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GATE 2024 MA – Question 11

Complex Analysis · Analytic and Harmonic Functions · 1 mark · Multiple choice

An entire f satisfies |f(z)|=1 whenever Im z=0. Which statement is correct?

  1. Some nonconstant polynomial satisfies this condition.
  2. Every such entire f is constant.
  3. Every such entire f is zero-free.
  4. Some such entire f has infinitely many zeros.

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Correct answer: (C) Every such entire f is zero-free.

Explanation

Define entire $g(z)=\overline{f(\bar z)}$. On the real axis f(z)g(z)=|f(z)|²=1. The identity theorem gives fg≡1 on ℂ, so f has no zeros. **C**. The function e^(iz) is a nonconstant entire example, refuting B; a polynomial with bounded modulus on the whole real axis must be constant.

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