Is Smoothness the Key to Understanding Complex Projective Space?

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SUMMARY

The discussion focuses on establishing the smoothness of the quotient map \(\pi: \mathbb{C}^{n+1}\setminus\{0\} \to \mathbb{CP}^n\) in the context of complex projective space. Participants emphasize that smoothness refers to infinite complex differentiability and question whether the function's entire nature on \(\mathbb{C}^{n+1}\) suffices for this property. They also highlight the importance of the complex structure over the real structure, particularly in relation to the Cauchy-Riemann equations and the definition of complex projective spaces.

PREREQUISITES
  • Understanding of complex analysis, specifically Cauchy-Riemann equations.
  • Familiarity with complex projective spaces, particularly \(\mathbb{CP}^n\).
  • Knowledge of smooth functions in the context of complex differentiability.
  • Basic concepts of quotient maps in topology.
NEXT STEPS
  • Study the properties of smooth complex functions and their differentiability.
  • Learn about the structure and properties of complex projective spaces, focusing on \(\mathbb{CP}^n\).
  • Explore the implications of the quotient map \(\pi\) and its well-defined nature in multiple complex variables.
  • Review the resources on smooth complex projective varieties provided in the discussion.
USEFUL FOR

Mathematicians, particularly those specializing in complex analysis, algebraic geometry, and topology, will benefit from this discussion, as well as students seeking to understand the intricacies of complex projective spaces.

Kreizhn
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So I have been tasked with what is likely a very simple problem, but have forgotten so much complex analysis that I would like to very the problem.

Let \mathbb{CP}^n denote the n-dimensional complex projective space. We want to show that the quotient map \pi: \mathbb C^{n+1}\setminus\{0\} \to \mathbb{CP}^n is smooth.

Essentially, I just want to ensure that nothing tricky is going on when we talk about "smooth" complex functions.

1) Having identified \mathbb{CP}^n as an n dimensional space as compared to a 2n dimensional would imply we are looking at it with a complex structure rather than a real one. Does this cause any problems? I'm thinking I cannot just work the solution for the real structure and directly apply it to the complex one, since something about Cauchy-Riemann equations is jumping out at me.

2) I assume smooth in this context is infinitely complex differentiable. For this would it be sufficient to show that the function is entire on \mathbb C^{n+1}?

3) How must I adjust working with a function of multiple complex variables?
 
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Are you sure ##\pi## is well-defined, since you cannot say which coordinate is unequal zero?

The answer depends a bit on how you define the complex projective spaces, which determines how the analytical structure is written. E.g. ##\mathbb{CP}^n = \mathbb{S}^{2n+1}/\mathbb{S}^1##
(https://ncatlab.org/nlab/show/complex+projective+space)

Smooth complex projective varieties can be found here (p.23 f):
http://page.mi.fu-berlin.de/groemich/complex.pdf

For a more general introduction see
http://homepages.math.uic.edu/~ddumas/work/survey/survey.pdf
 

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