Orientation covering

Revision as of 16:19, 2 January 2013

1 Construction

The orientation covering of a topological manifold ${{Stub}}{{Authors|Matthias Kreck}} == Construction == <wikitex include="TeXInclude:">; The orientation covering of a topological manifold $M$ is a canonical two-fold covering of $M$. {{beginthm|Theorem|c.f. \cite{Dold1995|VIII 2.11}}} Let $M$ be a $n$-dimensional topological manifold. There is an oriented manifold $\hat M$ and a $-fold covering $p \colon \hat M \to M$ called the orientation covering. If $M$ is a smooth, resp. piecewise linear, manifold then $\hat M$ and the covering map $p$ are smooth, resp. piecewise linear. {{endthm}} {{beginrem|Remark}} The covering $p \colon \hat M \to M$ is called the '''orientation covering'''. For more information, see \cite{Dold1995|VIII § 2}. {{endrem}} {{beginproof}} As a set $\hat M$ is the set of pairs $(x, o_x)$, where $o_x$ is a local orientation of $M$ at $x$, either given by a generator of $H_n(M, M-x;\mathbb Z)$ or by an orientation of $T_xM$ in the smooth case (for the equivalence of these data see the atlas page on orientation of manifold). The map $p$ assigns $x$ to $(x,o_x)$. Since there are precisely two local orientations, the fibers of this map have cardinality $. Next we define a topology on this set. Let $\varphi : U \to V\subset \mathbb R^n$ be a [[Wikipedia:hart_(topology)#Charts|chart]] of $M$ (smooth, if $M$ is smooth). We orient $\mathbb R^n$ by the standard orientation given by the standard basis $e_1$, $e_2$, ..., $e_n$, from which we define a continuous local orientation by identifying the tangent space with $\mathbb R^n$. Since for a smooth manifold a tangential orientation defines a homological orientation, this also gives a homological orientation (see atlas page on orientation of manifolds). We call the standard local orientation at $x \in \mathbb R^n$ by $sto_x$. Using the chart we transport this standard orientation to $U$ by the induced map on homology or the differential in the case of tangential orientations. The local orientations given by this orientation of $U$ is a subset of $\hat M$, which we require to be open. Doing the same starting with the non-standard orientation of $\mathbb R^n$ we obtain another subset, which we also call open. We give $\hat M$ the topology generated by these open subsets, where we vary over all charts of $M$ (smooth charts, if $M$ is smooth). By construction these open subsets are homeomorphic to an open subset of $\mathbb R^n$, and so we obtain an atlas of $\hat M$. In the smooth case this is a smooth atlas making $\hat M$ a smooth manifold. The map $p$ is by construction a $-fold covering, smooth, if $M$ is smooth. By construction $\hat M$ is oriented in a tautological way. Thus we have constructed a $-fold covering of $M$ by an oriented manifold $\hat M$, which is smooth, if $M$ is smooth. {{endproof}} </wikitex> == Properties == <wikitex include="TeXInclude:">; In this section we record the key properties of the orientation cover are given in Proposition \ref{prop:properties} below. The orientation covering of a manifold $M$ is very closely related to the [[Wikipedia:Orientation_characater|orientation character]] of $M$. This is a homomorphism $$ w \colon \pi_1(M) \to \Z/2 $$ which may be defined as follows. Fix a base-point $x \in M$ with lifts $\tilde x_1$ and $\tilde x_{-1}$ in $\hat M$. For a loop $\gamma \colon ([0, 1], \{0, 1\}) \to (M, x)$ based at $x$, let $\tilde \gamma \colon [0, 1] \to M$ be the lift of $\gamma$ with $\tilde{\gamma}(0) = 1$ and define $w$ on the homotopy class of $\gamma$ by $$ w([\gamma]) := \left\{ \begin{array}{rc} 1 & \text{if $\tilde{\gamma}(1) = x_1$} \ -1 & \text{if $\tilde{\gamma}(1) = x_{-1}$} \end{array} \right. $$ {{beginthm|Proposition}} \label{properties} Let $p \colon \hat M \to M$ be the orientation covering of a topological manifold $M$. # $M$ is orientable if and only if $\hat M = M \times \Z/2$ and $p$ is the projection to $M$. # Converely, if $M$ is connected then $M$ is non-orientable if and only if $\hat M$ is connected. # $M$ is orientable if and only if $w \colon \pi_1(M) \to \Z/2$ is the zero homomorphism. # By construction, the [[Wikipedia:Covering_space#Deck_transformation_group.2C_regular_covers|deck transformation]] of orientation covering is orientation reversing. # If $N $ is an oriented manifold and $p: N \to M$ is a $-fold covering with orientation reversing deck transformation, then $p \colon N \to M$ is isomorphic to the orientation covering. {{endthm}} {{beginproof}} (1.) If $M$ is orientable, we pick an orientation and see that $\hat M$ is the disjoint union of $\{(x,o_x)| \,\, o_x \,\, is \,\, the \,\, local \,\, orientation \,\, given \,\, by \,\, the \,\, orientation \,\, of \,\, M\}$ and its complement, so it is isomorphic to the trivial covering $ M \times \mathbb Z/2$. In turn if the orientation covering is trivial it decomposes $\hat M$ into to open (and thus oriented) subsets homeomorphic to $M$ and so $M$ is orientable. (2.) Follows immediately from (1.) since a two-fold cover of a connected space is non-trivial if and only if the total space of the covering is disconnected. (3.) Follows from (1.) since $w$ classifies the orientation cover: see the page [[Wikipedia:Orientation_character|orientation character]]. (4.) Is true by construciton as stated. <!-- By construction of $\hat M$ the deck transformation of the covering is orientation reversing. --> (5.) We have a map $N \to \hat M$ by mapping $y \in N$ to $(p(y), orientation \,\, induced \,\, by \,\, p)$. It is easily checked that his is an isomorphism of these two coverings. <!-- By the considerations above, $M$ is orientable if and only if this covering is trivial, or $M$ is non-orientable if and only if $N$ is connected. --> {{endproof}} </wikitex> == Examples == <wikitex>; We give a list of basic in interesting orientation double coverings. # If $M$ is orientable then $p \colon \hat M \to M$ is isomorphic to the projection $M \times \Z/2 \to M$. # If $n$ is even, $\Rr P^n$ is non-orienable and with orientation cover $S^n \to \Rr P^n$. The deck transformation is the [[Wikipedia:Antipodal_point|antipodal map]] on $S^n$. # The orientation cover of the Klein bottle $K^2$ is the projection from the [[$-torus]]; $T^2 \to K^2$. # The orientation of the open [[Wikipedia:Mobius_strip|Möbius strip]] $Mö$ is the cylinder; $S^1 \times \Rr \to Mö$. </wikitex> ==References== {{#RefList:}} == External links == * The Encylopedia of Mathematics article on [http://www.encyclopediaofmath.org/index.php/Orientation orientation]. * The Wikipedia page on the [[Wikipedia:Orientability#Orientable_double_cover|orientability]]. [[Category:Theory]]M$ is a canonical two-fold covering of $M$.

Proof. As a set $\hat M$ is the set of pairs $(x, o_x)$, where $o_x$ is a local orientation of $M$ at $x$, either given by a generator of $H_n(M, M-x;\mathbb Z)$ or by an orientation of $T_xM$ in the smooth case (for the equivalence of these data see the atlas page on orientation of manifold). The map $p$ assigns $x$ to $(x,o_x)$. Since there are precisely two local orientations, the fibers of this map have cardinality $2$.

Next we define a topology on this set. Let $\varphi : U \to V\subset \mathbb R^n$ be a chart of $M$ (smooth, if $M$ is smooth). We orient $\mathbb R^n$ by the standard orientation given by the standard basis $e_1$, $e_2$, ..., $e_n$, from which we define a continuous local orientation by identifying the tangent space with $\mathbb R^n$. Since for a smooth manifold a tangential orientation defines a homological orientation, this also gives a homological orientation (see atlas page on orientation of manifolds). We call the standard local orientation at $x \in \mathbb R^n$ by $sto_x$. Using the chart we transport this standard orientation to $U$ by the induced map on homology or the differential in the case of tangential orientations. The local orientations given by this orientation of $U$ is a subset of $\hat M$, which we require to be open. Doing the same starting with the non-standard orientation of $\mathbb R^n$ we obtain another subset, which we also call open. We give $\hat M$ the topology generated by these open subsets, where we vary over all charts of $M$ (smooth charts, if $M$ is smooth). By construction these open subsets are homeomorphic to an open subset of $\mathbb R^n$, and so we obtain an atlas of $\hat M$. In the smooth case this is a smooth atlas making $\hat M$ a smooth manifold. The map $p$ is by construction a $2$-fold covering, smooth, if $M$ is smooth. By construction $\hat M$ is oriented in a tautological way. Thus we have constructed a $2$-fold covering of $M$ by an oriented manifold $\hat M$, which is smooth, if $M$ is smooth.

$\square$

2 Properties

In this section we record the key properties of the orientation cover are given in Proposition \ref{prop:properties} below. The orientation covering of a manifold $M$ is very closely related to the orientation character of $M$. This is a homomorphism

$$\displaystyle w \colon \pi_1(M) \to \Z/2$$

which may be defined as follows. Fix a base-point $x \in M$ with lifts $\tilde x_1$ and $\tilde x_{-1}$ in $\hat M$. For a loop $\gamma \colon ([0, 1], \{0, 1\}) \to (M, x)$ based at $x$, let $\tilde \gamma \colon [0, 1] \to M$ be the lift of $\gamma$ with $\tilde{\gamma}(0) = 1$ and define $w$ on the homotopy class of $\gamma$ by

$$\displaystyle w([\gamma]) := \left\{ \begin{array}{rc} 1 & \text{if $\tilde{\gamma}(1) = x_1$} \\ -1 & \text{if $\tilde{\gamma}(1) = x_{-1}$} \end{array} \right.$$

Proof. (1.) If $M$ is orientable, we pick an orientation and see that $\hat M$ is the disjoint union of $\{(x,o_x)| \,\, o_x \,\, is \,\, the \,\, local \,\, orientation \,\, given \,\, by \,\, the \,\, orientation \,\, of \,\, M\}$ and its complement, so it is isomorphic to the trivial covering $M \times \mathbb Z/2$. In turn if the orientation covering is trivial it decomposes $\hat M$ into to open (and thus oriented) subsets homeomorphic to $M$ and so $M$ is orientable.

(2.) Follows immediately from (1.) since a two-fold cover of a connected space is non-trivial if and only if the total space of the covering is disconnected.

(3.) Follows from (1.) since $w$ classifies the orientation cover: see the page orientation character.

(4.) Is true by construciton as stated.

(5.) We have a map $N \to \hat M$ by mapping $y \in N$ to $(p(y), orientation \,\, induced \,\, by \,\, p)$. It is easily checked that his is an isomorphism of these two coverings.

$\square$

3 Examples

We give a list of basic in interesting orientation double coverings.

1. If $M$ is orientable then $p \colon \hat M \to M$ is isomorphic to the projection $M \times \Z/2 \to M$.
2. If $n$ is even, $\Rr P^n$ is non-orienable and with orientation cover $S^n \to \Rr P^n$. The deck transformation

is the antipodal map on $S^n$.

1. The orientation cover of the Klein bottle $K^2$ is the projection from the [[$2$-torus]]; $T^2 \to K^2$.
2. The orientation of the open Möbius strip $Mö$ is the cylinder; $S^1 \times \Rr \to Mö$.

4 References

• [Dold1995] A. Dold, Lectures on algebraic topology, Springer-Verlag, 1995. MR1335915 (96c:55001) Zbl 0872.55001

5 External links

• The Encylopedia of Mathematics article on orientation.
• The Wikipedia page on the orientability.