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GATE 2020 MA – Question 46

Complex Analysis · Analytic and Harmonic Functions · 2 marks · Multiple choice

Let f:C\{0}→C satisfy $f(z)=f(z/|z|)$ and have a continuous restriction to the unit circle. Then

  1. f is continuous but need not be analytic.
  2. f is analytic but need not be constant.
  3. f is constant.
  4. $\lim_{z\to0}f(z)$ exists.

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Show answer and explanation

Correct answer: (A) f is continuous but need not be analytic.

Explanation

The map z↦z/|z| is continuous away from zero. Composing it with the continuous circle restriction proves f is continuous there. It may vary with direction; for example f(z) = z/|z| satisfies the conditions but is nonconstant, nonanalytic and has no limit at zero. Hence A.

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