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GATE 2021 MA – Question 13

Complex Analysis · Conformal Mappings · 1 mark · Multiple choice

A Möbius map T(z)=(az+b)/(cz+d),ad−bc≠0, sends 0,−i,∞ to 10,5−5i,5+5i respectively. Find T({Re z<0}).

  1. {w:|w|<5}
  2. {w:|w|>5}
  3. {w:|w−5|<5}
  4. {w:|w−5|>5}

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Correct answer: (C) {w:|w−5|<5}

Explanation

The boundary imaginary axis passes through 0,−i,∞, so its image is the circle through 10,5−5i,5+5i: center 5, radius 5. Solving the map gives T(z)=(10−5iz)/(1−(1+i)z/2). Test z=−1 in the left half-plane: T(−1)=7+i, inside this circle. Therefore the left half-plane maps to the interior **C**.

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