Oriented cover
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(I explain the 'convention' for orienting orientation double covers) |
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− | == | + | == Orientations of the orientation double cover == |
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− | In the case that the manifold $M$ is already orientatable, the orientation double cover $(M^w,\Zz_2,\id_{\Zz_2})$ consists of two disjoint copies of $M$ | + | Let $M^m$ be a connected manifold and let $(M^w,\Zz_2,\id_{\Zz_2})$ be the orientation double cover. There is a short exact sequence of $\Z[\Z_2]$-modules |
+ | $$\xymatrix@R=1mm{0 \ar[r] &\Z^{-} \ar[r] & \Z[\Z_2] \ar[r] & \Z \ar[r] &0 \\ & 1 \ar@{|->}[r] & 1-T, && \\ &&a+bT \ar@{|->}[r] & a+b. &} $$ | ||
+ | Applying $-\otimes_{\Z[\Z_2]} S(M^w)$ we obtain another short exact sequence which induces the following exact sequence in homology: | ||
+ | $$\xymatrix{\ldots \ar[r] & H_{n+1}(M) \ar[r] & H_n(M;\Z^w) \ar[r] & H_n(M^w) \ar[r]^-{p_*} & H_n(M) \ar[r] & \ldots}$$ | ||
+ | As $M$ is $m$-dimensional we have that $H_{m+1}(M)=0$ and so by the long exact sequence $$ H_m(M;\Z^w)\cong \ker(p_*:H_m(M^w)\to H_m(M)).$$ | ||
+ | In other words, a $w$-twisted fundamental class of a connected manifold $M$ can be thought of as a fundamental class of the orientation double cover that projects to zero in $H_m(M)$. In the case that the manifold $M$ is already orientatable, the orientation double cover $(M^w,\Zz_2,\id_{\Zz_2})$ consists of two disjoint copies of $M$ and a $w$-twisted orientation corresponds to an orientation of $M^w$ where the two copies of $M$ are given opposite orientations. In the case that $M$ is non-orientable $H_m(M;\Z)=0$ so a $w$-twisted orientation is precisely an orientation of the cover $M^w$. | ||
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Revision as of 18:07, 28 May 2014
This page has not been refereed. The information given here might be incomplete or provisional. |
Contents |
1 Introduction
Lemma 1.1. The cover is orientable if and only if the orientation character factors through .
Proof. Consider the diagram
If there exists a such that , then . Conversely if then the map
for any representative of is well defined and factors .
In light of this we make the following definition.
2 Definition
An orientable cover of a (connected) space with an orientation character is a regular covering of with group of covering translations , together with an orientation character such that
3 Lifts correspond to orientations
Let be an oriented cover of a connected manifold . A choice of lift corresponds to a choice of -twisted fundamental class . Given a lift a fundamental class is uniquely determined by setting its restriction to to be and extending equivariantly. Conversely, given a fundamental class define to be the lift such that the restriction of to is .
4 Examples
The two most important examples of oriented covers are the universal cover and the orientation double cover . These correspond to the two extreme cases of factoring the orientation character via and respectively. Every oriented cover is a regular cover of and has as a regular cover - this is a consequence of the fact that if are normal subgroups of with a subgroup of then is a normal subgroup of . Thus for any oriented cover of the following diagram commutes
5 Orientations of the orientation double cover
Let be a connected manifold and let be the orientation double cover. There is a short exact sequence of -modules
Applying we obtain another short exact sequence which induces the following exact sequence in homology:
In other words, a -twisted fundamental class of a connected manifold can be thought of as a fundamental class of the orientation double cover that projects to zero in . In the case that the manifold is already orientatable, the orientation double cover consists of two disjoint copies of and a -twisted orientation corresponds to an orientation of where the two copies of are given opposite orientations. In the case that is non-orientable so a -twisted orientation is precisely an orientation of the cover .
6 References
- [Hatcher2002] A. Hatcher, Algebraic topology, Cambridge University Press, 2002. MR1867354 (2002k:55001) Zbl 1044.55001
- [Ranicki2002] A. Ranicki, Algebraic and geometric surgery, The Clarendon Press Oxford University Press, Oxford, 2002. MR2061749 (2005e:57075) Zbl 1003.57001
Lemma 1.1. The cover is orientable if and only if the orientation character factors through .
Proof. Consider the diagram
If there exists a such that , then . Conversely if then the map
for any representative of is well defined and factors .
In light of this we make the following definition.
2 Definition
An orientable cover of a (connected) space with an orientation character is a regular covering of with group of covering translations , together with an orientation character such that
3 Lifts correspond to orientations
Let be an oriented cover of a connected manifold . A choice of lift corresponds to a choice of -twisted fundamental class . Given a lift a fundamental class is uniquely determined by setting its restriction to to be and extending equivariantly. Conversely, given a fundamental class define to be the lift such that the restriction of to is .
4 Examples
The two most important examples of oriented covers are the universal cover and the orientation double cover . These correspond to the two extreme cases of factoring the orientation character via and respectively. Every oriented cover is a regular cover of and has as a regular cover - this is a consequence of the fact that if are normal subgroups of with a subgroup of then is a normal subgroup of . Thus for any oriented cover of the following diagram commutes
5 Orientations of the orientation double cover
Let be a connected manifold and let be the orientation double cover. There is a short exact sequence of -modules
Applying we obtain another short exact sequence which induces the following exact sequence in homology:
In other words, a -twisted fundamental class of a connected manifold can be thought of as a fundamental class of the orientation double cover that projects to zero in . In the case that the manifold is already orientatable, the orientation double cover consists of two disjoint copies of and a -twisted orientation corresponds to an orientation of where the two copies of are given opposite orientations. In the case that is non-orientable so a -twisted orientation is precisely an orientation of the cover .
6 References
- [Hatcher2002] A. Hatcher, Algebraic topology, Cambridge University Press, 2002. MR1867354 (2002k:55001) Zbl 1044.55001
- [Ranicki2002] A. Ranicki, Algebraic and geometric surgery, The Clarendon Press Oxford University Press, Oxford, 2002. MR2061749 (2005e:57075) Zbl 1003.57001
Lemma 1.1. The cover is orientable if and only if the orientation character factors through .
Proof. Consider the diagram
If there exists a such that , then . Conversely if then the map
for any representative of is well defined and factors .
In light of this we make the following definition.
2 Definition
An orientable cover of a (connected) space with an orientation character is a regular covering of with group of covering translations , together with an orientation character such that
3 Lifts correspond to orientations
Let be an oriented cover of a connected manifold . A choice of lift corresponds to a choice of -twisted fundamental class . Given a lift a fundamental class is uniquely determined by setting its restriction to to be and extending equivariantly. Conversely, given a fundamental class define to be the lift such that the restriction of to is .
4 Examples
The two most important examples of oriented covers are the universal cover and the orientation double cover . These correspond to the two extreme cases of factoring the orientation character via and respectively. Every oriented cover is a regular cover of and has as a regular cover - this is a consequence of the fact that if are normal subgroups of with a subgroup of then is a normal subgroup of . Thus for any oriented cover of the following diagram commutes
5 Orientations of the orientation double cover
Let be a connected manifold and let be the orientation double cover. There is a short exact sequence of -modules
Applying we obtain another short exact sequence which induces the following exact sequence in homology:
In other words, a -twisted fundamental class of a connected manifold can be thought of as a fundamental class of the orientation double cover that projects to zero in . In the case that the manifold is already orientatable, the orientation double cover consists of two disjoint copies of and a -twisted orientation corresponds to an orientation of where the two copies of are given opposite orientations. In the case that is non-orientable so a -twisted orientation is precisely an orientation of the cover .
6 References
- [Hatcher2002] A. Hatcher, Algebraic topology, Cambridge University Press, 2002. MR1867354 (2002k:55001) Zbl 1044.55001
- [Ranicki2002] A. Ranicki, Algebraic and geometric surgery, The Clarendon Press Oxford University Press, Oxford, 2002. MR2061749 (2005e:57075) Zbl 1003.57001