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GATE 2020 MA – Question 14

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

Consider I: $\log|z|$ is harmonic on $\mathbb C\setminus\{0\}$. II: it has a harmonic conjugate on that entire domain.

  1. Both I and II are true.
  2. I is true but II is false.
  3. I is false but II is true.
  4. Both I and II are false.

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Show answer and explanation

Correct answer: (B) I is true but II is false.

Explanation

Writing $\log|z|=\frac12\log(x^2+y^2)$ gives Laplacian zero away from the origin, so I is true. A global harmonic conjugate would be a single-valued argument function, which cannot exist around a loop enclosing zero: its change around the loop is $2\pi$. Thus only local conjugates exist on suitable slit domains. II is false, giving B.

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