Möbius maps and circles
T(z)=(az+b)/(cz+d), ad−bc≠0, extends to a bijection of C∪{∞}, with T(−d/c)=∞ and T(∞)=a/c when c≠0.
Key knowledge
T(z)=(az+b)/(cz+d), ad−bc≠0, extends to a bijection of C∪{∞}, with T(−d/c)=∞ and T(∞)=a/c when c≠0. It maps generalized circles (lines or circles) to generalized circles. For example (z−i)/(z+i) maps the upper half-plane bijectively to the unit disk.
Conditions and scope
- For c=0, T(∞)=∞; use the extended-plane convention at poles.
- On finite points away from its pole T′=(ad−bc)/(cz+d)²≠0.
Sources · 1
References
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Evidence and review
Sections 10.7–10.7.3, Example 10.14 and lines/circles theorem; inverse is (dw−b)/(a−cw), with extended-point convention.
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