Mathematics Asked by Udalricus.S. on February 10, 2021
Consider the annulus $$A:={(x,y)inmathbb{R}^{2}mid 1leq x^{2}+y^{2}leq 4}$$
I have to find all (connected) covering up to isomorphism.
My proof so far:
First of all, we have a homeomorphism $Acong S^{1}times [1,2]$. Therefore, we find $pi_{1}(A)cong pi_{1}(S^{1}times [1,2])cong pi_{1}(S^{1})=mathbb{Z}$. All the subgroups of $mathbb{Z}$ are of the form $mathbb{Z}n$ for $ninmathbb{N}$ and since $mathbb{Z}$ is abelian, all of them define distinct conjugacy classes of subgroups.
Now there is a theorem, which says that,if $A$ admits an universl cover, the then there are for each $n$ a covering $(widetilde{A},p)$ such that $p_{ast}(pi_{1}(widetilde{A}))cong nmathbb{Z}$ and this are all coverings up to isomorphism.
Now, I know that for $S^{1}$, we can define the maps $f_{n}:zmapsto z^{n}$, which define covering maps of $S^{1}$, such that $(f_{n})_{ast}(pi_{1}(S^{1}))cong nmathbb{Z}$. So therefore, my idea was to define the maps $g_{n}:S^{1}times [1,2]to Acong S^{1}times [1,2]$ exactly by $g_{n}(z,lambda):=(f_{n}(z),lambda)$. This should then be a covering with the claimed property $(g_{n})_{ast}(pi_{1}(S^{1}times [1,2]))cong nmathbb{Z}$.
Now to my question: Does this look right so far? and furthermore, the annulus $A$ is not simply connected, therefore all of my constructed coverings above are not universal. So I still have to find a universal cover, in order to show that I can apply the theorem above.
Thanks to the comments, I think I can answer it by myself.
All the coverings of the annulus are (up to isomorphism):
(1) The universal cover $$p:mathbb{R}times [1,2] to A, \ (t,lambda)mapsto (lambda cos(2pi t),lambdasin(2pi t))$$
(2) For each $ninmathbb{N}$ the cover $$g_{n}:S^{1}times [1,2] to A, \ (e^{itheta},lambda)mapsto (lambda cos(theta n ),lambdasin(theta n ))$$
Answered by Udalricus.S. on February 10, 2021
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