Fractional linear transformation--conformal mapping

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SUMMARY

The discussion focuses on the necessary and sufficient conditions for the fractional linear transformation defined by the equation \( f(z) = \frac{az + b}{cz + d} \) to map the upper half-plane to itself. The key condition established is that \( ad > bc \). Participants emphasized the importance of analyzing the imaginary part of the numerator after multiplying by the complex conjugate of the denominator to ensure it remains positive for \( z \) with a positive imaginary part.

PREREQUISITES
  • Understanding of fractional linear transformations
  • Knowledge of complex conjugates
  • Familiarity with the upper half-plane in complex analysis
  • Basic principles of imaginary and real parts of complex numbers
NEXT STEPS
  • Study the properties of fractional linear transformations in complex analysis
  • Learn about the implications of the condition \( ad > bc \) in mapping transformations
  • Explore the concept of complex conjugates and their applications in transformations
  • Investigate other mappings of the upper half-plane and their conditions
USEFUL FOR

Mathematicians, students of complex analysis, and anyone interested in understanding conformal mappings and their properties.

Dustinsfl
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Find necessary and sufficient conditions on the real numbers $a$, $b$, $c$, and $d$ such that the fractional linear transformation
$$
f(z) = \frac{az + b}{cz + d}
$$
maps the upper half plane to itself.

I just need some guidance on starting this one since I am not sure on how to begin.
 
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dwsmith said:
Find necessary and sufficient conditions on the real numbers $a$, $b$, $c$, and $d$ such that the fractional linear transformation
$$
f(z) = \frac{az + b}{cz + d}
$$
maps the upper half plane to itself.

I just need some guidance on starting this one since I am not sure on how to begin.
Begin by multiplying top and bottom by the complex conjugate of the denominator: $$f(z) = \dfrac{az + b}{cz + d} = \dfrac{(az + b)(c\overline{z} + d)}{(cz + d)(c\overline{z} + d)}.$$

The denominator in that last fraction is real, so you need to find the imaginary part of the numerator and investigate what makes it positive whenever $z$ has positive imaginary part.
 
Opalg said:
Begin by multiplying top and bottom by the complex conjugate of the denominator: $$f(z) = \dfrac{az + b}{cz + d} = \dfrac{(az + b)(c\overline{z} + d)}{(cz + d)(c\overline{z} + d)}.$$

The denominator in that last fraction is real, so you need to find the imaginary part of the numerator and investigate what makes it positive whenever $z$ has positive imaginary part.

So $ad > bc$
 
dwsmith said:
So $ad > bc$
(Yes)
 

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