1 and x2 of manifold M the tangent spaces T1 and T2 have no common vector...

satisfies

where

denotes the trivial line bundle. From this, the

Chern classIn mathematics, in particular in algebraic topology and differential geometry, the Chern classes are characteristic classes associated to complex vector bundles.Chern classes were introduced by .-Basic idea and motivation:...

es and characteristic numbers can be calculated.

### Classifying space

There is a space

**CP**^{∞} which, in a sense, is the limit of

**CP**^{n} as

*n* → ∞. It is BU(1), the

classifying spaceIn mathematics, specifically in homotopy theory, a classifying space BG of a topological group G is the quotient of a weakly contractible space EG by a free action of G...

of U(1), in the sense of homotopy theory, and so classifies complex

line bundleIn mathematics, a line bundle expresses the concept of a line that varies from point to point of a space. For example a curve in the plane having a tangent line at each point determines a varying line: the tangent bundle is a way of organising these...

s; equivalently it accounts for the first

Chern classIn mathematics, in particular in algebraic topology and differential geometry, the Chern classes are characteristic classes associated to complex vector bundles.Chern classes were introduced by .-Basic idea and motivation:...

. See, for instance, and . The space

**CP**^{∞} is also the same as the infinite-dimensional

projective unitary groupIn mathematics, the projective unitary group PU is the quotient of the unitary group U by the right multiplication of its center, U, embedded as scalars....

; see that article for additional properties and discussion.

## Geometry

The natural metric on

**CP**^{n} is the

Fubini-Study metricIn mathematics, the Fubini–Study metric is a Kähler metric on projective Hilbert space, that is, complex projective space CPn endowed with a Hermitian form. This metric was originally described in 1904 and 1905 by Guido Fubini and Eduard Study....

, and its isometry group is the

projective unitary groupIn mathematics, the projective unitary group PU is the quotient of the unitary group U by the right multiplication of its center, U, embedded as scalars....

PU(

*n*+1), where the stabilizer of a point is

It is a

Hermitian symmetric spaceIn mathematics, a Hermitian symmetric space is a Kähler manifold M which, as a Riemannian manifold, is a Riemannian symmetric space. Equivalently, M is a Riemannian symmetric space with a parallel complex structure with respect to which the Riemannian metric is Hermitian...

, represented as a coset space

The geodesic symmetry at a point

*p* is the unitary transformation that fixes

*p* and is the negative identity on the orthogonal complement of the line represented by

*p*.

### Geodesics

Through any two points

*p*,

*q* in complex projective space, there passes a unique

*complex* line (a

**CP**^{1}). A

great circleA great circle, also known as a Riemannian circle, of a sphere is the intersection of the sphere and a plane which passes through the center point of the sphere, as opposed to a general circle of a sphere where the plane is not required to pass through the center...

of this complex line that contains

*p* and

*q* is a

geodesicIn mathematics, a geodesic is a generalization of the notion of a "straight line" to "curved spaces". In the presence of a Riemannian metric, geodesics are defined to be the shortest path between points in the space...

for the Fubini–Study metric. In particular, all of the geodesics are closed (they are circles), and all have equal length. (This is always true of Riemannian globally symmetric spaces of rank 1.)

The cut locus of any point

*p* is equal to a hyperplane

**CP**^{n−1}. This is also the set of fixed points of the geodesic symmetry at

*p* (less

*p* itself). See .

### Sectional curvature pinching

It has

sectional curvatureIn Riemannian geometry, the sectional curvature is one of the ways to describe the curvature of Riemannian manifolds. The sectional curvature K depends on a two-dimensional plane σp in the tangent space at p...

ranging from 1/4 to 1, and is the roundest manifold that is not a sphere (or covered by a sphere): by the 1/4-pinched sphere theorem, any complete, simply connected Riemannian manifold with curvature strictly between 1/4 and 1 is homeomorphic to the sphere. Complex projective space shows that 1/4 is sharp. Conversely, any complete simply connected δ-pinched Riemannian manifold is either homeomorphic to the sphere, or diffeomorphic to the complex projective space, the

quaternionic projective spaceIn mathematics, quaternionic projective space is an extension of the ideas of real projective space and complex projective space, to the case where coordinates lie in the ring of quaternions H. Quaternionic projective space of dimension n is usually denoted byand is a closed manifold of dimension 4n...

, or else the

Cayley planeIn mathematics, the Cayley plane OP2 is a projective plane over the octonions. It was discovered in 1933 by Ruth Moufang, and is named after Arthur Cayley ....

F

_{4}/Spin(9); see .

## Algebraic geometry

Complex projective space is a special case of a

GrassmannianIn mathematics, a Grassmannian is a space which parameterizes all linear subspaces of a vector space V of a given dimension. For example, the Grassmannian Gr is the space of lines through the origin in V, so it is the same as the projective space P. The Grassmanians are compact, topological...

, and is a

homogeneous spaceIn mathematics, particularly in the theories of Lie groups, algebraic groups and topological groups, a homogeneous space for a group G is a non-empty manifold or topological space X on which G acts continuously by symmetry in a transitive way. A special case of this is when the topological group,...

for various

Lie groupIn mathematics, a Lie group is a group which is also a differentiable manifold, with the property that the group operations are compatible with the smooth structure...

s. It is a

Kähler manifoldIn mathematics, a Kähler manifold is a manifold with unitary structure satisfying an integrability condition.In particular, it is a Riemannian manifold, a complex manifold, and a symplectic manifold, with these three structures all mutually compatible.This threefold structure corresponds to the...

carrying the

Fubini-Study metricIn mathematics, the Fubini–Study metric is a Kähler metric on projective Hilbert space, that is, complex projective space CPn endowed with a Hermitian form. This metric was originally described in 1904 and 1905 by Guido Fubini and Eduard Study....

, which is essentially determined by symmetry properties. It also plays a central role in

algebraic geometryAlgebraic geometry is a branch of mathematics which combines techniques of abstract algebra, especially commutative algebra, with the language and the problems of geometry. It occupies a central place in modern mathematics and has multiple conceptual connections with such diverse fields as complex...

; by Chow's theorem, any compact complex submanifold of

**CP**^{n} is the zero locus of a finite number of polynomials, and is thus a projective

algebraic varietyIn mathematics, an algebraic variety is the set of solutions of a system of polynomial equations. Algebraic varieties are one of the central objects of study in algebraic geometry...

. See

### Zariski topology

In

algebraic geometryAlgebraic geometry is a branch of mathematics which combines techniques of abstract algebra, especially commutative algebra, with the language and the problems of geometry. It occupies a central place in modern mathematics and has multiple conceptual connections with such diverse fields as complex...

, complex projective space can be equipped with another topology known as the

Zariski topologyIn algebraic geometry, the Zariski topology is a particular topology chosen for algebraic varieties that reflects the algebraic nature of their definition. It is due to Oscar Zariski and took a place of particular importance in the field around 1950...

. Let denote the

commutative ringIn ring theory, a branch of abstract algebra, a commutative ring is a ring in which the multiplication operation is commutative. The study of commutative rings is called commutative algebra....

of polynomials in the (

*n*+1) variables

*Z*_{0},...,

*Z*_{n}. This ring is graded by the total degree of each polynomial:

Define a subset of

**CP**^{n} to be

*closed* if it is the simultaneous solution set of a collection of homogeneous polynomials. Declaring the complements of the closed sets to be open, this defines a topology (the Zariski topology) on

**CP**^{n}.

### Structure as a scheme

Another construction of

**CP**^{n} (and its Zariski topology) is possible. Let

*S*_{+} ⊂

*S* be the

idealIn ring theory, a branch of abstract algebra, an ideal is a special subset of a ring. The ideal concept allows the generalization in an appropriate way of some important properties of integers like "even number" or "multiple of 3"....

spanned by the homogeneous polynomials of positive degree:

Define Proj

*S* to be the set of all homogeneous

prime idealIn algebra , a prime ideal is a subset of a ring which shares many important properties of a prime number in the ring of integers...

s in

*S* that do not contain

*S*_{+}. Call a closed of Proj

*S* open if it has the form

for some ideal

*I* in

*S*. The complements of these closed sets define a topology on Proj

*S*. The ring

*S*, by

localization at a prime idealIn abstract algebra, localization is a systematic method of adding multiplicative inverses to a ring. Given a ring R and a subset S, one wants to construct some ring R* and ring homomorphism from R to R*, such that the image of S consists of units in R*...

, determines a

sheafIn mathematics, a sheaf is a tool for systematically tracking locally defined data attached to the open sets of a topological space. The data can be restricted to smaller open sets, and the data assigned to an open set is equivalent to all collections of compatible data assigned to collections of...

of

local ringIn abstract algebra, more particularly in ring theory, local rings are certain rings that are comparatively simple, and serve to describe what is called "local behaviour", in the sense of functions defined on varieties or manifolds, or of algebraic number fields examined at a particular place, or...

s on Proj

*S*. The space Proj

*S*, together with its topology and sheaf of local rings, is a

schemeIn mathematics, a scheme is an important concept connecting the fields of algebraic geometry, commutative algebra and number theory. Schemes were introduced by Alexander Grothendieck so as to broaden the notion of algebraic variety; some consider schemes to be the basic object of study of modern...

. The subset of closed points of Proj

*S* is homeomorphic to

**CP**^{n} with its Zariski topology. Local sections of the sheaf are identified with the

rational functionIn mathematics, a rational function is any function which can be written as the ratio of two polynomial functions. Neither the coefficients of the polynomials nor the values taken by the function are necessarily rational.-Definitions:...

s of total degree zero on

**CP**^{n}.

### Line bundles

All line bundles on complex projective space can be obtained by the following construction. A function is called

homogeneousIn mathematics, a homogeneous function is a function with multiplicative scaling behaviour: if the argument is multiplied by a factor, then the result is multiplied by some power of this factor. More precisely, if is a function between two vector spaces over a field F, and k is an integer, then...

of degree

*k* if

for all } and }. More generally, this definition makes sense in

conesIn linear algebra, a cone is a subset of a vector space that is closed under multiplication by positive scalars. In other words, a subset C of a real vector space V is a cone if and only if λx belongs to C for any x in C and any positive scalar λ of V .A cone is said...

in }. A set } is called a cone if, whenever , then for all }; that is, a subset is a cone if it contains the complex line through each of its points. If is an open set (in either the analytic topology or the

Zariski topologyIn algebraic geometry, the Zariski topology is a particular topology chosen for algebraic varieties that reflects the algebraic nature of their definition. It is due to Oscar Zariski and took a place of particular importance in the field around 1950...

), let } be the cone over

*U*: the preimage of

*U* under the projection . Finally, for each integer

*k*, let

*O*(

*k*)(

*U*) be the set of functions that are homogeneous of degree

*k* in

*V*. This defines a

sheafIn mathematics, a sheaf is a tool for systematically tracking locally defined data attached to the open sets of a topological space. The data can be restricted to smaller open sets, and the data assigned to an open set is equivalent to all collections of compatible data assigned to collections of...

of sections of a certain line bundle, denoted by

*O*(

*k*).

In the special case , the bundle

*O*(−1) is called the tautological line bundle. It is equivalently defined as the subbundle of the product

whose fiber over is the set

These line bundles can also be described in the language of

divisorsIn algebraic geometry, divisors are a generalization of codimension one subvarieties of algebraic varieties; two different generalizations are in common use, Cartier divisors and Weil divisors...

. Let

*H* =

**CP**^{n−1} be a given complex hyperplane in

**CP**^{n}. The space of

meromorphic functionIn complex analysis, a meromorphic function on an open subset D of the complex plane is a function that is holomorphic on all D except a set of isolated points, which are poles for the function...

s on

**CP**^{n−1} with at most a simple pole along

*H* (and nowhere else) is a one-dimensional space, denoted by

*O*(

*H*), and called the hyperplane bundle. The dual bundle is denoted

*O*(−

*H*), and the

*k*^{th} tensor power of

*O*(

*H*) is denoted by

*O*(

*kH*). This is the sheaf generated by holomorphic multiples of a meromorphic function with a pole of order

*k* along

*H*. It turns out that

Indeed, if is a linear defining function for

*H*, then

*L*^{−k} is a meromorphic section of

*O*(

*k*), and locally the other sections of

*O*(

*k*) are multiples of this section.

Since , the line bundles on

**CP**^{n} are classified up to isomorphism by their

Chern classIn mathematics, in particular in algebraic topology and differential geometry, the Chern classes are characteristic classes associated to complex vector bundles.Chern classes were introduced by .-Basic idea and motivation:...

es, which are integers: they lie in . In fact, the first Chern classes of complex projective space are generated under

Poincaré dualityIn mathematics, the Poincaré duality theorem named after Henri Poincaré, is a basic result on the structure of the homology and cohomology groups of manifolds...

by the homology class associated to a hyperplane

*H*. The line bundle

*O*(

*kH*) has Chern class

*k*. Hence every holomorphic line bundle on

**CP**^{n} is a tensor power of

*O*(

*H*) or

*O*(−

*H*). In other words, the

Picard group of

**CP**^{n} is generated as an abelian group by the hyperplane class [

*H*] .

## See also

- Gromov's inequality for complex projective space
- Projective Hilbert space
In mathematics and the foundations of quantum mechanics, the projective Hilbert space P of a complex Hilbert space H is the set of equivalence classes of vectors v in H, with v ≠ 0, for the relation given by...

- Quaternionic projective space
In mathematics, quaternionic projective space is an extension of the ideas of real projective space and complex projective space, to the case where coordinates lie in the ring of quaternions H. Quaternionic projective space of dimension n is usually denoted byand is a closed manifold of dimension 4n...

- Real projective space
In mathematics, real projective space, or RPn, is the topological space of lines through 0 in Rn+1. It is a compact, smooth manifold of dimension n, and a special case of a Grassmannian.-Construction:...