Complex integral Definition and 125 Threads

In mathematics, a line integral is an integral where the function to be integrated is evaluated along a curve. The terms path integral, curve integral, and curvilinear integral are also used; contour integral is used as well, although that is typically reserved for line integrals in the complex plane.
The function to be integrated may be a scalar field or a vector field. The value of the line integral is the sum of values of the field at all points on the curve, weighted by some scalar function on the curve (commonly arc length or, for a vector field, the scalar product of the vector field with a differential vector in the curve). This weighting distinguishes the line integral from simpler integrals defined on intervals. Many simple formulae in physics, such as the definition of work as



W
=

F



s



{\displaystyle W=\mathbf {F} \cdot \mathbf {s} }
, have natural continuous analogues in terms of line integrals, in this case




W
=



L



F

(

s

)

d

s




{\displaystyle \textstyle W=\int _{L}\mathbf {F} (\mathbf {s} )\cdot d\mathbf {s} }
, which computes the work done on an object moving through an electric or gravitational field F along a path



L


{\displaystyle L}
.

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  1. B

    Solving Complex Integral: How to Approach?

    \int_{-\infty}^{\infty}\frac{\ln{(a+ix)}}{x^2+1}dx I tried with integration by parts but go nowhere. I think it may require a branch cut and integrating along a contour. How would you approach this?
  2. J

    Finding Partial Fractions and Integrating over Unit Circle for Complex Integral?

    write f(z) in terms of partial fractions and integrate it counterclockwise over the unit circle where f(z)=\frac{2z+3\iota}{z^2+\frac{1}{4}} \\ 2\int\frac{z}{z^2 + \frac{1}{4}} dz \ + \ 3\iota \int\frac{1}{z^2 + \frac{1}{4}}dz \\ \ \mbox{. Also } \ z(t) = \exp{\iota t} (0 \leq \ t \...
  3. J

    Complex Integral Evaluation for f(z)=z^4 on |z|=2 using Theorems 1 and 2

    Evaluate \int_C f(z)dz by theorem 1 and check your result by theorem 2 where f(z) = z^4 and C is the semicircle |z|=2 from -2i to 2i in the right half-plane. Theorem 1 : \int_{z_0} ^{z_1} f(z)dz = F(z_1) - F(z_0) \ \ \frac{dF}{dz}=f(z) \\ \int_{-2\iota} ^{2\iota}z^4dz = \frac{z^5}{5} =...
  4. S

    Solve Complex Integral: \oint \frac{f(z)}{z^{2}+1}dz

    Homework Statement Let \gamma_{r} be the circle centered at 2i with a radius r. Compute: \oint \frac{f(z)}{z^{2}+1}dz Homework Equations 2 \pi i f(w)=\oint \frac{f(z)}{z-w}dz Cauchy's integral formula... maybe? The Attempt at a Solution I can see how to find solutions...
  5. malawi_glenn

    How Do You Solve This Complex Integral with a Curved Path?

    Homework Statement Evaluate: \int _{c} \dfrac{1- Log z}{z^{2}} dz where C is the curve: C : z(t) = 2 + e^{it} ; - \pi / 2 \leq t \leq \pi / 2 Homework Equations I know the independance of path in a domain where f(z) is analytical, but I tried the standard parametrization...
  6. P

    Complex Integral Homework: Calculate (z+(1/z))^n dz

    Homework Statement Homework Equations I hope there's someone who can help me with the following: I have to calculate the integral over C (the unit cicle) of (z+(1/z))^n dz, where z is a complex number. The Attempt at a Solution I tried to use the subtitution z=e^(i*theta), so...
  7. W

    Complex Integral: Solving the Equation $\oint _{|z+i|=1} \frac{e^z}{1+z^2} dz$

    Homework Statement \oint _{|z+i|=1} \frac{e^z}{1+z^2} dz =? The Attempt at a Solution I substituted z+i=z' and [itex]z'=e^{i\theta}[/tex] to arrive at e^{-i} \int _0 ^{2 \pi} \frac{e^{e^{i \theta}}}{-ie^{i \theta}-2} d \theta I have no clue how to solve such an integral, any...
  8. mattmns

    What is the Indefinite Integral of \sqrt{z} along a Path in the Third Quadrant?

    Here is the exercise: Use the indefinite integral to compute \int_{C} \sqrt{z}dz where C is a path from z = i to z = -1 and lying in the third quadrant. Note: \sqrt{z} = e^{(1/2)lnz} where the principal branch of lnz is defined on C \setminus [0,\infty]. ------- I am just a little unsure of...
  9. L

    Why is dealing with complex integrals so difficult?

    It's quite a "strange" thing..why people have so many difficulties with dealing with integrals of the form: \int _{c-i\infty}^{c+i\infty}dsf(s)e^{st}=I(t) ? You can always make the change of variable s=c+ix so you get: \int _{-\infty}^{+\infty}dxf(c+ix)e^{ixt}=I(t)e^{-ict} (2)...
  10. E

    Calculating Complex Integral for a Half Circle and Line - Step by Step Guide

    Hello, I have a question about a complex integral. The question is about the index of a curve. This curve is defined as: j = j1 * j2 with j1: r*exp(i*t) with t: [0,pi] and j2: [-r,r] This is quite simple: a half cirle followed by a line from (-r,0) through (0,0) to (r,0). To...
  11. E

    How to Calculate the Index of a Line in a Complex Integral?

    Hello, I have a question about a complex integral. The question is about the index of a curve. This curve is defined as: j = j1 * j2 with j1: r*exp(i*t) with t: [0,pi] and j2: [-r,r] This is quite simple: a half cirle followed by a line from (-r,0) through (0,0) to (r,0). To...
  12. Z

    Integral Problem: Solve Complex Integral on Exam

    Hey, I'm about to do an exam tomorrow and I seem to be a little stuck on how to answer this problem from a past paper. We've mainly been integrating by using the residue theorem, I forgot how to do something like this: \int_{C} \left( z^2 - 1 \right)^\frac{1}{2} dz C = \{ z : |z - 10| = 25\}...
  13. S

    Complex Integral: Struggling to Integrate [(lnx)^2](1+x^2)

    We have to integrate [(lnx)^2)/(1+(x^2)) from zero to infinity. I have set up the correct complex integral with a branch cut along the negative y-axis, but I end up with an integral of [(lnx)^2](1+x^2) from minus infinity to zero. I'm not sure how to deal with this.
  14. K

    Understanding Complex Integrals: A Step-by-Step Example with Truncated Waves

    I'm working through an example regarding the spectral content of a truncated wave, and came across this in the textbook. I have no idea what they mean by the cosine being even and the sine odd. If anyone can explain this step to me that would be great.
  15. G

    Solving Complex Integral Issues in Math Exam Prep

    I'm studying for a math exam for Complex Variable Analysis. We had a bunch of problems, and we keep having to let an integral just equal zero, and we're not sure why, but it always works. What we do is this... we're integrated over a closed contour. and we're supposed to chose a point z_0...
  16. E

    What is the result of the complex integral with Riemann zeta function?

    let be the integral..where \zeta(s) is the Riemann zeta function. \int_{c-i\infty}^{c+i\infty}ds\zeta(s)(x^{s}/s) then what would be the result?..there would be two singularities at the points s=0 and s=1 the problem is if there would be any other singularitiy on the integral
  17. K

    Can Anyone Help Solve This Complex Integral Equality?

    I attempted to prove the following equality, but to no avail. Anyone is willing to lend a hand? \int_0^{\infty} s^{2t-2v} e^{i w s} ds + \int_0^{\infty} s^{2t-2v} e^{-i w s} ds = \left[ \left( \frac{1}{-iw}\right)^{2t-2v+1} + \left( \frac{1}{iw}\right)^{2t-2v+1} \right] \Gamma(2t-2v + 1)...
  18. T

    Integrating a Complex Integral: Solving the Mystery of the Missing Factor 3

    I'm trying to perform the following integral \pi \int\limits_0^\pi {e^{2x} } \left( {\frac{1}{2} - \frac{1}{2}\cos 2x} \right)dx I split the integral and temporarely ignore the Pi so that I get \frac{1}{2}\int {e^{2x} dx} - \frac{1}{2}\int {e^{2x} \cdot \cos } \left( {2x} \right)dx...
  19. S

    Complex Integral: Solving from Ln to ArcTan

    Hi, I'm doing the following as an exercise to try and get my head around complex numbers. Specifically, I need to understand what it means to take the natural log of a complex number and what it involves. Say I wanted to integrate 1/ (1 +x^2) dx I know this is arcTan(x). I can also...
  20. quasar987

    A complex integral from the text

    In my book QM book, Gasiorowicz says that \int_{-\infty}^{\infty} e^{i(p-p')x/\hbar}dx = 2\pi \hbar \delta(p-p') Where does that come from? I mean, set i(p-p')/h = K. Then the solution is \frac{\hbar e^{i(p-p')x/\hbar}}{i(p-p')} and evaluate at infinity, it doesn't exists as the...
  21. quasar987

    Evaluating Complex Integral: 2$\int_0^{\infty} cos(-Ax) e^{-Bx^2} dx$

    Now I have to evaluate \int_{-\infty}^{\infty} e^{-Bx^2} e^{-iAx} dx Splitting it in two using Euler's identity show that the imaginary part is 0 (cuz integrand is odd). Remains the real part 2 \int_0^{\infty} cos(-Ax) e^{-Bx^2} dx for which integration by parts leads nowhere.
  22. L

    Solving a Complex Integral with Partial Fractions

    \int \frac {1}{x\sqrt{4x+1}}dx Here's what I have done so far on this problem I let u= \sqrt{4x+1} , so then u^2=4x+1 , du= \frac {2dx}{u} and x= \frac {u^2-1}{4} Substituting, I get \int \frac {1}{(\frac{u^2-1}{4})u}du Then moving stuff around, I get 4 \int \frac...
  23. E

    How do we calculate the complex integral with poles at 2npi?

    complex integral... let be the integral \int_{-i\infty}^{i\infty}\frac{1}{exp(s)-1}ds then their poles are 2n\pi my question is How would we calculate this integral? i think that the contribution from the poles is -{\pi}Res(z_0) the main problem i find is when i make the change of...
  24. L

    Complex Contour Integral: Does it Equal 0?

    Hi, Does this complex contour integral equal 0? [Int] (z^2)/(sin z) dz along the closed contour e^2(pi)(i)(t) 0<t<1 It should equal zero cause its analytic in the domain around te curve and the zero in the numerator is of higher order than the zero in the denominator at the point z=0...
  25. D

    Efficiently Solving a Complex Integral: A Scientist's Approach

    Here is the problem: {\mathop{\rm Im}\nolimits} \int {e^{x(2 + 3i)} } dx One sec, I'm having another go at it. = {\mathop{\rm Im}\nolimits} \int {e^2 } e^{3ix} dx = {\mathop{\rm Im}\nolimits} \int {e^2 } [\cos (3x) + i\sin (3x)]dx \begin{array}{l} = \frac{{ - e^2 \cos...
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