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Schwarz Lemma

The Schwarz Lemma is a fundamental result in complex analysis, particularly in the field of holomorphic functions. It states that if a function fff is holomorphic on the unit disk D\mathbb{D}D (where D={z∈C:∣z∣<1}\mathbb{D} = \{ z \in \mathbb{C} : |z| < 1 \}D={z∈C:∣z∣<1}) and maps the unit disk into itself, with the additional condition that f(0)=0f(0) = 0f(0)=0, then the following properties hold:

  1. Boundedness: The modulus of the function is bounded by the modulus of the input: ∣f(z)∣≤∣z∣|f(z)| \leq |z|∣f(z)∣≤∣z∣ for all z∈Dz \in \mathbb{D}z∈D.
  2. Derivative Condition: The derivative at the origin satisfies ∣f′(0)∣≤1|f'(0)| \leq 1∣f′(0)∣≤1.

Moreover, if these inequalities hold with equality, fff must be a rotation of the identity function, specifically of the form f(z)=eiθzf(z) = e^{i\theta} zf(z)=eiθz for some real number θ\thetaθ. The Schwarz Lemma provides a powerful tool for understanding the behavior of holomorphic functions within the unit disk and has implications in various areas, including the study of conformal mappings and the general theory of analytic functions.

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