# Mod 2 cohomology ring of a kind of orbit configuration space

Hao Li

###### Abstract.

In this paper we caculate mod 2 cohomology ring of
$F_{\mathbb{Z}_{2}^{m}}(\mathbb{R}^{m},n)$ , which is local representation of
orbit congfiguration spaces over small covers. We construct a differntial
graded algebra, and there is a ring isomorphism between its mod 2 cohomology
ring and $H^{*}(F_{\mathbb{Z}_{2}^{m}}(\mathbb{R}^{m},n),\mathbb{Z}_{2})$.
This idea can also be applied to calculate mod 2 cohomology ring of complement
space of real arrangements.

##  1\. Introduction

Let X be a topological space. We will consider configuration space of n
ordered distinct points in X:

$F(X,n)=\\{(x_{1},\ldots,x_{n})\in X^{n}|x_{i}\neq x_{j},\quad\forall\quad
i\neq j\\}$

If X admits a group action $G\times X\rightarrow X$, we consider orbit
configuration space:

$F_{G}(X,n)=\\{(x_{1},\ldots,x_{n})\in X^{n}|G(x_{i})\cap
G(x_{j})=\emptyset,\quad\forall\quad i\neq j\\}$

The notion of configuration space was introduced in physics in 1940’s. In
mathematics, configuration spaces were first introduced by Fadell and
Neuwirth[2] in 1962.

The classical configuration space is $F({\mathbb{R}}^{2},n)\cong
F({\mathbb{C}},n)$, it is exactly the complement space of the union of finite
hyperplanes in ${\mathbb{C}}^{n}$, its fundamental group eqauls to classical
pure braid group. In 1969, Arnol’d[3] computed the cohomology ring of
$F({\mathbb{C}},n)$, it is the form of Orlik-Solomon algebra in arrangement
theory. $F({\mathbb{R}}^{k},n)$ is the complement of finite union of linear
subspaces of codimension k in ${\mathbb{R}}^{nk}$, F.R.Cohen[4] calculated its
integral cohomology ring as a free Lie algebras with each generator
corresponding to a codimension-k subspace. In 2000, Feichtner and Ziegler[5]
determined $H^{*}(F(S^{k},n);{\mathbb{Z}})$; in 2001[6], they computed
$H^{*}(F_{{\mathbb{Z}}_{2}}(S^{k},n);{\mathbb{Z}})$ , where the group aciton
is the antipodar map.

If X is a smooth complex projective variety. In 1994, Fulton-MacPherson[7]
proved that the rational cohomology ring can be computed from the rational
cohomology ring of X and the Chern class of X. Totaro[8] improved their work
by proving that Chern class is actually irrevalent.

M.A. Xicoténcatl[9] did a lot of work in his Ph.D. thesis on orbit
configuration space where $G$ acts freely on $M$, he computed the cohomology
and loop space homology of some free action spaces, such as complements of
arrangements, spaces of polynomials and spaces of type $K(\pi,1)$.

But for other topological spaces with non-free group action, it becomes much
harder to compute their homotopy groups and cohomology rings, tools used in
above examples can no longer be applied to the computation.

There is a very typical kind of spaces with non-free group action. In 1991,
Davis and Januszkiewicz[1] introduced four classes of nicely behaving
manifolds over simple convex polytopes—small covers, quasi-toric manifolds,
(real) moment-angle manifolds which have become important objects in toric
topology. We are interested to study the orbit configuration spaces
$F_{G_{d}^{m}}(M,n)$ for a dm-dimensional $G_{d}^{m}$-manifold $M$ over a
simple convex m-polytope P —Where $M$ is a small cover and
$G_{d}^{m}={\mathbb{Z}}_{2}^{m}$ when $d=1$, and a quasi-toric manifold and
$G_{d}^{m}=T^{m}$ when $d=2$. We expect to find the relation between the
algebraic topology of $F_{G_{d}^{m}}(M,n)$ and combinatoric informations of
polytope P. In 2008, Junda Chen[10] gave an explicit formula for the Euler
characteristic of $F_{G_{d}^{m}}(M,n)$ in terms of the h-vector of P and gave
a description of homotopy type when $n=2$. But there is still some distance
between this work and our expectation.

In this paper, we focus on $F_{\mathbb{Z}_{2}^{m}}(\mathbb{R}^{m},n)$. Since
it is the local representation of $F_{\mathbb{Z}_{2}^{m}}(M,n)$, the results
in this paper will help to improve the study of $F_{\mathbb{Z}_{2}^{m}}(M,n)$.
Besides, ${\mathbb{Z}}_{2}^{m}\curvearrowright{\mathbb{R}}^{m}$ is a typical
non-free group action over Euclidean space, it can be an interesting example
for the computation of orbit configuration spaces.

In the pointview of arrangements, $F_{\mathbb{Z}_{2}^{m}}(\mathbb{R}^{m},n)$
can be regarded as complement space of a collection of subspaces in Euclidean
space. The study of complex arrangement is complete. In 1982, Richard Randell
[16] gave a nice description of the fundamental group of complement of
complexification of real arrangements. In 1995, De Concini and Procesi[11]
constructed a rational model using only labeled lattice proved that the
rational cohomology ring are determined by this lattice. In 1999, Sergey
Yuzvinsky [13] constructed rational model on atomic complex to simplify De
Concini and Procesi’s result.

There is little result for real arrangements. Goresky-MacPherson describe the
integral homology group. In 2000, Mark de Longueville and Carsten A. Schultz
[12] computed the integral cohomology ring of geometric ($\geq 2$) real
arrangements. For general real arrangements, there isn’t a good tool to get
its cohomology ring information.

Our main result is an description of
$H^{*}(F_{\mathbb{Z}_{2}^{m}}(\mathbb{R}^{m},n);{\mathbb{Z}}_{2})$. Let
$\mathcal{A}$ be a real arrangement in Euclidean space, $M(\mathcal{A})$
denote its complement space. A differential graded algebra $\widetilde{D}$ is
constructed based on the intersection poset of arrangement $\mathcal{A}$. And
$H^{*}(\widetilde{D},{\mathbb{Z}}_{2})$ is computable.

######  Theorem 1.1.

There is a ring isomorphism
$H^{*}(F_{\mathbb{Z}_{2}^{m}}(\mathbb{R}^{m},n);{\mathbb{Z}}_{2})\cong
H^{*}(\widetilde{D},{\mathbb{Z}}_{2})$

This method can be applied to calculation of mod 2 cohomology ring of any real
arrangements.

This paper is organised as follows. In section 2 and 3, we give a brief
introduction on the notions of small covers and quasi-toric manifolds,
subspace arrangements, Goresky-MacPherson isomorphism and intersection product
in arrangements theory; in section 4, we construct a differential graded
algebra to describe the cohomology ring of
$H^{*}(F_{\mathbb{Z}_{2}^{m}}(\mathbb{R}^{m},n);{\mathbb{Z}}_{2})$; in section
5, we give a simple example.

##  2\. Preliminary

###  2.1. Small covers and quasi-toric manifolds

By the definitions in [1], let $P^{m}$ be an $m$-dimensional simple convex
polytope.

Let $G_{d}^{m}$ be ${\mathbb{Z}}_{2}^{m}$, ${\mathbb{F}}_{d}={\mathbb{R}}$ if
d=1; and the torus $T^{m}$, ${\mathbb{F}}_{d}={\mathbb{C}}$ if $d=2$.

The natural action of $G_{d}^{m}$ on ${\mathbb{F}}_{d}^{m}$ is called the
standard representation , and the orbit space is ${\mathbb{R}}^{m}_{+}$.

A $dm$-dimensional $G_{d}^{m}$-manifold $M^{dm}$ over $P^{m}$, is a smooth
closed $dm$-dimensional manifold $M^{dm}$ with a locally standard
$G_{d}^{m}$-action such that the orbit space is $P^{m}$.

A $G_{d}^{m}$-manifold $M^{dm}$ is called small cover if $d=1$ and quasi-toric
manifold if $d=2$.

###  2.2. Local representation

Since it is difficult to describe the topology of $F_{G_{d}^{m}}(M^{dm},n)$,
we first consider its local representation.

For $d=1$,the orbit configuration space is
$F_{{\mathbb{Z}}_{2}^{m}}(M^{m},n)$, its local representation is
$F_{{\mathbb{Z}}_{2}^{m}}({\mathbb{R}}^{m},n)$.

For $d=2$, the orbit configuration space is $F_{T^{m}}(M^{2m},n)$, its local
representation is $F_{T^{m}}({\mathbb{R}}^{2m},n)$.

In this paper, we only consider the case when d=1.

We can observe that
$F_{{\mathbb{Z}}_{2}^{m}}(M^{m},n)=\\{(x_{1},x_{2},...,x_{n})\in({\mathbb{R}}^{m})^{n}|x_{i}\in{\mathbb{R}}^{m},{\mathbb{Z}}_{2}^{m}(x_{i})\cap{\mathbb{Z}}_{2}^{m}(x_{j})=\emptyset,\forall
i\neq j\\}\\\ \hskip 54.06006pt=({\mathbb{R}}^{m})^{n}\setminus\underset{1\leq
i<j\leq
n}{\bigcup}\\{(x_{1},x_{2},...,x_{n})\in({\mathbb{R}}^{m})^{n}|{\mathbb{Z}}_{2}^{m}(x_{i})={\mathbb{Z}}_{2}^{m}(x_{j})\\}$

Let
$A_{ij}\triangleq\\{(x_{1},x_{2},...,x_{n})\in(\mathbb{R}^{m})^{n}|x_{i}\in\mathbb{Z}_{2}^{m}(x_{j})\\}$,
it is the union of $2^{m}$ subspaces in $(\mathbb{R}^{m})^{n}$, each subspace
is in the form

$A_{ij}^{g}\triangleq\\{(x_{1},x_{2},...,x_{n})\in(\mathbb{R}^{m})^{n}|x_{i}=g(x_{j}),g\in\mathbb{Z}_{2}^{m}\\}$

with codimension $m$.

Thus $F_{\mathbb{Z}_{2}^{m}}(\mathbb{R}^{m},n)$ can be regarded as the
complement space of subspace arrangement $\mathcal{A}$ in
$(\mathbb{R}^{m})^{\times n}$. $\mathcal{A}$ consists of $C_{n}^{2}\times
2^{m}$ subspaces with codimension $m$. $\mathcal{A}=\\{A_{ij}^{g}|1\leq
i<j\leq n,g\in\mathbb{Z}_{2}^{m}\\}$.

###  2.3. Arrangement theory

In this part, we review some useful concepts about subspace arrangement in
[12].

####  2.3.1. Notations

Let $\mathcal{A}$ be a linear subspace arrangement in a finite-dimensional
$\mathbb{R}$-vector space W, let $\mathit{u}\subseteq\mathit{v}\subseteq W$ be
linear subspaces.

  * •

$\pi^{\mathit{u}}$: the quotient map $W\rightarrow W/\mathit{u}$

  * •

$\pi^{\mathit{u},\mathit{v}}$: the quotient map $W/\mathit{u}\rightarrow
W/\mathit{v}$

  * •

$\mathcal{A}_{\mathit{u}}\triangleq\\{\mathit{z}\in\mathcal{A}|\mathit{u}\subseteq\mathit{z}\\}$
the subarrangement in W

  * •

$\mathcal{A}/\mathit{u}\triangleq\\{\pi^{\mathit{u}}(\mathit{z})|\mathit{z}\in\mathcal{A}_{\mathit{u}}\\}$
the arrangement in $W/\mathit{u}$

  * •

$M(\mathcal{A})$ denote the complement space $W\setminus\bigcup\mathcal{A}$

  * •

$P$ intersection poset, denote the set of all intersections of subset of
$\mathcal{A}$.
The intersection poset $P$ is partially ordered by the reverse inclusion;
It has maximal element $\top\triangleq\bigcap\mathcal{A}$ and minimal element
$W\triangleq\bigcap\emptyset$;
The join operation $\vee$ in $P$ is given by intersection;
$P$ is furnished with a dimension function $d:P\rightarrow\mathbb{N}$;
For $\mathit{u},\mathit{v}\in P$, we denote by
$[\mathit{u},\mathit{v}],(\mathit{u},\mathit{v}],[\mathit{u},\mathit{v})$ the
respective intervals in $P$.

For any partially ordered set Q, denote by $\triangle(Q)$ the order complex of
Q whose simplices are given by chains in Q.

####  2.3.2. generic points

To establish a map between intersection poset $P$ and subspace arrangement, we
have to introduce the concept of generic points.

For $\mathit{u}\in P$, generic point $x^{\mathit{u}}$ means either a point in
$\mathit{u}\setminus\underset{\mathit{z}\in(\mathit{u},T]}{\bigcup}\mathit{z}$
or a map
$x^{\mathit{u}}:[W,\mathit{u}]$ $\rightarrow$ $W/\mathit{u}$ $\mathit{v}$
$\mapsto$ $x_{\mathit{v}}^{\mathit{u}}$ with
$x_{\mathit{v}}^{\mathit{u}}\in\pi^{\mathit{u}}(v)\setminus\underset{\mathit{v}^{\prime}\in(\mathit{v},\mathit{u}]}{\bigcup}\pi^{\mathit{u}}(\mathit{v}^{\prime})$

Let $\mathit{u}\in P$, with generic points $x^{\mathit{u}}$, define a affine
map $\phi^{x^{\mathit{u}}}:\triangle[W,\mathit{u}]\rightarrow W/\mathit{u}$
which is affine on simplices and satisfies
$\phi^{x^{\mathit{u}}}(\mathit{w})=x_{\mathit{w}}^{\mathit{u}}\qquad\mathit{w}\in[W,\mathit{u}]$

Note that we can identify the abstract simplicial complex with its geometric
realization.

##  3\. Goresky-MacPherson isomorphism and products

This chapter mainly states the results in [12].

###  3.1. Goresky-MacPherson isomorphism

Given an arrangement $\mathcal{A}$ in W. If $\epsilon>0$ is a real number.
$B_{\epsilon}$ denotes the open $\epsilon$-ball in W. Then

(3.1)
$H^{k}(W\setminus\bigcup\mathcal{A})\xrightarrow[\cong]{i^{*}}H^{k}(B_{\epsilon}\setminus\bigcup\mathcal{A})\xrightarrow[\cong]{\cap[W]}H_{d(W)-k}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$

here $\mathit{i}$ denotes inclusion, [W] is the orientation class of W, and
$\mathcal{C}B_{\epsilon}$ is the complement of $B_{\epsilon}$.

The first isomorphism is trivial induced by inclusion map, the second
isomorphism derives from Alexander Duality.

Now if we want to describe the cohomology ring of $M(\mathcal{A})$, we will
work mainly in $H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$ with
intersection product $\bullet$ given by
$(\alpha\cap[W])\bullet(\beta\cap[W])=(\alpha\cup\beta)\cap[W]$,
$\quad\alpha,\beta\in H^{*}(B_{\epsilon}\setminus\bigcup\mathcal{A})$.

Recall the map $\phi^{x^{\mathit{u}}}:\triangle[W,\mathit{u}]\rightarrow
W/\mathit{u}$

For a simplex $\sigma\in\triangle[W,\mathit{u}]$.
$\sigma=<v_{0},\dots,v_{k}>,v_{0}<\dots<v_{k}$,
$\phi^{x^{\mathit{u}}}(\sigma)\subseteq\pi^{\mathit{u}}({v_{0}})$,
$\phi^{x^{\mathit{u}}}(\sigma)\cap\pi^{\mathit{u}}(v_{k})=\\{x_{v_{k}}^{\mathit{u}}\\}$.

$\phi^{x^{\mathit{u}}}(\triangle(W,\mathit{u}])\subseteq\bigcup\mathcal{A}/\mathit{u}$,
$\phi^{x^{\mathit{u}}}(\triangle[W,\mathit{u}))\subseteq W/\mathit{u}\setminus
B_{\epsilon}^{W/\mathit{u}}$ (for small enough $\epsilon$).

Then we have map of pairs

$\phi^{x^{\mathit{u}}}:(\triangle[W,\mathit{u}],\triangle(W,\mathit{u}]\cup\triangle[W,\mathit{u}))\rightarrow(W/\mathit{u},\bigcup\mathcal{A}/\mathit{u}\cup\mathcal{C}B_{\epsilon}^{W/\mathit{u}})$

We simplify
$\Delta\Delta[W,\mathit{u}]\triangleq(\triangle[W,\mathit{u}],\triangle(W,\mathit{u}]\cup\triangle[W,\mathit{u}))$

Because
$(\pi^{\mathit{u}})^{-1}(\cup\mathcal{A}/\mathit{u})=\cup\mathcal{A}_{\mathit{u}}\subseteq\mathcal{A}$
and
$(\pi^{\mathit{u}})^{-1}(\mathcal{C}B_{\epsilon}^{W/\mathit{u}})\subset\mathcal{C}B_{\epsilon}$

Thus we can consider the following maps:

(3.2)
$H_{k}(\Delta\Delta[W,\mathit{u}])\xrightarrow{\phi_{*}^{x^{\mathit{u}}}}H_{k}(W/\mathit{u},\bigcup\mathcal{A}/\mathit{u}\cup\mathcal{C}B_{\epsilon}^{W/\mathit{u}})\xrightarrow{\pi_{!}^{\mathit{u}}}H_{d(\mathit{u})+k}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$

$\pi_{!}^{\mathit{u}}$ is given by
$\alpha\cap[W/\mathit{u}]\mapsto(\pi^{\mathit{u}})^{*}\alpha\cap[W]\qquad\alpha\in
H^{*}(W/\mathit{u})$

Then we can introduce well known Goresky-MacPherson isomorphism.

######  Theorem 3.1 (Goresky-MacPherson isomorphism).

Let $\mathcal{A}$ be an arrangement in W and $x^{\mathit{u}}$ be a choice of
gneric points. Then the map

$\sum_{\mathit{u}\in[W,\top]}\pi_{!}^{\mathit{u}}\circ\phi_{*}^{x^{\mathit{u}}}:\bigoplus_{\mathit{u}\in[W,T]}H_{*}(\Delta\Delta[W,\mathit{u}])\longrightarrow
H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$
is an isomorphism as group.

This proposition is originally proved in [15] by means of stratified Morse
theory. There is another elementary proof in [12].

####  3.1.1. Products

Let $P,Q$ be two intersection posets. $\triangle(P\times
Q)=\triangle(P)\times\triangle(Q)$

If $C_{*}$ denotes the ordered chain complex, there is the well known map

$\quad C_{*}(\triangle P)\bigotimes C_{*}(\triangle
Q)\xrightarrow{\times}C_{*}(\triangle(P\times Q))$ given by
$\langle u_{0},\dots,u_{k}\rangle\bigotimes\langle
v_{0},\dots,v_{l}\rangle\longmapsto\underset{\begin{subarray}{c}0=i_{0}\leq\dots\leq
i_{k+l}=k\\\ 0=j_{0}\leq\dots\leq j_{k+l}=l\\\ \forall
r\quad(i_{r-1},j_{r-1})\neq(i_{r},j_{r})\end{subarray}}{\sum}\sigma_{i,j}\langle(u_{i_{0}},v_{j_{0}}),\dots,(u_{i_{k+l}},v_{j_{k+l}})\rangle$
where the $\sigma_{i,j}$ are signs determined by $\sigma_{i,j}=1$ if k=0 or
l=0 and by $\partial(a\times b)=\partial a\times b+(-1)^{k}a\times\partial b$.

Since $\triangle\triangle P\times\triangle\triangle
Q=\triangle\triangle(P\times Q)$, this induces a product

$\times:H_{*}(\triangle\triangle P)\otimes H_{*}(\triangle\triangle
Q)\longrightarrow H_{*}(\triangle\triangle(P\times Q))$

Let $\mathcal{A}$ be an arrangement in W and $\mathit{u},\mathit{v}\in P$.

To describe the products on
$H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$, we have to know the
products on $H_{*}(\triangle\triangle[W,\mathit{u}])$ and
$H_{*}(\triangle\triangle[W,\mathit{v}])\quad\forall\quad\mathit{u},\mathit{v}\in
P$

When $\mathit{u}+\mathit{v}=W$,
$(\pi^{\mathit{u}\cap\mathit{v},\mathit{v}},\pi^{\mathit{u}\cap\mathit{v},\mathit{u}}):W/(\mathit{u}\cap\mathit{v})\rightarrow
W/\mathit{v}\times W\mathit{u}$ is an isomorphism.
$\epsilon_{\mathit{u},\mathit{v}}$ be the degree of this linear isomorphism.

The join operator

$\vee:$ $[W,\mathit{v}]$ $\times$ $[W,\mathit{u}]$ $\longrightarrow$
$[W,\mathit{v}\cap\mathit{u}]$ $(z$ , $w)$ $\longmapsto$ $z\cap w$

induces a simplicial map of pairs
$\vee:\Delta\Delta[W,\mathit{v}]\times\Delta\Delta[W,\mathit{u}]\rightarrow\Delta\Delta[W,\mathit{u}\cap\mathit{v}]$.

The product is given in the same way as that of intersection posets defined
above. We will get the following proposition.

######  Proposition 3.2.

Let $\mathcal{A}$ be an arrangement in W and $\mathit{u},\mathit{v}$
intersections in $\mathcal{A}$, such that $\mathit{u}+\mathit{v}=W$. Given
generic points $x^{\mathit{u}}$ and $x^{\mathit{v}}$ we have for $a\in
H_{k}(\Delta\Delta[W,\mathit{u}])$, $b\in H_{l}(\Delta\Delta[W,\mathit{v}])$,
and the generic points $x^{\mathit{u}\cap\mathit{v}}$ constructed above, that
$\pi_{!}^{\mathit{u}}(\phi_{*}^{x^{\mathit{u}}}(a))\bullet\pi_{!}^{\mathit{v}}(\phi_{*}^{x^{\mathit{v}}}(b))=\epsilon_{\mathit{u},\mathit{v}}(-1)^{l(d(W)-d(\mathit{u}))}\pi_{!}^{\mathit{u}\cap\mathit{v}}(\phi_{*}^{x^{\mathit{u}\cap\mathit{v}}}(\vee_{*}(a\times
b)))$.

When $\mathit{u}+\mathit{v}\neq W$, there exists a non-trivial linear
functional $\Lambda:W\rightarrow\mathbb{R}$ with the kernel containing
$\mathit{u}+\mathit{v}$. This induces functionals
$\Lambda_{\mathit{u}},\Lambda_{\mathit{v}}$ on $W/\mathit{u},W/\mathit{v}$
respectively. then we can choose generic points $x^{\mathit{u}}$ and
$y^{\mathit{v}}$ such that

$\Lambda_{\mathit{u}}(x_{\mathit{w}}^{\mathit{u}})\geq 0$ | $\forall\mathit{w}\in(W,\mathit{u}],$ |  $\Lambda_{\mathit{u}}(x_{\mathit{W}}^{\mathit{u}})>0$,
---|---|---
$\Lambda_{\mathit{v}}(y_{\mathit{w}}^{\mathit{v}})\leq 0$ | $\forall\mathit{w}\in(W,\mathit{v}],$ | $\Lambda_{\mathit{v}}(y_{\mathit{W}}^{\mathit{v}})<0.$

######  Proposition 3.3.

Let $\mathcal{A}$ be an arrangemnet in W and $\mathit{u},\mathit{v}$
intersections in $\mathcal{A}$ with $\mathit{u}+\mathit{v}\neq W$. Then for
generic points $x^{\mathit{u}}$ and $y^{\mathit{v}}$ constructed above, the
composition
$H_{*}(\Delta\Delta[W,\mathit{u}])\bigotimes
H_{*}(\Delta\Delta[W,\mathit{v}])\\\ \hskip
28.45274pt\xrightarrow{(\pi_{!}^{\mathit{u}}\circ\phi_{*}^{x^{\mathit{u}}})\bigotimes(\pi_{!}^{\mathit{v}}\circ\phi_{*}^{y^{\mathit{v}}})}H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})\bigotimes
H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})\\\ \hskip
227.62204pt\xrightarrow{\quad\bullet\quad}H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$
is the zero map.

The proof of Prop 3.2 and Prop 3.3 sees [12].

##  4\. the cohomology of local representation

In Prop 3.1, Prop 3.2 and Prop 3.3, Goresky-MacPherson isomorphism and
intersection product of homology groups depend on the choice of generic points
$x^{\mathit{u}}$. If the arrangements is a $(\geq 2)$-arrangement (that is
$\forall\mathit{u},\mathit{v}\in P$, if $\mathit{u}<\mathit{v}$, then
$d(\mathit{u})-d(\mathit{v})\geq 2$), by Lemma 5.1 in [12], Goresky-MacPherson
isomorphism is independent of the choice of generic points, in that case one
can obtain a complete combinatorial description of the intersection product,
furthermore one can describe the integral cohomology ring structure of
$W\setminus\bigcup\mathcal{A}$.

However, it is not easy for real arrangements to satisfy $(\geq 2)$ condition.
There are many cases for real arrangements such that the intersection of two
subspaces decreases by only one dimension, and this is the key reason why real
arrangements are much more difficult to deal with than complex arrangements.

In our concerning case, it is a pity that
$F_{\mathbb{Z}_{2}^{m}}(\mathbb{R}^{m},n)$ doesn’t satisfy the $(\geq 2)$
condition, so we can’t get rid of the influence of the choice of generic
points. But if we consider the mod 2 cohomology ring instead of the integral
cohomology ring, we can overcome this barrier. Thus we can get the following
lemma easily.

######  Lemma 4.1.

Under $\mathbb{Z}_{2}$-coefficient, Goresky-MacPherson isomorphism and
intersection products depend only on the intersection poset $P$.

###### Proof.

For arbitary two generic points $x^{\mathit{u}},\tilde{x}^{\mathit{u}}$, let
$a\in C_{k}(\Delta\Delta[W,\mathit{u}])$, $\phi^{x^{\mathit{u}}}(a)$ and
$\phi^{\tilde{x}^{\mathit{u}}}(a)$ differ merely by an orientation. Thus
$\phi^{x^{\mathit{u}}}_{*}=\phi^{\tilde{x}^{\mathit{u}}}_{*}(mod\quad 2)$. It
means the Goresky-MacPherson isomorphism and intersection product of homology
groups are independent of the choice of generic points over ${\mathbb{Z}}_{2}$
coefficient. ∎

Furthermore, the intersection products can be depicted as follows:

######  Theorem 4.2.

For an arrangement $\mathcal{A}$ with intersection poset $P$, under
$\mathbb{Z}_{2}$-coefficients, the intersection product is given by the
combinatorial data as follows.

$H_{k}(\Delta\Delta[W,\mathit{u}])\bigotimes H_{l}(\Delta\Delta[W,\mathit{v}])$ | $\longrightarrow$ | $H_{k+l}(\Delta\Delta[W,\mathit{u}\vee\mathit{v}])$
---|---|---
$a\bigotimes b$ | $\longmapsto$ | $\Big{\\{}\begin{matrix}\vee_{*}(a\times b)&\mathit{u}+\mathit{v}=W\\\ 0&otherwise\end{matrix}$

###### Proof.

Immediately get by Prop 3.1, Prop 3.2 and Prop 3.3 via below sequence of maps

$H_{*}(\Delta\Delta[W,\mathit{u}])\bigotimes
H_{*}(\Delta\Delta[W,\mathit{v}])\longrightarrow
H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})\otimes
H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$
$\overset{\bullet}{\longrightarrow}H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$
$\overset{\cong}{\longleftarrow}\underset{\mathit{w}\in[W,\top]}{\bigoplus}H_{*}(\Delta\Delta[W,\mathit{w}])$
∎

In [11], Yuzvinsky construct differential graded algebra based on atomic
complex to describe the rational cohomology ring of complement sapce of
complex subspace arrangement. In this paper, we adapt his construction with
different definition of order of elements.

Since $H_{k}(\Delta\Delta[W,\mathit{u}])\triangleq
H_{k}([W,\mathit{u}],(W,\mathit{u}]\cup[W,\mathit{u}))$

Let $\sigma\in
C_{k}(\Delta\Delta[W,\mathit{u}]),\sigma\sim(W<\mathit{u}_{1}<\mathit{u})$,
$\mathit{u}_{1}\in C_{k-2}(\triangle(W,\mathit{u}))$

and
$(W<\mathit{u}_{1}<\mathit{u})\sim(W<\mathit{u}_{2}<\mathit{u})\Longleftrightarrow\mathit{u}_{1}\sim\mathit{u}_{2}$

$\therefore
H_{k}(\Delta\Delta[W,\mathit{u}])\cong\widetilde{H}_{k-2}(\triangle(W,\mathit{u}))\qquad
k\geq 2$

$H_{1}(\Delta\Delta[W,\mathit{u}])=\bigg{\\{}\begin{matrix}\mathbb{Z}_{2}&\text{if
u is an atom}\\\ 0&otherwise\end{matrix}$

Now we determine the homology of $\triangle(W,\mathit{u})$.

Let $\mathcal{A}=\\{A_{1},\dots,A_{p}\\}$ be an subspace arrangement, which
are called atoms. $P$ be the intersection poset.

$\sigma\subset\mathcal{A}$. $\sigma=\\{A_{i_{1}},\dots,A_{i_{k}}\\}$,
$\vee(\sigma)=A_{i_{1}}\cap\dots\cap A_{i_{k}}$.

Construct atomic complex
$A(P)=\\{\sigma\subset\mathcal{A}|\vee(\sigma)<\top\\}$
($\top\triangleq\bigcap\mathcal{A}$ ).

By Lemma 2.1 in [13] , $A(P)$ is homotopy equivalent to order complex
$\triangle(W,\top)$

For $\mathit{u}\in P$, define
$\mathcal{A}_{u}=\\{A_{i}\in\mathcal{A}|\mathit{u}\subset A_{i}\\}$. Its
corresponding intesection poset is denoted by $P_{\mathit{u}}$. Then
$A(P_{\mathit{u}})\simeq\triangle(W,\mathit{u})$

######  Definition 4.3.

The relative atomic (chain) complex $D=D(P)$ is the free abelian group on all
subsets $\sigma=\\{A_{i_{1}},\ldots,A_{i_{p}}\\}\subset\mathcal{A}$,
$dim(\sigma)=|\sigma|$.with its differential

$\partial:$ $C_{n}$ $\rightarrow$ $C_{n-1}$ $\sigma$ $\mapsto$
$\sum(-1)^{j}\sigma\setminus{A_{i_{j}}}$ .

where the summation is taken over index j such that
$\vee(\sigma\setminus{A_{i_{j}}})=\vee(\sigma)$.

In fact, reative atomic complex $D$ can be represented as the direct sum of
complexes. Let $\sum(\mathit{u})$ denotes the simplicial complex whose
simplices are all the subsets of $\mathcal{A}_{\mathit{u}}$, denote
$\overline{A(\mathit{u})}=\sum(\mathit{u})/A(P_{\mathit{u}})=\\{\sigma\subset{\mathcal{A}_{\mathit{u}}}|\vee(\sigma)=\mathit{u}\\}$,
let $D(\mathit{u})=C(\overline{A(\mathit{u})})$. Obviously, $\sum(\mathit{u})$
is acyclic. The following lemma is immediate by easy calculation of simplicial
homology group.

######  Lemma 4.4.

  1. (1)

$\widetilde{H}_{p}(D(\mathit{u}))\simeq\widetilde{H}_{p-2}(A(P_{\mathit{u}}))$

  2. (2)

$D=\underset{\mathit{u}\in P}{\bigoplus}D(\mathit{u})$

So $\underset{\mathit{u}\in
P}{\bigoplus}H_{k}(\Delta\Delta[W,\mathit{u}])\cong\underset{\mathit{u}\in
P}{\bigoplus}\widetilde{H}_{k-2}(\triangle(W,\mathit{u}))\cong\underset{\mathit{u}\in
P}{\bigoplus}\widetilde{H}_{k-2}(A(P_{\mathit{u}}))$

$\cong\underset{\mathit{u}\in
P}{\bigoplus}\widetilde{H}_{k}D(\mathit{u})\cong\widetilde{H}_{k}(D)\cong
H_{k}(D)\qquad k\geq 2$

When k=1, $\underset{\mathit{u}\in
P}{\bigoplus}H_{1}(\Delta\Delta[W,\mathit{u}])\cong H_{1}(D)$

Combine them together, $\underset{\mathit{u}\in
P}{\bigoplus}H_{k}(\Delta\Delta[W,\mathit{u}])\cong H_{k}(D)$ for $k\geq 1$

Now turn to the product, look at $D(\mathit{u})$, it is generated by
$\sigma\in\mathcal{A}_{\mathit{u}},s.t.\vee(\sigma)=\mathit{u}$

Recall two maps (2.1) and (2.2).

$H_{k}(\Delta\Delta[W,\mathit{u}])\xrightarrow{\pi_{!}^{\mathit{u}}\circ\phi_{*}^{x^{\mathit{u}}}}H_{d(\mathit{u})+k}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})\xleftarrow[\cong]{\cap[W]\circ\mathit{i}^{*}}H^{d(W)-d(\mathit{u})-k}(W\setminus\bigcup\mathcal{A})$

To describe the cup product, we are to define a new differential graded
algebra $\widetilde{D}$ .

######  Definition 4.5.

differential graded algebra $\widetilde{D}$ is the free abelian group on all
subsets $\sigma=\\{A_{i_{1}},\ldots,A_{i_{p}}\\}\subset\mathcal{A}$ let
$deg(\sigma)=d(W)-\left|\sigma\right|-d(\vee(\sigma))$, with its differential

$\delta:$ $C^{n}$ $\rightarrow$ $C^{n+1}$ $\sigma$ $\mapsto$
$\sum(-1)^{j}\sigma\setminus{A_{i_{j}}}$ .

where the summation is taken over index j such that
$\vee(\sigma\setminus{A_{i_{j}}})=\vee(\sigma)$. and define the multiplication
on algebra $\widetilde{D}$ as following:
$\sigma\circ\tau=\bigg{\\{}\begin{matrix}\sigma\cup\tau&if\vee(\sigma)+\vee(\tau)=W\\\
0&otherwise\end{matrix}$

Thus according to Goresky-MacPherson isomorphism and Alexander Duality, we get
our result

######  Theorem 4.6.

there is a ring isomorphism $H^{*}(M(\mathcal{A});{\mathbb{Z}}_{2})\cong
H^{*}(\widetilde{D};{\mathbb{Z}}_{2})$

###### Proof.

We have proved $\underset{\mathit{u}\in
P}{\bigoplus}H_{k}(\Delta\Delta[W,\mathit{u}])\cong H_{k}(D)$ as group,
together with Goresky-MacPherson isomorphism:

$\sum_{\mathit{u}\in[W,\top]}\pi_{!}^{\mathit{u}}\circ\phi_{*}^{x^{\mathit{u}}}:\bigoplus_{\mathit{u}\in[W,T]}H_{*}(\Delta\Delta[W,\mathit{u}])\longrightarrow
H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$

we get $H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})\cong H_{*}(D)$
as groups.

Since we have

$H^{k}(W\setminus\bigcup\mathcal{A})\xrightarrow[\cong]{i^{*}}H^{k}(B_{\epsilon}\setminus\bigcup\mathcal{A})\xrightarrow[\cong]{\cap[W]}H_{d(W)-k}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$

And $H^{*}(\widetilde{D})$ is dual to $H_{*}(D)$, so
$H^{*}(W\setminus\bigcup\mathcal{A})\cong H^{*}(\widetilde{D})$ as group.

Now we turn to product. Intersection product in Theorem 4.2 corresponds to cup
product. In $H^{*}(\widetilde{D})$, the product defined in
$H^{*}(\widetilde{D})$ dual to intersection product in
$H_{*}(W,\bigcup\mathcal{A}\cup\mathcal{C}B_{\epsilon})$ , therefore agree
with cup product. so there is a ring isomorphism
$H^{*}(W\setminus\bigcup\mathcal{A})\cong H^{*}(\widetilde{D})$. ∎

The shortcoming of rational model method is that we can not read the
generators and relations explictly from the differential graded algebra. In my
opinion, this shortcoming comes from the complexity of real arrangements.

##  5\. Example

We take $F_{\mathbb{Z}_{2}^{2}}(\mathbb{R}^{2},2)$ as an example to verify the
Theorem 4.6.

Because
$F_{\mathbb{Z}_{2}^{2}}(\mathbb{R}^{2},2)={\mathbb{C}}^{2}\setminus\mathcal{A}\qquad\mathcal{A}=\\{H_{1},H_{2},H_{3},H_{4}\\}$
where

$H_{1}=\\{((x_{1},y_{1}),(x_{2},y_{2}))\in{\mathbb{C}}\times{\mathbb{C}}|x_{1}=x_{2},y_{1}=y_{2}\\}$

$H_{2}=\\{((x_{1},y_{1}),(x_{2},y_{2}))\in{\mathbb{C}}\times{\mathbb{C}}|x_{1}=x_{2},y_{1}=-y_{2}\\}$

$H_{3}=\\{((x_{1},y_{1}),(x_{2},y_{2}))\in{\mathbb{C}}\times{\mathbb{C}}|x_{1}=-x_{2},y_{1}=-y_{2}\\}$

$H_{4}=\\{((x_{1},y_{1}),(x_{2},y_{2}))\in{\mathbb{C}}\times{\mathbb{C}}|x_{1}=-x_{2},y_{1}=y_{2}\\}$

The differential graded algebra $\widetilde{D}$ is stated as following:

$deg(\emptyset)=4-0-4=0$

$\delta(\emptyset)=0$

$deg(\sigma)=d(W)-\left|\sigma\right|-d(\vee(\sigma))$

$deg(1)=deg(2)=deg(3)=deg(4)=4-1-2=1$

$\delta(1)=\delta(2)=\delta(3)=\delta(4)=0$

$deg(12)=deg(14)=deg(23)=deg(34)=4-2-1=1$

$deg(13)=deg(24)=4-2-0=2$

$\delta(12)=\delta(23)=\delta(34)=\delta(14)=\delta(24)=\delta(13)=0$

$deg(123)=deg(124)=deg(234)=deg(134)=4-3-0=1$

$\delta(123)=\delta(134)=13,\hskip 14.22636pt\delta(124)=\delta(234)=24$

$deg(1234)=4-4-0=0$

$\delta(1234)=123+124+134+234$

Through easy caculation, we choose
(1),(2),(3),(4),(12),(14),(23),(24),(123)+(134) as the generators of
$H^{1}(F_{\mathbb{Z}_{2}^{2}}(\mathbb{R}^{2},2),{\mathbb{Z}}_{2})$,
$(\emptyset)$ as the generators of
$H^{2}(F_{\mathbb{Z}_{2}^{2}}(\mathbb{R}^{2},2),{\mathbb{Z}}_{2})$, thus
$H^{k}(F_{\mathbb{Z}_{2}^{2}}(\mathbb{R}^{2},2),{\mathbb{Z}}_{2})=\bigg{\\{}\begin{matrix}0&k\geq
2\\\ {\mathbb{Z}}_{2}^{9}&k=1\\\ {\mathbb{Z}}_{2}&k=0\end{matrix}$, where
products vanish.

On the other hand, since
$F_{\mathbb{Z}_{2}^{2}}(\mathbb{R}^{2},2)={\mathbb{C}}^{2}\setminus\mathcal{A}$,
it can deforme retract to a torus with eight points removed, which has
homotopy type of the wedge of nine circles. Thus, the caculation of
$H^{k}(F_{\mathbb{Z}_{2}^{2}}(\mathbb{R}^{2},2),{\mathbb{Z}}_{2})$ agrees in
two different ways —geometric and combinatoric.

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