Smooth Mapping Between Unit Circle and Curve in R^2?

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SUMMARY

The discussion focuses on the smooth mapping between the unit circle defined by the equation {(x,y) | x^2 + y^2 = 1} and the curve defined by {(x,y) | x^4 + y^2 = 1} in R^2. Participants explore various mapping techniques, including the proposed map (x,y) → (sqrt(x), y) for non-negative x and (sqrt(-x), y) for negative x. The conversation highlights the importance of ensuring smoothness at x=0 and references Riemann's mapping theorem as a potential framework for establishing an analytic map. The conclusion emphasizes the need for geometric reasoning to address the smoothness concerns raised.

PREREQUISITES
  • Understanding of smooth mappings in differential geometry
  • Familiarity with the unit circle and polynomial curves in R^2
  • Knowledge of Riemann's mapping theorem and its applications
  • Basic concepts of continuity and differentiability in mathematical analysis
NEXT STEPS
  • Research the application of Riemann's mapping theorem in complex analysis
  • Study smooth mappings and their properties in differential geometry
  • Explore techniques for proving smoothness in mappings at critical points
  • Investigate geometric interpretations of polynomial curves in R^2
USEFUL FOR

Mathematicians, geometry enthusiasts, and students studying differential geometry or complex analysis who are interested in smooth mappings and their applications in R^2.

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Hi, I have been told that in R^2 the unit circle {(x,y) | x^2 + y^2 = 1} is smoothly mappable to the curve {(x,y) | x^4 + y^2 = 1}.

Can someone please tell me what this smooth map is between them? I can only think of using the map (x,y) --> (sqrt(x), y) if x is non-negative and (sqrt(-x), y) if x is negative. Thanks for any help.
 
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Both x^2+ y^2= 1 and x^4+ y^2= 1 loop around the origin. Draw the line from the origin through a point on the circle. Where that ray crosses the second graph is s(x,y).
 
is there a smoothness problem at x=0? (in answer #1)
 
The advantage of Halls' answer seems to be that he is projecting along a direction that never becomes tangent to the circle. I.e. #1 projects horizontally, and #2 projects radially. Still it is not so trivial for me to prove #2 is actually smooth, as the equation I am getting for r is still undefined at x=0, although it seems to extend.

An abstract approach is Riemann's mapping theorem, with extension to the boundary, that apparently gives an analytic map.
 
HallsofIvy: Thanks, that's a nice bijection. I clearly need to think more geometrically for this type of problem.

mathwonk: I don't think there is a problem at x=0 (for Halls' map) because you can just define r to be 1 for x=0 and then it is smooth on S1.
 
well you have prove it is smooth.
 

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