Questions on the evaluation of residues, on the evaluation of integrals using the method of residues or in the method's theory.

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Integration of a analytic function

here is the problem I currently try to solve: $\int\limits_{-\infty}^{+\infty}\left((1+ixa^2)^{-\frac{n_1}{2}}\cdot(1+ixb^2)^{-\frac{n_2}{2}}\right)e^{icx} \mathrm{d}x $ with $a,b,c\geq0$ (real ...
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0answers
35 views

How to solve this equation with implicit sum

I want to know how the authors of this arxiv paper (p. 10) solved the equation \begin{align} g\left(\lambda\right) ={}& ...
2
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1answer
82 views

My complex integral cancels at the end; how can I modify the integrand to prevent this?

$$\int_0^\infty \frac{x^a}{x^2 + b^2}$$ for $-1< a < 1$ and b>0 -- these constraints help with estimating the integral on the big circle and small circle of a keyhole contour that I chose to ...
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4answers
139 views

Can I use an upper semi-circle to integrate this function?

I'm trying to integrate $$\int_{-\infty}^{\infty} \frac{e^{iz}}{e^z + e^{-z}}dz$$ Do I have have to integrate this over a box, or can I use my first guess at a contour and use an upper semi-circle ...
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0answers
63 views

Contour integral mystery: why is the answer different from Maple/Matlab?

The mystery is that here is a fairly standard contour integral which can be done by the residue theorem. Yet when I tried to evaluate it using numerical softwares like Maple or Matlab, the answer is ...
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1answer
41 views

What is a residue?

I've heard of residues in complex analysis, contour integration, etc. but all I really know it to be is the $c_{-1}$ term in the Laurent series for a function. Is there some sort of intuition on what ...
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1answer
31 views

Inverse Laplacetransform of rational function with multiple pole

I have to calculate the inverse Laplacetransorm of this function using Residue calculus $$ \frac{s^4 + 6s^3 - 10s^2 + 1}{s^5} $$ but I can't find any Residue rule that would solve this. Can you show ...
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1answer
60 views

Finding the residues of poles

Consider the equation $\mathcal{F}(\lambda)=0\ \ \ \forall\ \lambda = \lambda_{n},\ n \in \mathbb{N}$. I understand that the expression $\frac{d}{d\lambda}\ ...
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49 views

Multivariate Residue Theorem

Let $G(s,t)$ be a complex valued function in two variables that converges absolutely for $Re(s), Re(t)>1$. Suppose we can analytically continue $G$ in such a way that $$G(s,t) = f(s)g(t)H(s,t)$$ ...
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2answers
38 views

Two different answers with Laplace

Find the solution for the equation $$ -u'' + u = \delta'(t)$$ for which it "disappears" for $t<0$ By using residuals! So I used Laplace transformation for this. $$Y(-s^2 + 1) = ...
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60 views

Why does this example of global residue theorem not work?

This question is related to and inspired by a previous question What is the residue obtained from this integral? , but note that the appearing functions are slightly different. Consider the following ...
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1answer
42 views

What is the residue obtained from this integral?

Consider the following integral in two complex variables $z_1$ and $z_2$: $$\frac{1}{(2\pi i)^2}\oint_{{|z_1|=\epsilon}\atop{|z_2|=\epsilon}}dz_1 dz_2\frac{1}{z_1 ...
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2answers
73 views

A Residue problem [duplicate]

Preparing myself for qualifying exam, I found this problem in residues in some previous qualifying exam, and I am stuck and don't know how to solve it, any helps? ...
2
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2answers
39 views

What is a “Contour Integral” and how do I evaluate one?

A very general question, I apologize, but as you read this, hopefully you get what I'm asking. Recently, Bernoulli Numbers have caught my eye, for I am studying infinite series' and it is a part of ...
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3answers
101 views

Residue Problem

I am trying to find residues for all singularities of the function: $$f(z)= \frac{\tanh z}{z^2}$$ Here is what I did: $$f(z)= \frac{\sinh z}{z^2\cosh z}$$ when $$\cosh z=0$$ then $z_k =i( ...
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1answer
93 views

A Tough Problem about Residue

I tried my best to solve this problem from what I learned in residues, but the solution seems very far from what I was doing!! Is there any way other than using Laurent series expansion? Here is the ...
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1answer
54 views

The residue of $\frac{f(z)}{g(z)^2}$

Let $f$ and $g$ be analytic near $a$, and suppose $g$ has a simple zero at $a$. Find a formula for the residue of $\frac{f(z)}{g(z)^2}$ at $a$ in terms of $f(a)$ and the derivatives of $f$ and $g$ at ...
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1answer
27 views

Complex integration, showing that arc integral vanishes

I stumbled upon the following integral in QFT: $$\int_{\mathbb{R}} \frac{k e^{ikr}}{k^2+m^2} dk$$ If We turn this into complex variable integral along semi-circle arc in the upper complex plane: ...
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2answers
82 views

my computation of a real integral still has an imaginary number in it,

I have four residues that I have found. I multiplied each by $2\pi i$, using the Residue Theorem. But my final answer still has an $i$ in it. Needless to say, it is not the right answer, since the ...
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0answers
36 views

Is there a faster way to compute the residues of this function,

$$\frac{z^2log(z)}{1+z^4}$$ I have that this function has simple poles at $$e^{i\pi/4},e^{i3\pi/4},e^{i5\pi/4},e^{i7\pi/4}$$ which are the zeroes of the denominator (1+$z^4$). The computation of ...
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0answers
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Integral of ratio of complex polynomials

Let $p(z),q(z) \in \mathbb{C}[z]$ two polynomials with coefficients in $\mathbb{C}$ s.t. $deg(p) = m$, $deg(q) = n$ and $n \ge m +2$. I need to show that $$ \lim_{R \to \infty} \int_{|z| = R} ...
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2answers
58 views

$\int_0^\infty \frac{\log(x)}{x^2+\alpha^2}$ using residues

I'm trying to find $\int_0^\infty \frac{\log(x)}{x^2+\alpha^2}dx$ where $\alpha>0$ is real. My approach was to take an integral along the real line from $1/R$ to $R$, around the circle ...
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2answers
52 views

Is there a simpler way to compute the residue of a function at a pole of order 3?

The function $$\frac {1}{z^2(e^{i2\pi z}-1)}$$ has a triple pole at z = 0. To compute the residue of f at z = 0, I can compute the Laurent expansion of f about z = 0, and then read off the ...
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1answer
98 views

Computing residues of $\cot(\pi z)/z(z+1)$ with symmetries

I would like to know if there is a quick way of computing the residues of $$f(z) = \frac{\cot \pi z}{z(z+1)}$$at the points $z = 0$ and $z = -1$. They are double poles. Expanding this in Laurent ...
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2answers
37 views

Help evaluating residue with simple poles

I am having a bit of trouble evaluating $$\sum_{k=1}^3{ \rm Res}\left(\frac{\log(z)}{z^3+8};z_k\right)$$ where $z_1=2e^{i\pi}$, $z_2=2e^{i\pi/3}$ and $z_3=2e^{i5\pi/3}$. I know that each $z_k$ is a ...
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2answers
466 views

Compute this integral, using a method other than the Residue Theorem,

$\int_0^\infty$ $\frac{1}{1+x}$$\frac{dx}{\sqrt{x}}$ Part (a) asks to compute the integral by means of the residue at x = -1. I have done this just now, and the answer is $\pi$. Part (b) asks, "can ...
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1answer
30 views

What does complexification mean in complex analysis, .e.g., in residue calculus,

I've learned complexification formally in a graduate linear algebra class. But what does the word mean in the setting of complex analysis? If I consider a real integral on the positive half line, ...
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1answer
119 views

A particular integral: $\int_{-\infty}^{+\infty}\frac{\sin(\pi x)}{\prod_{k=-n}^{n}(x-k)}\,dx$

I have to show summability, then compute the following integral: $$\int\limits_{-\infty}^{+\infty} \frac{\sin(\pi\,x)}{\prod_{k = - n}^n (x - k)}\,dx = \frac{(-4)^n}{(2\,n)!}\,\pi $$ for every $n\in ...
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How to calculate this Ei(x)-involved definite integral?

I want to solve the integral attached below by means of residue theorem. I tried the common integration ways and seeked references(e.g, Rjadov, et. al). Finally, I decided to solve this integral by ...
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How to calculate this residue which has a pole of order n-r?

So I have this complex integral: $$ \oint \frac{dz}{2\pi}\frac{e^{iz(br-(n-r)a)}}{\left(1-(1-q)e^{-ik_{1}-iza}\right)^{n-r}}$$ b,r,q,a,n are all constants in this context. However I'm not entirely ...
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Reciprocals of theta functions

I've spent the last few months with partial fraction expansions, and thought to create a function with simple poles over a lattice of zeros, like that of any of the Jacobi theta functions... but I ...
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1answer
38 views

Find the residue of $e^{\frac{1}{z^2-1}}\sin(\pi z)$ at $z=1$

I'm dealing with the following problem (from an old qualifying exam): Let $\gamma$ be a closed curve in the right half-plane that has index $N$ with respect to the point 1. Find $$ ...
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1answer
38 views

How can I calculate the singularities and residues of…?

$$\frac{e^z}{z^3(z-1)}+\frac{1}{z^3}$$ I have problems specially for $z=0$ Can anyone show me how to do it?
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1answer
40 views

Evaluating $\int_{\gamma} \frac{z}{\cosh (z) -1}dz$

Evaluate $\int_{\gamma} \frac{z}{\cosh (z) -1}dz$ where $\gamma$ is the positively oriented boundary of $\{x+iy \in \Bbb{C} : y^2 < (4\pi^2 -1)(1-x^2)\}$. I just learned the residue theorem, ...
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4answers
227 views

Help with the contour for this integral using residues

$$ PV \int_0^\infty \frac{dx}{\sqrt{x}(x^2-1)} $$ A keyhole contour can't be used because we have a pole in the real positive axis, isn't it?
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1answer
36 views

Solving $\int_{0}^{+ \infty} \frac{x \cos(x)}{x^4 + 4 a^4} dx$ with residues

We also have the condition $a > 0$. My attempt was to, as usual, define $f(z) = \displaystyle\frac{z e^{iz}}{z^4 + 4 a^4}$. Then I tried to integrate $f$ over a curve $\gamma$ which goes from $0$ ...
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3answers
110 views

Solve integrals using residue theorem? [closed]

$$\int_{0}^{\pi}\frac{d\theta }{2+\cos\theta}$$ $$\int_{0}^{\infty}\frac{x }{(1+x)^6} dx$$ My problem is that I don't know how to start solving these integrals, or how to convert them into usual ...
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1answer
38 views

Residue of $\frac{e^{iz}}{z^2+4z+5}$ [closed]

I need to find the residue of $\dfrac{e^{iz}}{z^2+4z+5}$ at its singular points. How do I do that?
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0answers
63 views

Find $\int_0^{2\pi} \frac{d\theta}{2\pi\cos^{2n}(\theta)} \ n=1,2,3,\dots$ via Residue Theorem

So the question is as follows: Use the Residue Theorem to calculate $$\int_0^{2\pi} \frac{1}{2\pi\cos^{2n}(\theta)} d\theta \quad\quad n=1,2,3,\dots.$$ Now I believe the first step would be to use the ...
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1answer
71 views

Computing the residue of $\frac{z-2}{z^2} \sin\left(\frac{1}{1-z}\right)$ for $z = 1$.

Consider the function $$f(z) = \frac{z-2}{z^2} \sin\left(\frac{1}{1-z}\right)$$ We have that $0$ is a double pole and $1$ is a single pole (essential singularity) of $f$. It is simple to compute ...
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1answer
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Finding the residue, $z=n\pi$, and $e^{n\pi}$

I have reached the following point in a residue calculation and am now unsure what to do: $$Res_{z= n\pi}=\lim_{z\to n\pi}\{(z-n\pi)\frac{ e^z}{\sin(z) } \}$$ $$=\lim_{z\to ...
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2answers
179 views

Complex Integration with trignometric function

Verify that $\int_0^{\frac{\pi}{2}}\frac{d\theta}{a+\sin^2\theta}=\frac{\pi}{2[(a(a+1)]^\frac{1}{2}}$ I know that $\sin\theta=\frac{e^{i\theta}-e^{-i\theta}}{2}$ then I did ...
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2answers
45 views

Improper integral and residues

Evaluate $\int_0^\infty \frac{dx}{x^4+1}$ By the residue theorem $$\int_{-R}^Rf(x)dx+\int_{C_R}dz=2\pi i\sum Res(f,z_i)$$ but I have problems to evaluate it because $$z^4+1=0\Rightarrow ...
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1answer
68 views

Poles on the curve

Say I have this integral: $$\oint_\gamma f(z)\,{\rm d}z,$$and $f$ has a pole on $\gamma$. I understand that we "cut around" the pole with an arc of radius $\epsilon$ and then make $\epsilon \to 0$. ...
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1answer
31 views

Evaluate the improper integral with residues

Evaluate $\displaystyle\int_0^\infty\frac{dx}{x^2+1}$ I have that $z_0=i$ and $z_1=-i$ are singularity points but just $z_0=i$ is in the upper plane then ...
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1answer
21 views

Improper integrals and residues

I'm already read Conway, Churchill and Marsden but I'm still with doubts when it comes to improper integrals. Where come from this relation ...
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2answers
42 views

Computing the residue of $\phi/\psi$ given conditions.

Let $\phi$ and $\psi$ be holomorphic functions around $z = a$, where $\phi(a) \neq 0$ and $a$ is a double root of $\psi(z) = 0$. Prove that the residue of $\phi(z)/\psi(z)$ at $z = a$ is: ...
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1answer
69 views

What enclosure should I choose to evaluate the next integral using residues?

I have to solve the next integral: $$\int_{-\infty}^{\infty} e^{ibx}(e^{ia/x}-1)dx$$ where $a,b$ are real parameters. I can use Jordan´s Theorem to show that as $f(z)=e^{ibz}g(z)$ where $g(z)=(e^{ ia ...
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1answer
20 views

Residues and poles show that

Show that i) $\displaystyle\operatorname{Res}\limits_{z=\pi i}\frac{z-\sinh z}{z^2\sinh z}=\frac{i}{\pi}$ ii) $\displaystyle\operatorname{Res}\limits_{z=\pi i}\frac{\exp(zt)}{\sinh z}+ ...
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0answers
63 views

Using residue theorem find $\int_{\theta-\pi}^{\theta-2\pi}1/(r^2+1-2r\cos u)$

Find the value of $I$, using residue theorem. $$ I= \int_{\theta-\pi}^{\theta-2\pi} \frac{du}{r^2+1-2r\cos(u)} - \int_{\theta}^{\theta-\pi} \frac{du}{r^2+1-2r\cos(u)} $$ with $r<1$ and ...