Real projective space

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In mathematics, real projective space, or RPⁿ, is the topological space of lines passing through the origin 0 in Rⁿ⁺¹. It is a compact, smooth manifold of dimension n, and is a special case Gr(1, Rⁿ⁺¹) of a Grassmannian space.

Basic properties

Construction

As with all projective spaces, RPⁿ is formed by taking the quotient of Rⁿ⁺¹\{0} under the equivalence relation x ∼ λx for all real numbers λ ≠ 0. For all x in Rⁿ⁺¹\{0} one can always find a λ such that λx has norm 1. There are precisely two such λ differing by sign.

Thus RPⁿ can also be formed by identifying antipodal points of the unit n-sphere, Sⁿ, in Rⁿ⁺¹.

One can further restrict to the upper hemisphere of Sⁿ and merely identify antipodal points on the bounding equator. This shows that RPⁿ is also equivalent to the closed n-dimensional disk, Dⁿ, with antipodal points on the boundary, ∂Dⁿ = Sⁿ⁻¹, identified.

Low-dimensional examples

RP¹ is called the real projective line, which is topologically equivalent to a circle.

RP² is called the real projective plane. This space cannot be embedded in R³. It can however be embedded in R⁴ and can be immersed in R³. The questions of embeddability and immersibility for projective n-space have been well-studied.^[1]

RP³ is (diffeomorphic to) SO(3), hence admits a group structure; the covering map S³ → RP³ is a map of groups Spin(3) → SO(3), where Spin(3) is a Lie group that is the universal cover of SO(3).

Topology

The antipodal map on the n-sphere (the map sending x to −x) generates a Z₂ group action on Sⁿ. As mentioned above, the orbit space for this action is RPⁿ. This action is actually a covering space action giving Sⁿ as a double cover of RPⁿ. Since Sⁿ is simply connected for n ≥ 2, it also serves as the universal cover in these cases. It follows that the fundamental group of RPⁿ is Z₂ when n > 1. (When n = 1 the fundamental group is Z due to the homeomorphism with S¹). A generator for the fundamental group is the closed curve obtained by projecting any curve connecting antipodal points in Sⁿ down to RPⁿ.

The projective n-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 n-sphere, a simply connected space. It is a double cover. The antipode map on R^p has sign $(-1)^{p}$ , so it is orientation-preserving iff p is even. The orientation character is thus: the non-trivial loop in $\pi _{1}({\mathbf {RP}}^{n})$ acts as $(-1)^{{n+1}}$ on orientation, so RPⁿ is orientable iff n+1 is even, i.e., n is odd.

The projective n-space is in fact diffeomorphic to the submanifold of R^(n+1)² consisting of all symmetric (n+1) × (n+1) 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 manifolds. On Sⁿ, in homogeneous coordinates, (x₁...x_n+1), consider the subset U_i with x_i ≠ 0. Each U_i is homeomorphic to the open unit ball in Rⁿ and the coordinate transition functions are smooth. This gives RPⁿ a smooth structure.

CW structure

Real projective space RPⁿ admits a CW structure with 1 cell in every dimension.

In homogeneous coordinates (x₁ ... x_n+1) on Sⁿ, the coordinate neighborhood U₁ = {(x₁ ... x_n+1) | x₁ ≠ 0} can be identified with the interior of n-disk Dⁿ. When x_i = 0, one has RPⁿ⁻¹. Therefore the n−1 skeleton of RPⁿ is RPⁿ⁻¹, and the attaching map f : Sⁿ⁻¹ → RPⁿ⁻¹ is the 2-to-1 covering map. One can put

${\mathbf {RP}}^{n}={\mathbf {RP}}^{{n-1}}\cup _{f}D^{n}.$

Induction shows that RPⁿ is a CW complex with 1 cell in every dimension up to n.

The cells are Schubert cells, as on the flag manifold. That is, take a complete flag (say the standard flag) 0 = V₀ < V₁ <...< V_n; then the closed k-cell is lines that lie in V_k. Also the open k-cell (the interior of the k-cell) is lines in V_k\V_k-1(lines in V_k but not V_k−1).

In homogeneous coordinates (with respect to the flag), the cells are

$[*:0:0:\dots :0]$

$[*:*:0:\dots :0]$

$\vdots$

$[*:*:*:\dots :*].$

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 would show RPⁿ is a CW complex. One such function is given by, in homogeneous coordinates,

$g(x_{1},\cdots ,x_{{n+1}})=\sum _{1}^{{n+1}}i\cdot |x_{i}|^{2}.$

On each neighborhood U_i, g has nongenerate critical point (0,...,1,...,0) where 1 occurs in the i-th position with Morse index i. This shows RPⁿ is a CW complex with 1 cell in every dimension.

Tautological bundles

Real projective space has a natural line bundle 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 RPⁿ are exactly the higher homotopy groups of Sⁿ, via the long exact sequence on homotopy associated to a fibration.

Explicitly, the fiber bundle is:

${\mathbf {Z}}_{2}\to S^{n}\to {\mathbf {RP}}^{n}.$

You might also write this as

$S^{0}\to S^{n}\to {\mathbf {RP}}^{n}$

$O(1)\to S^{n}\to {\mathbf {RP}}^{n}$

by analogy with complex projective space.

The homotopy groups are:

$\pi _{i}({\mathbf {RP}}^{n})={\begin{cases}0&i=0\\{\mathbf {Z}}&i=1,n=1\\{\mathbf {Z}}/2{\mathbf {Z}}&i=1,n>1\\\pi _{i}(S^{n})&i>1,n>0.\end{cases}}$

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 d_k : δD^k → 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):

${\mathrm {deg}}(d_{k})=1+(-1)^{k}.$

Thus the integral homology is

$H_{i}({\mathbf {RP}}^{n})={\begin{cases}{\mathbf {Z}}&i=0{\mbox{ or }}i=n{\mbox{ odd,}}\\{\mathbf {Z}}/2{\mathbf {Z}}&0<i<n,\ i\ {\mbox{odd,}}\\0&{\mbox{else.}}\end{cases}}$

RPⁿ is orientable iff n is odd, as the above homology calculation shows.

Infinite real projective space

The infinite real projective space is constructed as the direct limit or union of the finite projective spaces:

${\mathbf {RP}}^{\infty }:=\lim _{n}{\mathbf {RP}}^{n}.$

Topologically, this space is double-covered by the infinite sphere $S^{\infty }$ , which is contractible. The infinite projective space is therefore the Eilenberg-MacLane space K(Z₂, 1) and it is BO(1), the classifying space for line bundles. More generally, the infinite Grassmannians are the classifying spaces for finite rank vector bundles.

Its cohomology ring modulo 2 is

$H^{*}({\mathbf {RP}}^{\infty };{\mathbf {Z}}/2{\mathbf {Z}})={\mathbf {Z}}/2{\mathbf {Z}}[w_{1}],$

where $w_{1}$ is the first Stiefel–Whitney class: it is the free ${\mathbf {Z}}/2{\mathbf {Z}}$ -algebra on $w_{1}$ , which has degree 1.

Notes

↑ See the table of Don Davis for a bibliography and list of results.

References

Bredon, G. Topology and geometry
Davis, Donald. "Table of immersions and embeddings of real projective spaces". Retrieved 22 Sep 2011.
Hatcher, Allen (2001). Algebraic Topology. Cambridge University Press. ISBN 978-0-521-79160-1.

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