Covering space

A covering of a topological space $X$ is a continuous map $\pi :E\rightarrow X$ with special properties.

Definition

Let $X$ be a topological space. A covering of $X$ is a continuous map

$\pi :E\rightarrow X$

s.t. there exists a discrete space $D$ and for every $x\in X$ an open neighborhood $U\subset X$, s.t. $\pi ^{-1}(U)=\displaystyle \bigsqcup _{d\in D}V_{d}$ and $\pi |_{V_{d}}:V_{d}\rightarrow U$ is a homeomorphism for every $d\in D$.

Intuitively, a covering locally project a "stack of pancakes" above an open neighborhood $U$ onto $U$

Often, the notion of a covering is used for the covering space $E$ as well as for the map $\pi :E\rightarrow X$. The open sets $V_{d}$ are called sheets, which are uniquely determined if $U$ is connected.$\,^{p.56}$ For a $x\in X$ the discrete subset $\pi ^{-1}(x)$ is called the fiber of $x$. The degree of a covering is the cardinality of the space $D$. If $E$ is path-connected, then the covering $\pi :E\rightarrow X$ is denoted as a path-connected covering.

Examples

• For every topological space $X$ there exists the covering $\pi :X\rightarrow X$ with $\pi (x)=x$, which is denoted as the trivial covering of $X.$

The space $Y=[0,1]\times \mathbb {R}$ is the covering space of $X=[0,1]\times S^{1}$. The disjoint open sets $S_{i}$ are mapped homeomorphically onto $U$. The fiber of $x$ consists of the points $y_{i}$.

• The map $r\colon \mathbb {R} \to S^{1}$ with $r(t)=(\cos(2\pi t),\sin(2\pi t))$ is a covering of the unit circle $S^{1}$. For an open neighborhood $U\subset X$ of an $x\in S^{1}$, which has positiv $\cos(2\pi t)$-value, one has: $r^{-1}(U)=\displaystyle \bigsqcup _{n\in \mathbb {Z} }(n-{\frac {1}{4}},n+{\frac {1}{4}})$.
• Another covering of the unit circle is the map $q\colon S^{1}\to S^{1}$ with $q(z)=z^{n}$ for some $n\in \mathbb {N}$. For an open neighborhood $U\subset X$ of an $x\in S^{1}$, one has: $q^{-1}(U)=\displaystyle \bigsqcup _{i=1}^{n}U$.
• A map which is a local homeomorphism but not a covering of the unit circle is $p\colon \mathbb {R_{+}} \to S^{1}$ with $p(t)=(\cos(2\pi t),\sin(2\pi t))$. There is a sheet of an open neighborhood of $(1,0)$, which is not mapped homeomorphically onto $U$.

Properties

Local homeomorphism

Since a covering $\pi :E\rightarrow X$ maps each of the disjoint open sets of $\pi ^{-1}(U)$ homeomorphically onto $U$ it is a local homeomorphism, i.e. $\pi$ is a continuous map and for every $e\in E$ there exists an open neighborhood $V\subset E$ of $e$, s.t. $\pi |_{V}:V\rightarrow \pi (V)$ is a homeomorphism.

It follows that the covering space $E$ and the base space $X$ locally share the same properties.

• If $X$ is a connected and non-orientable manifold, then there is a covering $\pi :{\tilde {X}}\rightarrow X$ of degree $2$, whereby ${\tilde {X}}$ is a connected and orientable manifold.$\,^{p.234}$
• If $X$ is a connected Lie group, then there is a covering $\pi :{\tilde {X}}\rightarrow X$ which is also a Lie group homomorphism and ${\tilde {X}}:=\{\gamma :\gamma {\text{ is a path in X with }}\gamma (0)={\boldsymbol {1_{X}}}{\text{ modulo homotopy with fixed ends}}\}$ is a Lie group. $^{p.174}$
• If $X$ is a graph, then it follows for a covering $\pi :E\rightarrow X$ that $E$ is also a graph. $^{p.85}$
• If $X$ is a connected manifold, then there is a covering $\pi :{\tilde {X}}\rightarrow X$, whereby ${\tilde {X}}$ is a connected and simply connected manifold. $^{p.32}$
• If $X$ is a connected Riemann surface, then there is a covering $\pi :{\tilde {X}}\rightarrow X$ which is also a holomorphic map $^{p.22}$ and ${\tilde {X}}$ is a connected and simply connected Riemann surface. $^{p.32}$

Factorisation

Let $p,q$ and $r$ be continuous maps, s.t. the diagram

commutes.

• If $p$ and $q$ are coverings, so is $r$. $^{p.485}$
• If $p$ and $r$ are coverings, so is $q$. $^{p.485}$

Product of coverings

Let $X$ and $X'$ be topological spaces and $p:E\rightarrow X$ and $p':E'\rightarrow X'$ be coverings, then $p\times p':E\times E'\rightarrow X\times X'$ with $(p\times p')(e,e')=(p(e),p'(e'))$ is a covering. $^{p.339}$

Equivalence of coverings

Let $X$ be a topological space and $p:E\rightarrow X$ and $p':E'\rightarrow X$ be coverings. Both coverings are called equivalent, if there exists a homeomorphism $h:E\rightarrow E'$, s.t. the diagram

commutes. If such a homeomorphism exists, then one calls the covering spaces $E$ and $E'$ isomorphic.

Lifting property

An important property of the covering is, that it satisfies the lifting property, i.e.:

Let $I$ be the unit interval and $p:E\rightarrow X$ be a covering. Let $F:Y\times I\rightarrow X$ be a continuous map and ${\tilde {F}}_{0}:Y\times \{0\}\rightarrow E$ be a lift of $F|_{Y\times \{0\}}$, i.e. a continuous map such that $p\circ {\tilde {F}}_{0}=F|_{Y\times \{0\}}$. Then there is a uniquely determined, continuous map ${\tilde {F}}:Y\times I\rightarrow E$, which is a lift of $F$, i.e. $p\circ {\tilde {F}}=F$. $^{p.60}$

If $X$ is a path-connected space, then for $Y=\{0\}$ it follows that the map ${\tilde {F}}$ is a lift of a path in $X$ and for $Y=I$ it is a lift of a homotopy of paths in $X$.

Because of that property one can show, that the fundamental group $\pi _{1}(S^{1})$ of the unit circle is an infinite cyclic group, which is generated by the homotopy classes of the loop $\gamma :I\rightarrow S^{1}$ with $\gamma (t)=(\cos(2\pi t),\sin(2\pi t))$. $^{p.29}$

Let $X$ be a path-connected space and $p:E\rightarrow X$ be a connected covering. Let $x,y\in X$ be any two points, which are connected by a path $\gamma$, i.e. $\gamma (0)=x$ and $\gamma (1)=y$. Let ${\tilde {\gamma }}$ be the unique lift of $\gamma$, then the map

$L_{\gamma }:p^{-1}(x)\rightarrow p^{-1}(y)$ with $L_{\gamma }({\tilde {\gamma }}(0))={\tilde {\gamma }}(1)$

is bijective. $^{p.69}$

If $X$ is a path-connected space and $p:E\rightarrow X$ a connected covering, then the induced group homomorphism

$p_{\#}:\pi _{1}(E)\rightarrow \pi _{1}(X)$ with $p_{\#}([\gamma ])=[p\circ \gamma ]$,

is injective and the subgroup $p_{\#}(\pi _{1}(E))$ of $\pi _{1}(X)$ consists of the homotopy classes of loops in $X$, whose lifts are loops in $E$. $^{p.61}$

Branched covering

Definitions

Holomorphic maps between Riemann surfaces

Let $X$ and $Y$ be Riemann surfaces, i.e. one dimensional complex manifolds, and let $f:X\rightarrow Y$ be a continuous map. $f$ is holomorphic in a point $x\in X$, if for any charts $\phi _{x}:U_{1}\rightarrow V_{1}$ of $x$ and $\phi _{f(x)}:U_{2}\rightarrow V_{2}$ of $f(x)$, with $\phi _{x}(U_{1})\subset U_{2}$, the map $\phi _{f(x)}\circ f\circ \phi _{x}^{-1}:\mathbb {C} \rightarrow \mathbb {C}$ is holomorphic.

If $f$ is for all $x\in X$ holomorphic, we say $f$ is holomorphic.

The map $F=\phi _{f(x)}\circ f\circ \phi _{x}^{-1}$ is called the local expression of $f$ in $x\in X$.

If $f:X\rightarrow Y$ is a non-constant, holomorphic map between compact Riemann surfaces, then $f$ is surjective $^{p.11}$ and an open map $^{p.11}$, i.e. for every open set $U\subset X$ the image $f(U)\subset Y$ is also open.

Ramification point and branch point

Let $f:X\rightarrow Y$ be a non-constant, holomorphic map between compact Riemann surfaces. For every $x\in X$ there exist charts for $x$ and $f(x)$ and there exists a uniquely determined $k_{x}\in \mathbb {N_{>0}}$, s.t. the local expression $F$ of $f$ in $x$ is of the form $z\mapsto z^{k_{x}}$. $^{p.10}$ The number $k_{x}$ is called the ramification index of $f$ in $x$ and the point $x\in X$ is called a ramification point if $k_{x}\geq 2$. If $k_{x}=1$ for an $x\in X$, then $x$ is unramified. The image point $y=f(x)\in Y$ of a ramification point is called a branch point.

Degree of a holomorphic map

Let $f:X\rightarrow Y$ be a non-constant, holomorphic map between compact Riemann surfaces. The degree $deg(f)$ of $f$ is the cardinality of the fiber of an unramified point $y=f(x)\in Y$, i.e. $deg(f):=|f^{-1}(y)|$.

This number is well-defined, since for every $y\in Y$ the fiber $f^{-1}(y)$ is discrete $^{p.20}$ and for any two unramified points $y_{1},y_{2}\in Y$, it is: $|f^{-1}(y_{1})|=|f^{-1}(y_{2})|.$ $^{p.29}$

It can be calculated by:

$\sum _{x\in f^{-1}(y)}k_{x}=deg(f)$ $^{p.29}$

Branched covering

Definition

A continuous map $f:X\rightarrow Y$ is called a branched covering, if there exists a closed set with dense complement $E\subset Y$, s.t. $f_{|X\smallsetminus f^{-1}(E)}:X\smallsetminus f^{-1}(E)\rightarrow Y\smallsetminus E$ is a covering.

Examples

• Let $n\in \mathbb {N}$ and $n\geq 2$, then $f:\mathbb {C} \rightarrow \mathbb {C}$ with $f(z)=z^{n}$ is branched covering of degree $n$, whereby $z=0$ is a branch point.
• Every non-constant, holomorphic map between compact Riemann surfaces $f:X\rightarrow Y$ of degree $d$ is a branched covering of degree $d$.

Universal covering

Let $p:{\tilde {X}}\rightarrow X$ be a simply connected covering and $\beta :E\rightarrow X$ be a covering, then there exists a uniquely determined covering $\alpha :{\tilde {X}}\rightarrow E$, s.t. the diagram

commutes. $^{p.486}$

Definition

Let $p:{\tilde {X}}\rightarrow X$ be a simply connected covering. If $\beta :E\rightarrow X$ is another simply connected covering, then there exists a uniquely determined homeomorphism $\alpha :{\tilde {X}}\rightarrow E$, s.t. the diagram

commutes. $^{p.482}$

This means that $p$ is, up to equivalence, uniquely determined and because of that universal property denoted as the universal covering of the space $X$.

Existence

A universal covering does not always exists, but the following properties guarantee the existence:

Let $X$ be a connected, locally simply connected, then there exists a universal covering $p:{\tilde {X}}\rightarrow X$.

${\tilde {X}}$ is defined as ${\tilde {X}}:=\{\gamma :\gamma {\text{ is a path in }}X{\text{ with }}\gamma (0)=x_{0}\}/{\text{ homotopy with fixed ends}}$ and $p:{\tilde {X}}\rightarrow X$ by $p([\gamma ]):=\gamma (1)$. $^{p.64}$

The topology on ${\tilde {X}}$ is constructed as follows: Let $\gamma :I\rightarrow X$ be a path with $\gamma (0)=x_{0}$. Let $U$ be a simply connected neighborhood of the endpoint $x=\gamma (1)$, then for every $y\in U$ the paths $\sigma _{y}$ inside $U$ from $x$ to $y$ are uniquely determined up to homotopy. Now consider ${\tilde {U}}:=\{\gamma .\sigma _{y}:y\in U\}/{\text{ homotopy with fixed ends}}$, then $p_{|{\tilde {U}}}:{\tilde {U}}\rightarrow U$ with $p([\gamma .\sigma _{y}])=\gamma .\sigma _{y}(1)=y$ is a bijection and ${\tilde {U}}$ can be equipped with the final topology of $p_{|{\tilde {U}}}$.

The fundamental group $\pi _{1}(X,x_{0})=\Gamma$ acts freely through $([\gamma ],[{\tilde {x}}])\mapsto [\gamma .{\tilde {x}}]$ on ${\tilde {X}}$ and $\psi :\Gamma \backslash {\tilde {X}}\rightarrow X$ with $\psi ([\Gamma {\tilde {x}}])={\tilde {x}}(1)$ is a homeomorphism, i.e. $\Gamma \backslash {\tilde {X}}\cong X$.

Examples

• $r\colon \mathbb {R} \to S^{1}$ with $r(t)=(\cos(2\pi t),\sin(2\pi t))$ is the universal covering of the unit circle $S^{1}$.
• $p\colon S^{n}\to \mathbb {R} P^{n}\cong \{+1,-1\}\backslash S^{n}$ with $p(x)=[x]$ is the universal covering of the projective space $\mathbb {R} P^{n}$ for $n>1$.
• $q\colon SU(n)\ltimes \mathbb {R} \to U(n)$ with $q(A,t)={\begin{bmatrix}\exp(2\pi it)&0\\0&I_{n-1}\end{bmatrix}}A$ is the universal covering of the unitary group $U(n)$.
• Since $SU(2)\cong S^{3}$, it follows that the quotient map $f:SU(2)\rightarrow \mathbb {Z_{2}} \backslash SU(2)\cong SO(3)$ is the universal covering of the $SO(3)$.

  

  The Hawaiian earring. Only the ten largest circles are shown.

• A topological space, which has no universal covering is the Hawaiian earring:

$X=\bigcup _{n\in \mathbb {N} }\left\{(x_{1},x_{2})\in \mathbb {R} ^{2}:{\Bigl (}x_{1}-{\frac {1}{n}}{\Bigr )}^{2}+x_{2}^{2}={\frac {1}{n^{2}}}\right\}$

One can show, that no neighborhood of the origin $(0,0)$ is simply connected. $^{p.487\,Example\,1}$

Deck transformation

Definition

Let $p:E\rightarrow X$ be a covering. A deck transformation is a homeomorphism $d:E\rightarrow E$, s.t. the diagram of continuous maps

commutes. Together with the composition of maps, the set of deck transformation forms a group $Deck(p)$, which is the same as $Aut(p)$.

Examples

• Let $q\colon S^{1}\to S^{1}$ be the covering $q(z)=z^{n}$ for some $n\in \mathbb {N}$, then the map $d:S^{1}\rightarrow S^{1}:z\mapsto z\,e^{2\pi i/n}$ is a deck transformation and $Deck(q)\cong \mathbb {Z} /\mathbb {nZ}$.
• Let $r\colon \mathbb {R} \to S^{1}$ be the covering $r(t)=(\cos(2\pi t),\sin(2\pi t))$, then the map $d_{k}:\mathbb {R} \rightarrow \mathbb {R} :t\mapsto t+k$ with $k\in \mathbb {Z}$ is a deck transformation and $Deck(r)\cong \mathbb {Z}$.

Properties

Let $X$ be a path-connected space and $p:E\rightarrow X$ be a connected covering. Since a deck transformation $d:E\rightarrow E$ is bijective, it permutes the elements of a fiber $p^{-1}(x)$ with $x\in X$ and is uniquely determined by where it sends a single point. In particular, only the identity map fixes a point in the fiber. $^{p.70}$Because of this property every deck transformation defines a group action on $E$, i.e. let $U\subset X$ be an open neighborhood of a $x\in X$ and ${\tilde {U}}\subset E$ an open neighborhood of an $e\in p^{-1}(x)$, then $Deck(p)\times E\rightarrow E:(d,{\tilde {U}})\mapsto d({\tilde {U}})$ is a group action.

Normal coverings

Definition

A covering $p:E\rightarrow X$ is called normal, if $Deck(p)\backslash X\cong E$. This means, that for every $x\in X$ and any two $e_{0},e_{1}\in p^{-1}(x)$ there exists a deck transformation $d:E\rightarrow E$, s.t. $d(e_{0})=e_{1}$.

Properties

Let $X$ be a path-connected space and $p:E\rightarrow X$ be a connected covering. Let $H=p_{\#}(\pi _{1}(E))$ be a subgroup of $\pi _{1}(X)$, then $p$ is a normal covering iff $H$ is a normal subgroup of $\pi _{1}(X)$. $^{p.71}$

If $p:E\rightarrow X$ is a normal covering and $H=p_{\#}(\pi _{1}(E))$, then $Deck(p)\cong \pi _{1}(X)/H$. $^{p.71}$

If $p:E\rightarrow X$ is a path-connected covering and $H=p_{\#}(\pi _{1}(E))$, then $Deck(p)\cong N(H)/H$, whereby $N(H)$ is the normaliser of $H$. $^{p.71}$

Let $E$ be a topological space. A group $\Gamma$ acts discontinuously on $E$, if every $e\in E$ has an open neighborhood $V\subset E$ with $V\neq \emptyset$, such that for every $\gamma \in \Gamma$ with $\gamma V\cap V\neq \emptyset$ one has $d_{1}=d_{2}$.

If a group $\Gamma$ acts discontinuously on a topological space $E$, then the quotient map $q:E\rightarrow \Gamma \backslash E$ with $q(e)=\Gamma e$ is a normal covering. $^{p.72}$ Hereby $\Gamma \backslash E=\{\Gamma e:e\in E\}$ is the quotient space and $\Gamma e=\{\gamma (e):\gamma \in \Gamma \}$ is the orbit of the group action.

Examples

• The covering $q\colon S^{1}\to S^{1}$ with $q(z)=z^{n}$ is a normal coverings for every $n\in \mathbb {N}$.
• Every simply connected covering is a normal covering.

Calculation

Let $\Gamma$ be a group, which acts discontinuously on a topological space $E$ and let $q:E\rightarrow \Gamma \backslash E$ be the normal covering.

• If $E$ is path-connected, then $Deck(q)\cong \Gamma$. $^{p.72}$

• If $E$ is simply connected, then $Deck(q)\cong \pi _{1}(X)$. $^{p.71}$

Examples

• Let $n\in \mathbb {N}$. The antipodal map $g:S^{n}\rightarrow S^{n}$ with $g(x)=-x$ generates, together with the composition of maps, a group $D(g)\cong \mathbb {Z/2Z}$ and induces a group action $D(g)\times S^{n}\rightarrow S^{n},(g,x)\mapsto g(x)$, which acts discontinuously on $S^{n}$. Because of $\mathbb {Z_{2}} \backslash S^{n}\cong \mathbb {R} P^{n}$ it follows, that the quotient map $q\colon S^{n}\rightarrow \mathbb {Z_{2}} \backslash S^{n}\cong \mathbb {R} P^{n}$ is a normal covering and for $n>1$ a universal covering, hence $Deck(q)\cong \mathbb {Z/2Z} \cong \pi _{1}({\mathbb {R} P^{n}})$ for $n>1$.
• Let $SO(3)$ be the special orthogonal group, then the map $f:SU(2)\rightarrow SO(3)\cong \mathbb {Z_{2}} \backslash SU(2)$ is a normal covering and because of $SU(2)\cong S^{3}$, it is the universal covering, hence $Deck(f)\cong \mathbb {Z/2Z} \cong \pi _{1}(SO(3))$.
• With the group action $(z_{1},z_{2})*(x,y)=(z_{1}+(-1)^{z_{2}}x,z_{2}+y)$ of $\mathbb {Z^{2}}$ on $\mathbb {R^{2}}$, whereby $(\mathbb {Z^{2}} ,*)$ is the semidirect product $\mathbb {Z} \rtimes \mathbb {Z}$, one gets the universal covering $f:\mathbb {R^{2}} \rightarrow (\mathbb {Z} \rtimes \mathbb {Z} )\backslash \mathbb {R^{2}} \cong K$ of the klein bottle $K$, hence $Deck(f)\cong \mathbb {Z} \rtimes \mathbb {Z} \cong \pi _{1}(K)$.
• Let $T=S^{1}\times S^{1}$ be the torus which is embedded in the $\mathbb {C^{2}}$. Then one gets a homeomorphism $\alpha :T\rightarrow T:(e^{ix},e^{iy})\mapsto (e^{i(x+\pi )},e^{-iy})$, which induces a discontinuous group action $G_{\alpha }\times T\rightarrow T$, whereby $G_{\alpha }\cong \mathbb {Z/2Z}$. It follows, that the map $f:T\rightarrow G_{\alpha }\backslash T\cong K$ is a normal covering of the klein bottle, hence $Deck(f)\cong \mathbb {Z/2Z}$.
• Let $S^{3}$ be embedded in the $\mathbb {C^{2}}$. Since the group action $S^{3}\times \mathbb {Z/pZ} \rightarrow S^{3}:((z_{1},z_{2}),[k])\mapsto (e^{2\pi ik/p}z_{1},e^{2\pi ikq/p}z_{2})$ is discontinuously, whereby $p,q\in \mathbb {N}$ are coprime, the map $f:S^{3}\rightarrow \mathbb {Z_{p}} \backslash S^{3}=:L_{p,q}$ is the universal covering of the lens space $L_{p,q}$, hence $Deck(f)\cong \mathbb {Z/pZ} \cong \pi _{1}(L_{p,q})$.

Galois correspondence

Let $X$ be a connected and locally simply connected space, then for every subgroup $H\subseteq \pi _{1}(X)$ there exists a path-connected covering $\alpha :X_{H}\rightarrow X$ with $\alpha _{\#}(\pi _{1}(X_{H}))=H$. $^{p.66}$

Let $p_{1}:E\rightarrow X$ and $p_{2}:E'\rightarrow X$ be two path-connected coverings, then they are equivalent iff the subgroups $H=p_{1\#}(\pi _{1}(E))$ and $H'=p_{2\#}(\pi _{1}(E'))$ are conjugate to each other. $^{p.482}$

Let $X$ be a connected and locally simply connected space, then, up to equivalence between coverings, there is a bijection:

${\begin{matrix}\qquad \displaystyle \{{\text{Subgroup of }}\pi _{1}(X)\}&\longleftrightarrow &\displaystyle \{{\text{path-connected covering }}p:E\rightarrow X\}\\H&\longrightarrow &\alpha :X_{H}\rightarrow X\\p_{\#}(\pi _{1}(E))&\longleftarrow &p\\\displaystyle \{{\text{normal subgroup of }}\pi _{1}(X)\}&\longleftrightarrow &\displaystyle \{{\text{normal covering }}p:E\rightarrow X\}\\H&\longrightarrow &\alpha :X_{H}\rightarrow X\\p_{\#}(\pi _{1}(E))&\longleftarrow &p\end{matrix}}$

For a sequence of subgroups $\displaystyle \{{\text{e}}\}\subset H\subset G\subset \pi _{1}(X)$ one gets a sequence of coverings ${\tilde {X}}\longrightarrow X_{H}\cong H\backslash {\tilde {X}}\longrightarrow X_{G}\cong G\backslash {\tilde {X}}\longrightarrow X\cong \pi _{1}(X)\backslash {\tilde {X}}$. For a subgroup $H\subset \pi _{1}(X)$ with index $\displaystyle [\pi _{1}(X):H]=d$, the covering $\alpha :X_{H}\rightarrow X$ has degree $d$.

Classification

Definitions

Category of coverings

Let $X$ be a topological space. The objects of the category ${\boldsymbol {Cov(X)}}$ are the coverings $p:E\rightarrow X$ of $X$ and the morphisms between two coverings $p:E\rightarrow X$ and $q:F\rightarrow X$ are continuous maps $f:E\rightarrow F$, s.t. the diagram

commutes.

G-Set

Let $G$ be a topological group. The category ${\boldsymbol {G-Set}}$ is the category of sets which are G-sets. The morphisms are G-maps $\phi :X\rightarrow Y$ between G-sets. They satisfy the condition $\phi (gx)=g\,\phi (x)$ for every $g\in G$.

Equivalence

Let $X$ be a connected and locally simply connected space, $x\in X$ and $G=\pi _{1}(X,x)$ be the fundamental group of $X$. Since $G$ defines, by lifting of paths and evaluating at the endpoint of the lift, a group action on the fiber of a covering, the functor $F:{\boldsymbol {Cov(X)}}\longrightarrow {\boldsymbol {G-Set}}:p\mapsto p^{-1}(x)$ is an equivalence of categories. $^{p.68-70}$

Applications

Gimbal lock occurs because any map T³ → RP³ is not a covering map. In particular, the relevant map carries any element of T³, that is, an ordered triple (a,b,c) of angles (real numbers mod 2π), to the composition of the three coordinate axis rotations R_x(a)∘R_y(b)∘R_z(c) by those angles, respectively. Each of these rotations, and their composition, is an element of the rotation group SO(3), which is topologically RP³. This animation shows a set of three gimbals mounted together to allow three degrees of freedom. When all three gimbals are lined up (in the same plane), the system can only move in two dimensions from this configuration, not three, and is in gimbal lock. In this case it can pitch or yaw, but not roll (rotate in the plane that the axes all lie in).

An important practical application of covering spaces occurs in charts on SO(3), the rotation group. This group occurs widely in engineering, due to 3-dimensional rotations being heavily used in navigation, nautical engineering, and aerospace engineering, among many other uses. Topologically, SO(3) is the real projective space RP³, with fundamental group Z/2, and only (non-trivial) covering space the hypersphere S³, which is the group Spin(3), and represented by the unit quaternions. Thus quaternions are a preferred method for representing spatial rotations – see quaternions and spatial rotation.

However, it is often desirable to represent rotations by a set of three numbers, known as Euler angles (in numerous variants), both because this is conceptually simpler for someone familiar with planar rotation, and because one can build a combination of three gimbals to produce rotations in three dimensions. Topologically this corresponds to a map from the 3-torus T³ of three angles to the real projective space RP³ of rotations, and the resulting map has imperfections due to this map being unable to be a covering map. Specifically, the failure of the map to be a local homeomorphism at certain points is referred to as gimbal lock, and is demonstrated in the animation at the right – at some points (when the axes are coplanar) the rank of the map is 2, rather than 3, meaning that only 2 dimensions of rotations can be realized from that point by changing the angles. This causes problems in applications, and is formalized by the notion of a covering space.

See also

• Bethe lattice is the universal cover of a Cayley graph
• Covering graph, a covering space for an undirected graph, and its special case the bipartite double cover
• Covering group
• Galois connection
• Quotient space (topology)

Literatur

• Allen Hatcher: Algebraic Topology. Cambridge Univ. Press, Cambridge, ISBN 0-521-79160-X
• Otto Forster: Lectures on Riemann surfaces. Springer Berlin, München 1991, ISBN 978-3-540-90617-9
• James Munkres: Topology. Upper Saddle River, NJ: Prentice Hall, Inc., ©2000, ISBN 978-0-13-468951-7
• Wolfgang Kühnel: Matrizen und Lie-Gruppen. Springer Fachmedien Wiesbaden GmbH, Stuttgart, ISBN 978-3-8348-9905-7
• Maximiliano Aguilar and Miguel Socolovsky: The Universal Covering Group of U(n) and Projective Representations. Hrsg.: International Journal of Theoretical Physics. Dezember 1999