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GATE 2015 EE (EE2) – Question 11

Engineering Mathematics · Complex Variables: Analytic functions, Cauchy integral formula, Taylor series, Laurent series, Residue theorem · 1 mark · Multiple choice

Let f(z)=g(z)+h(z), with complex-valued functions of complex z. Which statement is true?

  1. If f is differentiable at z0, g and h are also differentiable there.
  2. If g and h are differentiable at z0, f is differentiable there.
  3. If f is continuous at z0, it is differentiable there.
  4. If f is differentiable at z0, its real and imaginary parts are also differentiable there.

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Show answer and explanation

Correct answer: (B) If g and h are differentiable at z0, f is differentiable there.

Explanation

Differentiation is linear: if g and h have complex derivatives at z0, their sum has derivative g′(z0)+h′(z0). Thus **B** is always true.

The converse fails because nondifferentiable terms can cancel, and continuity alone is weaker than complex differentiability. Real and imaginary parts viewed as separate complex-valued functions need not be holomorphic; for example Re(z) is not, although f(z)=z is.

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