Question:hard

Consider the following statements:
(I) The function \(f(z)=\dfrac{x^2-y^2+2ixy}{x^2+y^2}\) (\(z=x+iy\)) has a limit as \(z\to0\).
(II) The function \(f(z)=\begin{cases}\operatorname{Re}(z)/|z|, & z\ne0\\ 1, & z=0\end{cases}\) is continuous at \(z=0\).
Choose the correct answer:

Show Hint

Check the limit along different paths approaching the origin.
Updated On: Jul 3, 2026
  • Only (I) is true
  • Only (II) is true
  • Both (I) and (II) are true
  • Neither (I) nor (II) is true
Show Solution

The Correct Option is D

Solution and Explanation

Step 1: For (I), substitute the line \(y=mx\) directly into the original expression instead of simplifying algebraically first. With \(y=mx\), \[f(x,mx)=\frac{x^2-m^2x^2+2imx^2}{x^2+m^2x^2}=\frac{x^2(1-m^2+2im)}{x^2(1+m^2)}=\frac{1-m^2+2im}{1+m^2},\] which is independent of \(x\) and depends only on the slope \(m\).

Step 2: For \(m=0\) (the x-axis) this gives \(1\); for \(m=1\) (the line \(y=x\)) it gives \((0+2i)/2=i\). Since approaching along different straight lines through the origin gives different values, the limit as \(z\to0\) does not exist. This confirms (I) is false using a family of straight-line paths rather than the polar substitution.

Step 3: For (II), apply the sequential criterion for limits: if \(\lim_{z\to0}f(z)\) existed, every sequence \(z_n\to0\) would give the same limit \(f(z_n)\).

Step 4: Take \(z_n=1/n\to0\); then \(f(z_n)=\operatorname{Re}(1/n)/|1/n|=1\) for every \(n\), so this sequence suggests limit \(1\). Now take \(w_n=i/n\to0\); then \(f(w_n)=\operatorname{Re}(i/n)/|i/n|=0/(1/n)=0\) for every \(n\), suggesting limit \(0\). Since the two sequences give different results (\(1\) versus \(0\)), the limit does not exist, so \(f\) is not continuous at \(z=0\) even though \(f(0)=1\) was assigned.

Step 5: Both statements fail. \[\boxed{\text{Neither (I) nor (II) is true}}\]

Was this answer helpful?
0

Top Questions on Complex Analysis