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The number of Mobius transformations which map the real line onto the unit circle is :

  • A.0
  • B.1
  • C.2
  • D.Infinitely many

Answer: D

There are infinitely many such Mobius transformations.

One example is the Cayley-type map \(T(z) = \dfrac{z - i}{z + i}\), which sends every real \(z\) to a point with \(|T(z)| = 1\) (real \(z\) is equidistant from \(i\) and \(-i\)) and the real line onto the unit circle.

Composing T with any Mobius map of the real line to itself (for example \(z \mapsto az + b\) with real \(a \ne 0\) and \(b\)), or with any rotation \(w \mapsto e^{i\theta}w\) of the circle, gives another such map. These form a three-parameter real family, so there are infinitely many, not 0, 1 or 2.

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