# ∫dx/((x^(2/3)(x+1)), integrated over [0,∞]

by Jamin2112
Tags: , ∫dx or x2 or 3x, integrated
 P: 862 1. The problem statement, all variables and given/known data As in thread title. 2. Relevant equations Residue Theorem. 3. The attempt at a solution I just need help figuring out the circle C I'll be using. Suggestions?
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,524 What does the presence of z2/3 tell you?
P: 862
 Quote by vela What does the presence of z2/3 tell you?
Other than that there's a pole at z=0?

Emeritus
HW Helper
Thanks
PF Gold
P: 11,524

## ∫dx/((x^(2/3)(x+1)), integrated over [0,∞]

Yes, other than that. In particular, what's the effect of the fractional power?
P: 862
 Quote by vela Yes, other than that. In particular, what's the effect of the fractional power?
Change the distance between z and the origin from r to r2/3
Change the angle between z and the x-axis from ø to 2ø/3
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,524 Right. Do you know what a branch point and a branch cut are?
P: 862
 Quote by vela Right. Do you know what a branch point and a branch cut are?
Yeah, I somehow need a loop that avoid z=-1 and z=0. Right?
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,524 It's more that you want to avoid crossing the branch cut than avoiding z=0, and you obviously want a piece or pieces of the contour to correspond to the original integral.
P: 862
 Quote by vela It's more that you want to avoid crossing the branch cut than avoiding z=0, and you obviously want a piece or pieces of the contour to correspond to the original integral.
So I'd take R>1 and make a half circle of radius R in the upper half of the plane. Then I'd make two little half circles that jump over z=-1 and z=0. Then I'd look at ∫C f(z)dz as the sum of several integrals, one of which can written as a real-valued integral and see what happens as R→∞ and the radii of the little half circles go to zero. Right?
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,524 No, that's too complicated. Take a look at http://en.wikipedia.org/wiki/Methods...93_branch_cuts. Also, rewrite the integrand as $$\frac{z^{1/3}}{z(z+1)}$$to make it clear how to calculate the residue at z=0.
P: 862
 Quote by vela No, that's too complicated. Take a look at http://en.wikipedia.org/wiki/Methods...93_branch_cuts. Also, rewrite the integrand as $$\frac{z^{1/3}}{z(z+1)}$$to make it clear how to calculate the residue at z=0.
So in my case the path should resemble a backwards Pacman?
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,524 Doesn't the answer to that question depend on which way Pacman is moving?
P: 862
 Quote by vela Doesn't the answer to that question depend on which way Pacman is moving?
I forgot that PacMan is in perpetual motion.

But yeah, how am I gonna do this? I need C to be formed from a series of paths, each of which will have a line integral that approaches a real value after I take some limit.
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,524 Start by taking the keyhole contour and break it into four pieces and evaluate the line integral for each piece.
P: 862
 Quote by vela Start by taking the keyhole contour and break it into four pieces and evaluate the line integral for each piece.
How would that work? I want ∫f(x)dx (integrated on [0, R]) to be one of the four line integrals.
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,524 That's what you're supposed to figure out. Did you understand the example on Wikipedia? That's pretty much the recipe you want to follow.

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