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For a simple pole at \( z = a \), the residue of \( f \) is

  • A.\( \lim_{z \to a} f(z) \)
  • B.\( \lim_{z \to a} (z - a)^{2}f(z) \)
  • C.\( f'(a) \)
  • D.\( \lim_{z \to a} (z - a)f(z) \)

Answer: D

Multiplying by the vanishing factor cancels the pole and leaves the coefficient \( a_{-1} \):

\[ \mathrm{Res}_{z=a} f \;=\; \lim_{z \to a} (z - a)f(z). \]

The square would be needed for a double pole, together with one differentiation, and the plain limit of \( f \) does not exist at a pole at all

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