Homology of real projective space

This article describes the value (and the process used to compute it) of some homotopy invariant(s) for a topological space or family of topological spaces. The invariant is homology group and the topological space/family is real projective space
Get more specific information about real projective space | Get more computations of homology group

Statement

Odd-dimensional projective space with coefficients in integers

$H_p({\mathbb{P}}^n(\mathbb{R}))=\{\begin{array}{cc}\mathbb{Z}& p=0,n\\ \mathbb{Z}/2\mathbb{Z},& p\mathrm{o}\mathrm{d}\mathrm{d},0<p<n\\ 0,& \mathrm{o}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{w}\mathrm{i}\mathrm{s}\mathrm{e}\end{array}$

Even-dimensional projective space with coefficients in integers

$H_p({\mathbb{P}}^n(\mathbb{R}))=\{\begin{array}{cc}\mathbb{Z}& p=0\\ \mathbb{Z}/2\mathbb{Z},& p\mathrm{o}\mathrm{d}\mathrm{d},0<p<n\\ 0,& \mathrm{o}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{w}\mathrm{i}\mathrm{s}\mathrm{e}\end{array}$

Thus the key difference between even and odd dimensional projective spaces is that the top homology vanishes in even-dimensional projective spaces. This is related to the fact that even-dimensional projective space is non-orientable, while odd-dimensional projective space is orientable.

Odd-dimensional projective space with coefficients in an abelian group or module

$H_p({\mathbb{P}}^n(\mathbb{R});M)=\{\begin{array}{cc}M& p=0,n\\ M/2M,& p\mathrm{o}\mathrm{d}\mathrm{d},0<p<n\\ T,& p\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n},0<p\le n\\ 0,& \mathrm{o}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{w}\mathrm{i}\mathrm{s}\mathrm{e}\end{array}$

where $T$ is the 2-torsion submodule of $M$, i.e. the submodule comprising elements whose double is zero.

Even-dimensional projective space with coefficients in an abelian group or module $M$

$H_p({\mathbb{P}}^n(\mathbb{R});M)=\{\begin{array}{cc}M& p=0\\ M/2M,& p\mathrm{o}\mathrm{d}\mathrm{d},0<p<n\\ T,& p\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n},0<p\le n\\ 0,& \mathrm{o}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{w}\mathrm{i}\mathrm{s}\mathrm{e}\end{array}$

where $T$ is the 2-torsion submodule of $M$, i.e. the submodule comprising elements whose double is zero.

Coefficients in a 2-divisible ring

If we consider the homology with coefficients in a module $M$ over a ring $R$ where 2 is invertible, then we have:

$H_p({\mathbb{P}}^n(\mathbb{R});M):=\{\begin{array}{cc}M,& p=0\\ M,& p=n,n\mathrm{o}\mathrm{d}\mathrm{d}\\ 0,& p=n,n\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}\\ 0,& p\ne 0,n\\ \end{array}$

In particular, these results are valid over the field of rational numbers or over any field of characteristic zero.

Related invariants

These are all invariants that can be computed in terms of the homology groups.

Invariant	General description	Description of value for real projective space
Betti numbers	The $k^{th}$ Betti number $b_k$ is the rank of the torsion-free part of the $k^{th}$ homology group.	$b_0=1$. $b_n=1$ if $n$ is odd and $b_n=0$ if $n$ is even. All other $b_k$s are zero.
Poincare polynomial	Generating polynomial for Betti numbers	$1+x^n$ if $n$ is odd. $1$ if $n$ is even.
Euler characteristic	$\sum _{k=0}^{\mathrm{\infty }}(-1{)}^k{b}_k$	$0$ if $n$ is odd. $1$ if $n$ is even. Note that the Euler characteristic is half the Euler characteristic of the sphere $S^n$, which is its double cover.

Facts used

1. CW structure of real projective space

Proof

Explication of chain complex

The proof follows from fact (1). By fact (1), we note that the homology of real projective space ${\mathbb{P}}^n(\mathbb{R})$ is the same as the homology of the following chain complex, obtained as its cellular chain complex:

• For $n$ even:

$0\to 0\to \dots \to 0\to \mathbb{Z}\stackrel{\cdot 2}{\to }\mathbb{Z}\stackrel{\cdot 0}{\to }\mathbb{Z}\to \dots \stackrel{\cdot 2}{\to }\mathbb{Z}\stackrel{\cdot 0}{\to }\mathbb{Z}$

where the largest nonzero chain group is the $n^{th}$ chain group.

• For $n$ odd:

$0\to 0\to \dots \to 0\to \mathbb{Z}\stackrel{\cdot 0}{\to }\mathbb{Z}\stackrel{\cdot 2}{\to }\mathbb{Z}\to \dots \stackrel{\cdot 2}{\to }\mathbb{Z}\stackrel{\cdot 0}{\to }\mathbb{Z}$

Note that the multiplication maps alternate between multiplication by two and multiplication by zero. In particular, in both cases, the map is multiplication by two if going down from an even to an odd index and multiplication by zero if going down from an odd to an even index. The key difference between the odd and even case is whether we start with a multiplication by two map or a multiplication by zero map.

Homology computation over integers

Case for $n$	Case for $p$	Fragment of relevance in chain complex ($(p+1{)}^{th}$ to $p^{th}$ to $(p-1{)}^{th}$)	Cycle group (kernel from $p^{th}$ to $(p-1{)}^{th}$	Boundary group (image from $(p+1{)}^{th}$ group to $p^{th}$ group	Homology group = cycle group/boundary group
both odd and even	0	$\mathbb{Z}\stackrel{\cdot 0}{\to }\mathbb{Z}\stackrel{0}{\to }0$	$\mathbb{Z}$	0	$\mathbb{Z}$
both odd and even	odd, strictly less than $n$	$\mathbb{Z}\stackrel{\cdot 2}{\to }\mathbb{Z}\stackrel{\cdot 0}{\to }\mathbb{Z}$	$\mathbb{Z}$	$2\mathbb{Z}$	$\mathbb{Z}/2\mathbb{Z}$
both odd and even	even, positive, strictly less than $n$	$\mathbb{Z}\stackrel{\cdot 0}{\to }\mathbb{Z}\stackrel{\cdot 2}{\to }\mathbb{Z}$	0	0	0
even	equal to $n$	$0\stackrel{0}{\to }\mathbb{Z}\stackrel{\cdot 2}{\to }\mathbb{Z}$	0	0	0
odd	equal to $n$	$0\stackrel{0}{\to }\mathbb{Z}\stackrel{\cdot 0}{\to }\mathbb{Z}$	$\mathbb{Z}$	0	$\mathbb{Z}$
both odd and even	greater than $n$	$0\stackrel{0}{\to }\mathbb{Z}\stackrel{\cdot 0}{\to }0$ (the last term becomes $\mathbb{Z}$ when $p=n+1$)	0	0	0

Homology computation over an abelian group or module $M$

The chain complex remains the same, but each $\mathbb{Z}$ is replaced by $M$.

Denote by $T$ the 2-torsion submodule of $M$ and by $M/2M$ the quotient of $M$ by the submodule $2M$ comprising the doubles of elements.

Case for $n$	Case for $p$	Fragment of relevance in chain complex ($(p+1{)}^{th}$ to $p^{th}$ to $(p-1{)}^{th}$)	Cycle group (kernel from $p^{th}$ to $(p-1{)}^{th}$	Boundary group (image from $(p+1{)}^{th}$ group to $p^{th}$ group	Homology group = cycle group/boundary group
both odd and even	0	$M\stackrel{\cdot 0}{\to }M\stackrel{0}{\to }0$	$M$	0	$M$
both odd and even	odd, strictly less than $n$	$M\stackrel{\cdot 2}{\to }M\stackrel{\cdot 0}{\to }\mathbb{Z}$	$M$	$2M$	$M/2M$
both odd and even	even, positive, strictly less than $n$	$M\stackrel{\cdot 0}{\to }M\stackrel{\cdot 2}{\to }M$	$T$	0	$T$
even	equal to $n$	$0\stackrel{0}{\to }M\stackrel{\cdot 2}{\to }M$	$T$	0	$T$
odd	equal to $n$	$0\stackrel{0}{\to }M\stackrel{\cdot 0}{\to }M$	$M$	0	$M$
both odd and even	greater than $n$	$0\stackrel{0}{\to }M\stackrel{\cdot 0}{\to }0$ (though the last term becomes $M$ when $p=n+1$)	0	0	0