Questions on the evaluation of integrals along a locus in the complex plane.

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2
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1answer
47 views

Assumptions on functions so that integral is zero

Let $f:\mathbb{R}\to\mathbb{R}$ and $g:\mathbb{R}\to\mathbb{R}$ be two arbitrary functions. Assume $g\in L^2(\mathbb{R})$. I'm looking to find out the minimal set of assumptions on $f$ and $g$ such ...
0
votes
1answer
28 views

Calculate complex integral

Let $C$ be a circle $\gamma=\partial B (0,2)$ oriented positively. I have to calculate $$\int_\gamma \frac{-\cos(1/z)}{\sin(1/z)z^2}dz$$ My attempt: Notice that $\sin(1/z)$ is meromorphic inside ...
0
votes
0answers
36 views

Show the integration with a complex variable

I want to show that there exists inverse Laplace transform, $f(t)$ of the function $F(\lambda)$. In other word, given $F(\lambda)$, existence of function $f(t)$ such that $$ ...
2
votes
1answer
59 views

$\int_0^{\infty} \exp(i(t-\alpha)^2) dt$

It's fairly straight forward to show that $$ \int_0^{\infty} \exp(it^2) dt = \frac{\sqrt{\pi}}{2}\exp\left(i\frac{\pi}{4}\right) $$ via complex contour integration over a contour shaped like a piece ...
6
votes
6answers
272 views

Compute definite integral

Question: Compute $$\int_0^1 \frac{\sqrt{x-x^2}}{x+2}dx.$$ Attempt: I've tried various substitutions with no success. Looked for a possible contour integration by converting this into a rational ...
1
vote
0answers
27 views

Series involving Laguerre polynomials

Given the series \begin{align} S_{x}(a) = \sum_{k=1}^{\infty} (-1)^{k+1} \, \binom{x-1}{k} \, L_{k+n-1}(a) \end{align} where $L_{m}(x)$ is the Laguerre polynomial. By using \begin{align} L_{n}(z) = ...
2
votes
1answer
53 views

Integral with complex variable

I want to compute $$ \int_{-\infty}^{\infty} \frac{1}{\sqrt{x+yi +2}} dy $$ where $i$ is the imaginary number. How to compute this integral??
1
vote
1answer
61 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 ...
3
votes
0answers
45 views

Do we have a inverse Laplace transform of $\frac{1}{\arctan s}$

Do we have a closed form of this seemingly very simple inverse transformation? If no closed form, what about its asymptotic form? Does this satisfies the criterion to have its inverse ...
0
votes
2answers
48 views

Contour Integration of a Complex function

In the context of contour integration: For positive real values of $\alpha$ the following integral is $$ I(\alpha)=\int_{-\infty}^{\infty}\frac{e^{it\alpha}}{1+t^2}dt=\frac{\pi}{e^{\alpha}} $$ Why ...
2
votes
2answers
64 views

evaluate $\int_0^{2\pi} \frac{1}{\cos x + \sin x +2}\, dx $

This is supposed to be a very easy integral, however I cannot get around. Evaluate: $$\int_0^{2\pi} \frac{1}{\cos x + \sin x +2}\, dx$$ What I did is: $$\int_{0}^{2\pi}\frac{dx}{\cos x + \sin x ...
5
votes
0answers
64 views

Evaluate $\int_{-\infty}^{\infty}\frac{\sqrt{a+ix}}{a^2+x^2}\,dx$ using residue calculus

I'm asked to evaluate $$\int_{-\infty}^{\infty}\frac{\sqrt{a+ix}}{a^2+x^2}\,dx$$ $\mathbb{R}\ni a>0$, using residue calculus (where $\sqrt{\cdot}$ is the PV $\sqrt{}$). My approach is as follows: ...
0
votes
0answers
46 views

Integration using residue theorem

Can you find, using residue theorem, ($\epsilon >0$), the value of this integral ($I$)? \begin{equation} I=\lim_{\epsilon->0^{+}} \int_{-\infty}^{\infty} \frac{dw}{w+i\epsilon} \end{equation} ...
0
votes
1answer
26 views

Contour Integration of Simple Closed Contour

Let C be any simple closed contour inside the annulus 4 < |z| < 6. Show that there holds: $$ \int_C \frac{dz}{z^2+1} = 0$$ To begin: I know that there are poles at $\pm i$ and that the ...
2
votes
3answers
58 views

Why $\lim_{R\to\infty}\int_{0}^{\pi}\sin(R^{2}e^{2i\theta})iRe^{i\theta}\:\mathrm{d}\theta = -\sqrt{\frac{\pi}{2}}$

This is a short question, but I'm simply not sure where to start, I know by Jordan's Lemma that the integral is not $0$, but I only know the below result due to Mathematica. ...
2
votes
1answer
25 views

Complex contour integral of fraction of polynomials

Let $n \in \mathbb{N}_0$ and set $p(z) = z^n + a_1 z^{n-1} + \cdots$ and $q(z) = z^{n+1} + b_1 z^{n} + \cdots$ to be two monic complex polynomials with no common zeros. I want to prove that ...
1
vote
2answers
20 views

Locations of singularities of a function with respect to given contours

Show that $\int_{C_1}f=\int_{C_2}f$, where $C_1:|z|=1$, $C_2:|z|=2$, and $f(z)=\frac{2z+1}{\sin z}$. Hint: Locate the singularities of $f$ in each case and indicate their location with respect ...
0
votes
1answer
33 views

Complex Integrations and Contours

Show that $$\int_C \frac{2 z^2-5}{(z^2+1)(z^2+4)} dz \le \frac{\pi R (2 R^2+5)}{(R^2-1)(R^2-4)} $$ Let $C$ be the upper half of the circle $z=R$ for any $R>2$. Do I need to actually find the ...
0
votes
0answers
52 views

Prove: $\int_C \frac{dz}{z^2+1} =0$ on the annulus $6\lt |z| \lt 8$

Let $D$ be the annulus $6\lt |z| \lt 8$ and let $C$ be any simple closed contour inside $D$. Show that there holds: $$\int_C \frac{dz}{z^2+1} =0$$ This has two singular points, $z=\pm i$, these are ...
4
votes
1answer
38 views

$\int_{\gamma}f(z)\log\left(\frac{z+1}{z-1}\right)dz = 2\pi i\int_{x=-1}^{x=1}f(x)dx$ on an ellipse

I am a self-studier and this is a problem from a course I've been doing. I would appreciate help showing: $$\int_{\gamma}f(z)\log\left(\frac{z+1}{z-1}\right)dz = 2\pi i\int_{x=-1}^{x=1}f(x)dx$$ ...
0
votes
2answers
49 views

Closed form for $\int_0^\infty\frac{1}{(1+x^2)^s}\,dx$ when $s\in (0.5,\infty)\setminus\mathbb{N}$

I know that the improper integral $$ \int_0^\infty\frac{1}{(1+x^2)^s}\,dx $$ is convergent for $s>0.5$ and divergent otherwise. Furthermore, it has a closed form for $s \in \mathbb{N}$ (this can ...
2
votes
2answers
40 views

Calculating residue $\int_C \frac{8-z}{z(4-z)}dz$

I want to calculate the following: $$\int_C \frac{8-z}{z(4-z)}dz$$ $C$ is a circle of radius $7$, centered at the origin,negative oriented. I want to do this via finding the residues at $z=0,4$. I ...
3
votes
4answers
82 views

Integral using contour integration [duplicate]

Here is the integral I want to evaluate: $$\int_{0}^{2\pi} \frac{dx}{a+b \cos x }, \quad a>b >0$$ Apparently there are limitations as to what values $a, b$ are supposed to take but let us not ...
3
votes
1answer
68 views

Steepest descent method with movable maximum

Suppose we want to find the asymptotic behavior as $n \rightarrow \infty$ of the integral $$\int_C \frac{dz}{z} \frac{e^z}{z^n}=\int_C \frac{dz}{z} \exp(z-n \ln z)$$ where $C$ is some contour in the ...
3
votes
1answer
32 views

Polynomial Inequality via Contour Integration

Problem. Let $P(z)=\sum_{k=1}^{n}a_{k}z^{k}$ be a polynomial which is real on the real axis. Prove the inequality ...
3
votes
3answers
67 views

Evaluating past exam problem: $\int_C \frac{\sin z}{(z+1)^7} \mathrm{d}z$

I want to evaluate the following: $$\int_C \frac{\sin z}{(z+1)^7} \mathrm{d}z$$ Where $C$ is the circle of radius $5$, centre $0$, positively oriented. Now this has one root at $z=-1$. Now I should ...
1
vote
0answers
21 views

real integral using residue theorem

Edit before posting: my result didn't match with the solution, found the error while posting, figured I would post it anyway because someone else might find it useful I'm trying to solve: ...
3
votes
1answer
69 views

How to evaluate such integral with pole structure?

Let's have integral: $$ I = \int \limits_{-\infty}^{\infty} \frac{e^{-\frac{x^{2}}{2}}}{x - a - i0} $$ How to evaluate it? I tried to do following: $$ \frac{1}{x -a - i0} = \int ...
3
votes
2answers
73 views

How to integrate $e^{-\cos(\theta)}\cos(\theta + \sin(\theta))$

I am struggling to find a way to evaluate the following real integral: $$\int_{0}^{2\pi}e^{-\cos(\theta)}\cos(\theta + \sin(\theta))\:\mathrm{d}\theta$$ The exercise started by asking me to ...
4
votes
3answers
108 views

Evaluating $\int_{-\infty}^{\infty}\frac{\cos x}{e^x + e^{-x}}$ using the Residue Theorem

I consider the complexification $$f(z)=\frac{e^{iz}}{e^z+e^{-z}}$$ Poles of $f$: $\text{Denominator}=e^{-z}(e^{2z}+1)=0\Rightarrow e^{2z}=-1=e^{i(\pi + 2\pi k)}\Rightarrow z=\frac{i\pi(1+ 2k)}{2}$, ...
1
vote
0answers
23 views

Question about an boundary integral equation with a jump in the boundary

I have the following problem: $$\Delta u = 0\;in\;\Omega$$ with several boundary conditions. Applying Green's second identity the representation formula can be derived: ...
5
votes
2answers
163 views

A (basic?) contour integration problem

I am trying to prove the following using complex analysis: $$\sum_{n=-\infty}^{\infty}\frac{(-1)^{n}}{a^{2}+n^{2}}=\frac{\pi}{a\sinh(a\pi)}$$ I am told to use the following function: ...
6
votes
1answer
67 views

Evaluating $\int_0 ^{\infty}\frac{dx}{x^{1/3}(1+x)}$ using Complex Analysis

I am trying to use the residue theorem to evaluate $$I=\int_0 ^{\infty}\frac{dx}{x^{1/3}(1+x)}$$ I'll explain my difficulty in finding a contour, then I explain my difficulty in finding a new contour ...
0
votes
1answer
26 views

removable singularity

Let $C$ be the positively oriented boundary of the square with vertices $(1,0)$, $(1,-i)$, $(-1,-i)$ and $(-1,0)$. If $$ f(z)=\frac{\sin(z)}{z}, $$ then clearly $f$ has a removable singularity on ...
0
votes
0answers
18 views

Change of variables in contour integrals

Lets say I had a keyhole contour around a branch cut on +x axis. Is the path of integration supposed to remain the same? for the value of the integral to remain the same, am I supposed to go around ...
1
vote
0answers
129 views

How to compute the definite integrals of special functions?

How can these integrals be solved: $${1\over \pi} \int_{0}^{\infty}\left({{\sqrt{x}(a-bx)}\over {x^{3}+(a-bx)^{2}}}\right)\cos(\sqrt{\alpha x}) \exp(-xt)\,\mathrm{d} x, $$ and $${1\over \pi} ...
3
votes
0answers
41 views

How to Solve this Improper Integral with six poles?

I'm trying to solve the following integral, where $a>0$, $b>0$, $y\in\mathbb{R}$ and $z\in\mathbb{R}$ are given constants: $$ \int_{-\infty}^{0} \left[ ...
1
vote
1answer
48 views

ML-inequality: How to show that $e^{i2x} = e^{i2z}$ when evaluating $\int_{-\infty}^\infty \frac {\cos^2 (x)}{x^2 + 1} dx$

I am to solve the following integral: $$\int_{-\infty}^\infty \frac {\cos^2 (x)}{x^2 + 1} dx$$ We use contour integration in combination with residue calculus, so for $R > 1$ ($R$ is the radius ...
3
votes
2answers
101 views

Showing $ \int_0^{2 \pi } \frac{dt}{a^2 \cos^2 t + b^2 \sin^2 t} = \frac{2 \pi}{ab}$ [duplicate]

The question: Let $\gamma$ be a contour such that $0 \in I(\gamma),$ where $I$ is the interior of the contour. Show that $$\int_\gamma z^n \, \text{d}z = \begin{cases} 2\pi i & \text{if } n = ...
2
votes
1answer
53 views

Numerical or analytical or exisistence: Inverse Laplace Transform

Edit 1: With the hint of Ron, we can simplify the question to : $$\bar{f}(s)=\frac{1}{(s^2+1)\arctan s }$$ So what about this function's inverse Laplace Transform? Or can anyone tell me that the ...
2
votes
1answer
50 views

Contour integral of $\int_0^\infty \log(x) e^{-x} dx$

Is it possible to resolve this integral using integral contour? What should be the contour? \begin{equation} \int_0^\infty \log(x) e^{-x} dx = -\gamma \approx -0.577216 \end{equation} where $\gamma$ ...
1
vote
3answers
46 views

Higher order poles, how high?

Clarification: I claim, for function $g(z)$, analytic and nonzero at $z=0$, if I have the function $f(z)=g(z)/z^n$ there is no use in trying to find poles of order smaller than n. And I would be ...
1
vote
0answers
19 views

Recovering cosh(ax) from it's fourier transform

Let's say $f(x)=\cosh(ax)$, where $a$ is a complex number and $x$ is real. Then the fourier transform is $F(\omega)=\sqrt \frac{\pi}{2} \delta(\omega-ia)+\sqrt \frac{\pi}{2} \delta(\omega+ia)$. So ...
2
votes
0answers
38 views

Does the dirac delta function have a residue?

I came to this question by looking at the fourier transform of a hyperbolic cosine. Let's say $f(x)=\cosh(ax)$, where $a$ is a complex number and $x$ is real. Then the fourier transform is ...
1
vote
1answer
38 views

definite integral of form G(cos(x), sin(x)) by complex integration

Given: the following integral: $$ \int_0^{2\pi} \frac{\mathrm{sin}(3x)}{5-3\mathrm{cos}(x)}\,\mathrm{d}x = 0 $$ Prove it by using complex integration and the residue theorem. But I do something wrong ...
0
votes
0answers
39 views

Contour integral with a different contour

This question is from the post: Contour integral with branch cut. My question is: if we choose the key hole contour with branch cut on the positive x axis, it seems that we have an addtional term: ...
3
votes
0answers
60 views

Solution of $\int_0^{\pi} \frac{ y \cos y}{s^2+y^2} dy$

Is there a solution for the following integral (even in terms of Bessel or Struve functions)? $$ \int_0^{\pi} \frac{ y \cos(y)}{s^2+y^2} \,dy $$
1
vote
1answer
41 views

$\int_c \frac{8-z}{z(4-z)} dz$

I want to calculate the following contour integral: $$\int_c \frac{8-z}{z(4-z)} dz$$ where $C$ is the circle of radius $7$, center $0$, negatively oriented. Do I have to do this the long and ...
2
votes
1answer
73 views

integral $\int_0^\pi \frac{\cos(2t)}{a^2\sin^2(t)+b^2\cos^2(t)}dt$

I want to compute this integral $$\int_0^\pi \frac{\cos(2t)}{a^2\sin^2(t)+b^2\cos^2(t)}dt$$ which can help me,where $b \leq a$. Thanks for all.
2
votes
1answer
74 views

Non trivial integral with the Bose-Einstein distribution and Cosine function

Do you have any idea how to solve this integral? $$\int\limits_0^\infty {\frac{{\cos \left( mx \right)}}{{x + {x_0}}}\left( {1 + n\left( x \right)} \right)} - \int\limits_0^\infty {\frac{{\cos ...