Question about covering map of punctured unit disk

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Discussion Overview

The discussion revolves around the properties of a specific function defined as a covering map from the upper half-plane to the punctured unit disk. Participants explore the conditions necessary for this function to be a covering map, including the need for a suitable domain and the implications of branch cuts in complex analysis.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes that to show the function ##p(z) = e^{\frac{2 \pi i z}{|a|}}## is a covering map, they need to start with a point in the punctured unit disk and consider a neighborhood around it.
  • Another participant questions the notation ##\mathbb{R}^*## and asks if ##\mathbb{H}## refers to the half-plane.
  • A third participant clarifies that ##\mathbb{R}^*## means the reals excluding zero and confirms that ##\mathbb{H}## is indeed the upper half-plane.
  • One participant suggests that the domain of ##p## may need to include the real line to adequately cover the punctured unit disk, raising concerns about the branch cut.
  • A later reply corrects an earlier assumption, noting that the positive reals in the punctured unit disk will be covered by the image of a specific point in the upper half-plane.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of including the real line in the domain of the function to ensure it is a covering map. The discussion remains unresolved regarding the implications of branch cuts and the completeness of the proposed domain.

Contextual Notes

There are limitations regarding the assumptions about the domain of the function and the implications of branch cuts in complex analysis that have not been fully explored or resolved.

MathLearner123
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Let ##\alpha \in \mathbb{R}^*,\, p:\mathbb{H} \to \mathbb{D} \setminus \{0\}, \, p(z) = e^\frac{2 \pi i z}{|a|}##. I want to show that ##p## is a covering map but I dont't know how to make this. I think I need to start with an ##y \in \mathbb{D} \setminus \{0\}## and take an open disk ##D \subset \mathbb{D} \setminus \{0\}## to be a neighbourhood of ##y##. Now, because disk is open and connected, there exists a holomorphic branch of logarithm in ##D##. It's ok this start? How I can continue? Thanks!
 
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Sorry for my ignorance -- What is ##\mathbb{R}^*##? Is ##\mathbb{H}## a half-plane?
 
@FactChecker ##\mathbb{R}^* = \mathbb{R} \setminus \{0\}## and ##\mathbb{H}## is the upper half-plane
 
I think you may need to include more at the branch cut. I think that ##\mathbb{H}## does not include any of the real line. Don't you need to include the real line in the domain of ##p## in addition to ##\mathbb{H}## in order to cover ##\mathbb{D} \setminus \{0\}##? And doesn't the domain need to include an open set that includes the reals?

CORRECTION: My mistake. The positive reals in ##\mathbb{D} \setminus \{0\}## will be covered by the image of ##z=x+|\alpha|i##.
 
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