An archived instance of discourse for discussion in undergraduate Complex Variables.

An integral using the extension of Cauchy’s integral formula

mark

Use Cauchy's integral formula (Theorem 4.2.4) along with its extension (Theorem 5.1) to compute
$$\int_{C_3(0)}\frac{e^{2z}}{(z-1)^2(z-2)}dz.$$
Note that this is part (i) of problem 3 from chapter 5. There are lots of similar problems there.

dgallimo

Edited.

Performing a partial fractions decomposition on the integrand, we find that
$$
\frac{1}{(z-1)^2(z-2)}=\frac{-1}{(z-1)^2}+\frac{-1}{z-1}+\frac{1}{z-2}.
$$
So the integral can be expressed
$$
\int_{C_3(0)}\frac{-e^{2z}}{(z-1)^2}\ dz+\int_{C_3(0)}\frac{-e^{2z}}{z-1}\ dz+\int_{C_3(0)}\frac{e^{2z}}{z-2}\ dz.
$$
Note both the poles $1$ and $2$ lie inside the circle of radius $3$ centered at $0$.
Using the Cauchy Integral Formula, we find that the second integral evaluates to $2\pi i(-e^{2\cdot1})=-2\pi ie^2$ and the third integral is $2\pi i(e^{2\cdot2})=2\pi ie^4$.
To solve the first integral, recall that
$$
f'(z_0)=\frac{1}{2\pi i}\int_\gamma\frac{f(z)}{(z-z_0)^2}\ dz.
$$
This implies
$$
\int_{C_3(0)}\frac{-e^{2z}}{(z-1)^2}\ dz=-2\pi i\bigg(\frac{1}{2\pi i}\bigg)\int_{C_3(0)}\frac{e^{2z}}{(z-1)^2}\ dz,
$$
which evaluates to $-2\pi i(2e^{2\cdot1})=-4\pi ie^2$.
So the sum of the integrals is $-4\pi ie^2-2\pi ie^2+2\pi ie^4=2\pi ie^2(e^2-3)$.


















Thanks for the tip, @felyahia.

felyahia

The fraction decomposition you made is not correct, check your calculations again :slight_smile:

felyahia

Let's add an additional path that separates 1 and 2. and let's integrate on two closed path $\gamma_1$ and $\gamma_2$ counterclockwise ($C_3(0)=\gamma_1\gamma_2$), the point 1 is inside $\gamma_1$ and outside $\gamma_2$, and the point 2 is inside $\gamma_2$ and outside $\gamma_1$.

$\int_{C_3(0)} \frac{exp(2z)}{(z-1)^2(z-2)}dz=\int_{\gamma_1} \frac{exp(2z)}{(z-1)^2(z-2)}dz+\int_{\gamma_2} \frac{exp(2z)}{(z-1)^2(z-2)}dz$

$\int_{\gamma_1} \frac{exp(2z)}{(z-1)^2(z-2)}dz=\int_{\gamma_1} \frac{exp(2z)/z-2}{(z-1)^2}dz$

the function $f_1(z)=\frac{exp(2z)}{z-2}$ is homolorphic on and inside $\gamma_1$ then by appying Cauchy theorem extension we get:
$\int_{\gamma_1} \frac{f_1(z)}{(z-1)^2}dz=2\pi i f'_1(1)$
= -6$\pi$i $exp(2)$
$\int_{\gamma_2} \frac{exp(2z)}{(z-1)^2(z-2)}dz=\int_{\gamma_2} \frac{exp(2z)/(z-1)^2}{(z-2)}dz$
=$\int_{\gamma_2} \frac{f_2(z)}{z-2}dz$



the function $f_2(z)=\frac{exp(2z)}{(z-1)^2}$ is homolorphic on and inside $\gamma_2$ then by appying Cauchy theorem we get:
$\int_{\gamma_2} \frac{f_2(z)}{(z-2)}dz=2\pi i f_2(2)$
=$2\pi i exp(4)$

Therefore

$\int_{C_3(0)} \frac{exp(2z)}{(z-1)^2(z-2)}dz=2\pi i$ exp(2)$(exp(2)-3)$

dgallimo

I used Mathematica to evaluate the integral with contour $\gamma(t)=3e^{it}$ and it came out to be the same as your answer. Nicely done.

hjoseph

Trying to solve this in python using:

$$\int_\gamma f(z)dz=\int_a^bf(\gamma(t))\cdot\gamma'(t)dt$$

Since Python's numerical integration does not support complex arguments, I have tried breaking the integral into real and imaginary parts. For some reason when I try to use scipy.real() and scipy.imag() to break the function values into their respective parts, I still get:
TypeError: can't convert complex to float
My code is below, pretty short but still does not work. I was to believe that real and imag both returned floats since the real and imaginary parts of a complex numbers can be represented as ordered pairs of real numbers...

from scipy import integrate,real,imag
from numpy import pi
from math import exp

def complex_quad(integrand, a, b): #attempt to break the integral into its real and imaginary parts
    def r(x):
        return real(integrand(x))            #for some reason I get 'can't convert complex to float' here
    def i(x):
        return imag(integrand(x))            #probably here too, but the other one throws exception first
    ri = integrate.quad(r, a, b)   #real part of integral
    ii = integrate.quad(i, a, b)   #imaginary part
    return (ri[0] + 1j*ii[0], ri[1:], ii[1:])


integrand=lambda t: ((exp(2*3*exp(t*1j)))/(((3*exp(t*1j)-1)**2)*(3*exp(t*1j)-2)))*3j*exp(t*1j)#integrand is(f(path(z))*dpath(z))
complex_quad(integrand,0,2*pi)##integral(f(path(z))*dpath(z)) from 0 to 2*pi