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In
mathematics Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...
, real projective space, denoted or is the
topological space In mathematics, a topological space is, roughly speaking, a Geometry, geometrical space in which Closeness (mathematics), closeness is defined but cannot necessarily be measured by a numeric Distance (mathematics), distance. More specifically, a to ...
of lines passing through the origin 0 in the real space It is a
compact Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to: * Interstate compact, a type of agreement used by U.S. states * Blood compact, an ancient ritual of the Philippines * Compact government, a t ...
,
smooth manifold In mathematics, a differentiable manifold (also differential manifold) is a type of manifold that is locally similar enough to a vector space to allow one to apply calculus. Any manifold can be described by a collection of charts (atlas). One may ...
of
dimension In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus, a line has a dimension of one (1D) because only one coo ...
, and is a special case of a
Grassmannian In mathematics, the Grassmannian \mathbf_k(V) (named in honour of Hermann Grassmann) is a differentiable manifold that parameterizes the set of all k-dimension (vector space), dimensional linear subspaces of an n-dimensional vector space V over a ...
space.


Basic properties


Construction

As with all projective spaces, is formed by taking the quotient of \R^\setminus \ under the
equivalence relation In mathematics, an equivalence relation is a binary relation that is reflexive, symmetric, and transitive. The equipollence relation between line segments in geometry is a common example of an equivalence relation. A simpler example is equ ...
for all
real number In mathematics, a real number is a number that can be used to measure a continuous one- dimensional quantity such as a duration or temperature. Here, ''continuous'' means that pairs of values can have arbitrarily small differences. Every re ...
s . For all in \R^\setminus \ one can always find a such that has norm 1. There are precisely two such differing by sign. Thus can also be formed by identifying antipodal points of the unit -
sphere A sphere (from Ancient Greek, Greek , ) is a surface (mathematics), surface analogous to the circle, a curve. In solid geometry, a sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...
, , in \R^. One can further restrict to the upper hemisphere of and merely identify antipodal points on the bounding equator. This shows that is also equivalent to the closed -dimensional disk, , with antipodal points on the boundary, \partial D^n=S^, identified.


Low-dimensional examples

* is called the
real projective line In geometry, a real projective line is a projective line over the real numbers. It is an extension of the usual concept of a line that has been historically introduced to solve a problem set by visual perspective: two parallel lines do not int ...
, which is topologically equivalent to a
circle A circle is a shape consisting of all point (geometry), points in a plane (mathematics), plane that are at a given distance from a given point, the Centre (geometry), centre. The distance between any point of the circle and the centre is cal ...
. Thinking of points of as unit-norm complex numbers z up to sign, the diffeomorphism is given by z \mapsto z^2. Geometrically, a line in \mathbb^2 is parameterized by an angle \theta \in , \pi/math> and the endpoints of this closed interval correspond to the same line. * is called the real projective plane. This space cannot be embedded in . It can however be embedded in and can be immersed in (see here). The questions of embeddability and immersibility for projective -space have been well-studied. * is diffeomorphic to
SO(3) In mechanics and geometry, the 3D rotation group, often denoted SO(3), is the group of all rotations about the origin of three-dimensional Euclidean space \R^3 under the operation of composition. By definition, a rotation about the origin is a ...
, hence admits a group structure; the covering map is a map of groups Spin(3) → SO(3), where Spin(3) is a
Lie group In mathematics, a Lie group (pronounced ) is a group (mathematics), group that is also a differentiable manifold, such that group multiplication and taking inverses are both differentiable. A manifold is a space that locally resembles Eucli ...
that is the
universal cover In topology, a covering or covering projection is a map between topological spaces that, intuitively, locally acts like a projection of multiple copies of a space onto itself. In particular, coverings are special types of local homeomorphism ...
of SO(3).


Topology

The antipodal map on the -sphere (the map sending to ) generates a Z2
group action In mathematics, a group action of a group G on a set S is a group homomorphism from G to some group (under function composition) of functions from S to itself. It is said that G acts on S. Many sets of transformations form a group under ...
on . As mentioned above, the orbit space for this action is . This action is actually a
covering space In topology, a covering or covering projection is a continuous function, map between topological spaces that, intuitively, Local property, locally acts like a Projection (mathematics), projection of multiple copies of a space onto itself. In par ...
action giving as a double cover of . Since is
simply connected In topology, a topological space is called simply connected (or 1-connected, or 1-simply connected) if it is path-connected and every Path (topology), path between two points can be continuously transformed into any other such path while preserving ...
for , it also serves as the
universal cover In topology, a covering or covering projection is a map between topological spaces that, intuitively, locally acts like a projection of multiple copies of a space onto itself. In particular, coverings are special types of local homeomorphism ...
in these cases. It follows that the fundamental group of is when . (When n=1 the fundamental group is due to the homeomorphism with ). A generator for the fundamental group is the closed
curve In mathematics, a curve (also called a curved line in older texts) is an object similar to a line, but that does not have to be straight. Intuitively, a curve may be thought of as the trace left by a moving point. This is the definition that ...
obtained by projecting any curve connecting antipodal points in down to . The projective -space is compact, connected, and has a fundamental group isomorphic to the cyclic group of order 2: its universal covering space is given by the antipody quotient map from the -sphere, a
simply connected In topology, a topological space is called simply connected (or 1-connected, or 1-simply connected) if it is path-connected and every Path (topology), path between two points can be continuously transformed into any other such path while preserving ...
space. It is a double cover. The antipode map on has sign (-1)^p, so it is orientation-preserving if and only if is even. The orientation character is thus: the non-trivial loop in \pi_1(\mathbb^n) acts as (-1)^ on orientation, so is orientable if and only if is even, i.e., is odd. The projective -space is in fact diffeomorphic to the submanifold of \R^ consisting of all symmetric matrices of trace 1 that are also idempotent linear transformations.


Geometry of real projective spaces

Real projective space admits a constant positive scalar curvature metric, coming from the double cover by the standard round sphere (the antipodal map is locally an isometry). For the standard round metric, this has sectional curvature identically 1. In the standard round metric, the measure of projective space is exactly half the measure of the sphere.


Smooth structure

Real projective spaces are
smooth manifold In mathematics, a differentiable manifold (also differential manifold) is a type of manifold that is locally similar enough to a vector space to allow one to apply calculus. Any manifold can be described by a collection of charts (atlas). One may ...
s. On ''Sn'', in homogeneous coordinates, (''x''1, ..., ''x''''n''+1), consider the subset ''Ui'' with ''xi'' ≠ 0. Each ''Ui'' is homeomorphic to the disjoint union of two open unit balls in R''n'' that map to the same subset of RP''n'' and the coordinate transition functions are smooth. This gives RP''n'' a smooth structure.


Structure as a CW complex

Real projective space RP''n'' admits the structure of a
CW complex In mathematics, and specifically in topology, a CW complex (also cellular complex or cell complex) is a topological space that is built by gluing together topological balls (so-called ''cells'') of different dimensions in specific ways. It generali ...
with 1 cell in every dimension. In homogeneous coordinates (''x''1 ... ''x''''n''+1) on ''Sn'', the coordinate neighborhood ''U''1 = can be identified with the interior of ''n''-disk ''Dn''. When ''xi'' = 0, one has RP''n''−1. Therefore the ''n''−1 skeleton of RP''n'' is RP''n''−1, and the attaching map ''f'' : ''S''''n''−1 → RP''n''−1 is the 2-to-1 covering map. One can put \mathbf^n = \mathbf^ \cup_f D^n. Induction shows that RP''n'' is a CW complex with 1 cell in every dimension up to ''n''. The cells are Schubert cells, as on the
flag manifold In mathematics, a generalized flag variety (or simply flag variety) is a homogeneous space whose points are flags in a finite-dimensional vector space ''V'' over a field F. When F is the real or complex numbers, a generalized flag variety is a sm ...
. That is, take a complete
flag A flag is a piece of textile, fabric (most often rectangular) with distinctive colours and design. It is used as a symbol, a signalling device, or for decoration. The term ''flag'' is also used to refer to the graphic design employed, and fla ...
(say the standard flag) 0 = ''V''0 < ''V''1 <...< ''Vn''; then the closed ''k''-cell is lines that lie in ''Vk''. Also the open ''k''-cell (the interior of the ''k''-cell) is lines in (lines in ''Vk'' but not ''V''''k''−1). In homogeneous coordinates (with respect to the flag), the cells are \begin :0:0:\dots:0\\ *:*:0:\dots:0] \\ \vdots \\ *:*:*:\dots:*]. \end This is not a regular CW structure, as the attaching maps are 2-to-1. However, its cover is a regular CW structure on the sphere, with 2 cells in every dimension; indeed, the minimal regular CW structure on the sphere. In light of the smooth structure, the existence of a
Morse function In mathematics, specifically in differential topology, Morse theory enables one to analyze the topology of a manifold by studying differentiable functions on that manifold. According to the basic insights of Marston Morse, a typical differenti ...
would show RP''n'' is a CW complex. One such function is given by, in homogeneous coordinates, g(x_1, \ldots, x_) = \sum_ ^ i \cdot , x_i, ^2. On each neighborhood ''Ui'', ''g'' has nondegenerate critical point (0,...,1,...,0) where 1 occurs in the ''i''-th position with Morse index ''i''. This shows RP''n'' is a CW complex with 1 cell in every dimension.


Tautological bundles

Real projective space has a natural
line bundle In 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 organis ...
over it, called the tautological bundle. More precisely, this is called the tautological subbundle, and there is also a dual ''n''-dimensional bundle called the tautological quotient bundle.


Algebraic topology of real projective spaces


Homotopy groups

The higher homotopy groups of RP''n'' are exactly the higher homotopy groups of ''Sn'', via the long exact sequence on homotopy associated to a fibration. Explicitly, the fiber bundle is: \mathbf_2 \to S^n \to \mathbf^n. You might also write this as S^0 \to S^n \to \mathbf^n or O(1) \to S^n \to \mathbf^n by analogy with
complex projective space In mathematics, complex projective space is the projective space with respect to the field of complex numbers. By analogy, whereas the points of a real projective space label the lines through the origin of a real Euclidean space, the points of a ...
. The homotopy groups are: \pi_i (\mathbf^n) = \begin 0 & i = 0\\ \mathbf & i = 1, n = 1\\ \mathbf/2\mathbf & i = 1, n > 1\\ \pi_i (S^n) & i > 1, n > 0. \end


Homology

The cellular chain complex associated to the above CW structure has 1 cell in each dimension 0, ..., ''n''. For each dimensional ''k'', the boundary maps ''dk'' : δ''Dk'' → RP''k''−1/RP''k''−2 is the map that collapses the equator on ''S''''k''−1 and then identifies antipodal points. In odd (resp. even) dimensions, this has degree 0 (resp. 2): \deg(d_k) = 1 + (-1)^k. Thus the integral homology is H_i(\mathbf^n) = \begin \mathbf & i = 0 \text i = n \text\\ \mathbf/2\mathbf & 0 RP''n'' is orientable if and only if ''n'' is odd, as the above homology calculation shows.


Infinite real projective space

The infinite real projective space is constructed as the
direct limit In mathematics, a direct limit is a way to construct a (typically large) object from many (typically smaller) objects that are put together in a specific way. These objects may be groups, rings, vector spaces or in general objects from any cate ...
or union of the finite projective spaces: \mathbf^\infty := \lim_n \mathbf^n. This space is classifying space of ''O''(1), the first
orthogonal group In mathematics, the orthogonal group in dimension , denoted , is the Group (mathematics), group of isometry, distance-preserving transformations of a Euclidean space of dimension that preserve a fixed point, where the group operation is given by ...
. The double cover of this space is the infinite sphere S^\infty, which is contractible. The infinite projective space is therefore the Eilenberg–MacLane space ''K''(Z2, 1). For each nonnegative integer ''q'', the modulo 2 homology group H_q(\mathbf^\infty; \mathbf/2) = \mathbf/2. Its cohomology ring
modulo In computing and mathematics, the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another, the latter being called the '' modulus'' of the operation. Given two positive numbers and , mo ...
2 is H^*(\mathbf^\infty; \mathbf/2\mathbf) = \mathbf/2\mathbf _1 where w_1 is the first Stiefel–Whitney class: it is the free \mathbf/2\mathbf-algebra on w_1, which has degree 1. Its cohomology ring with \mathbf coefficients is H^*(\mathbf^;\mathbf) = \mathbf
alpha Alpha (uppercase , lowercase ) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter ''aleph'' , whose name comes from the West Semitic word for ' ...
(2\alpha), where \alpha has degree 2. It can be deduced from the chain map between cellular cochain complexes with \mathbf and \mathbf/2 coefficients, which yield a ring homomorphism H^*(\mathbf^;\mathbf) \rightarrow H^*(\mathbf^;\mathbf/2\mathbf) injective in positive dimensions, with image the even dimensional part of H^*(\mathbf^;\mathbf/2\mathbf). Alternatively, the result can also be obtained using the Universal coefficient theorem.


See also

*
Complex projective space In mathematics, complex projective space is the projective space with respect to the field of complex numbers. By analogy, whereas the points of a real projective space label the lines through the origin of a real Euclidean space, the points of a ...
* Quaternionic projective space * Lens space * Real projective plane


Notes


References

* Bredon, Glen. ''Topology and geometry'', Graduate Texts in Mathematics, Springer Verlag 1993, 1996 * * {{DEFAULTSORT:Real Projective Space Algebraic topology Differential geometry Projective geometry