Question:hard

Which one of the following options is true?
(A) If \(\operatorname{Log}(z)\) denotes the principal value of the logarithm, then \(\operatorname{Log}(zw)=\operatorname{Log}(z)+\operatorname{Log}(w)\) for all \(z,w\in\mathbb{C}\setminus\{0\}\).
(B) The solution set of the equation \(e^{iz}=-1\) is \(\{(2k+1)\pi:k\in\mathbb{Z}\}\).
(C) \(\cos(z)\) is bounded in the whole complex plane.
(D) A Mobius transformation with three fixed points is a constant.

Show Hint

Test each option with a specific case or the explicit fixed-point equation.
Updated On: Jul 3, 2026
  • If \(\operatorname{Log}(z)\) denotes the principal value of the logarithm, then \(\operatorname{Log}(zw)=\operatorname{Log}(z)+\operatorname{Log}(w)\) for all \(z,w\in\mathbb{C}\setminus\{0\}\)
  • The solution set of the equation \(e^{iz}=-1\) is \(\{(2k+1)\pi:k\in\mathbb{Z}\}\)
  • \(\cos(z)\) is bounded in the whole complex plane
  • A Mobius transformation with three fixed points is a constant
Show Solution

The Correct Option is B

Solution and Explanation

Step 1: For (A), use the general fact that \(\operatorname{Arg}(zw)\equiv\operatorname{Arg}(z)+\operatorname{Arg}(w)\pmod{2\pi}\), but the principal argument is restricted to \((-\pi,\pi]\). Whenever \(\operatorname{Arg}(z)+\operatorname{Arg}(w)\) falls outside \((-\pi,\pi]\), the principal value wraps around by \(\pm2\pi\), so \(\operatorname{Log}(zw)\) differs from \(\operatorname{Log}(z)+\operatorname{Log}(w)\) by \(2\pi i\). Since this wrap-around genuinely occurs for some \(z,w\), the identity cannot hold for all \(z,w\), so (A) is false.

Step 2: For (B), solve using the periodicity of the complex exponential instead of splitting into real and imaginary parts. Since \(\cos\pi+i\sin\pi=-1\), \(z_0=\pi\) is one solution of \(e^{iz}=-1\). Because \(e^{iz}=e^{iz_0}\) if and only if \(i(z-z_0)=2k\pi i\) for some \(k\in\mathbb{Z}\), i.e. \(z=z_0+2k\pi\), the full solution set is \(z=\pi+2k\pi=(2k+1)\pi\), \(k\in\mathbb{Z}\), matching option (B).

Step 3: For (C), use the modulus formula \(|\cos z|^2=\cos^2x\cosh^2y+\sin^2x\sinh^2y\). Since \(\sinh^2y=\cosh^2y-1\), \[|\cos z|^2=\cos^2x\cosh^2y+\sin^2x(\cosh^2y-1)=\cosh^2y-\sin^2x.\] As \(y\to\infty\), \(\cosh^2y\to\infty\) while \(\sin^2x\) stays bounded between 0 and 1, so \(|\cos z|\to\infty\). This confirms \(\cos(z)\) is unbounded, so (C) is false.

Step 4: For (D), write a general Mobius map as \(w=(az+b)/(cz+d)\) with \(ad-bc\neq0\). Fixed points satisfy \(z(cz+d)=az+b\), i.e. \[cz^2+(d-a)z-b=0.\] If \(c\neq0\) this is a genuine quadratic with at most 2 roots; if \(c=0\) it is linear (or identically zero) with at most 1 root, unless \(d=a\) and \(b=0\) as well, in which case every \(z\) is fixed and the map is the identity \(w=z\). So three or more fixed points force the identity map, not a constant, confirming (D) is false.

Step 5: Only (B) survives all checks. \[\boxed{\text{(B)}}\]

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