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Question

Given $f(z) = \frac{1}{z+1} - \frac{2}{z+3}$. If $C$ is a counterclockwise path in the $z$-plane such that $|z+1|=1$, the value of $\frac{1}{2\pi j}\oint_C f(z)dz$ is

The correct answer is
1

To evaluate the integral \(\frac{1}{2\pi j}\oint_C f(z)dz\) where f(z) = \frac{1}{z+1} - \frac{2}{z+3} and C is the path |z+1|=1, we can use the residue theorem from complex analysis.

The residue theorem states that if you have a function f(z) analytic inside and on some simple closed contour C, except for isolated singularities, then:

\[\frac{1}{2\pi j}\oint_C f(z) dz = \sum \text{Res}(f, z_k)\]

where the sum is over all residues inside the contour C.

Step-by-step Solution:

  1. Identify singularities of f(z):
    • The function \frac{1}{z+1} has a singularity at z = -1.
    • The function \frac{2}{z+3} has a singularity at z = -3.
  2. Determine which singularities lie inside the contour |z+1|=1:
    • The circle |z+1|=1 is centered at z = -1 with a radius of 1, so it will include z = -1 but not z = -3.
  3. Calculate the residue at z = -1:
    • The function \frac{1}{z+1} has a residue of 1 at z = -1.
    • The function \frac{2}{z+3} does not contribute since z = -3 is not inside the contour.
    • Thus, the total residue inside the contour is simply 1.
  4. Apply the residue theorem:
    • The value of the integral is the sum of the residues inside the contour times 2\pi j, thus:
    • \(\frac{1}{2\pi j}\oint_C f(z)dz = 1\).

After evaluating the integral, the correct answer is 1.

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Important Questions from Complex Variables

  1. Let $z = x + iy$ be a complex variable and $\bar{z}$ be its complex conjugate. The equation$z^2 + \bar{z}^2 = 2$ represents a
  2. Consider the complex function $f(z) = \cos z + e^{z^2}$. The coefficient of $z^5$ in the Taylor series expansion of $f(z)$ about the origin is ________ (rounded off to 1 decimal place).
  3. Let $f(z) = j \frac{1-z}{1+z}$, where z denotes a complex number and j denotes $\sqrt{-1}$. The inverse function $f^{-1}(z)$ maps the real axis to the ______.
  4. The contour integral $$\oint_C e^{1/z} dz$$ with C as the counter-clockwise unit circle in the z-plane is equal to

  5. Let $$ I = \int_0^1 \frac{1}{1+t} dt + \frac{\pi i}{2} \int_0^1 \frac{1}{1+e^2} dt - i \int_0^1 \frac{1}{1+it} dt, $$ where $ t $ is a real variable and $ i = \sqrt{-1} $. The value of $ I $ is ______________

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