I am trying to show ∫∞0dxx3+1=2π3√3.
Any help?
(I am having troubles using the half circle infinite contour)
Or more specifically, what is the residue res(1z3+1,z0=eπi3)
Thanks!
Answer
There are already two answers showing how to find the integral using just calculus. It can also be done by the Residue Theorem:
It sounds like you're trying to apply RT to the closed curve defined by a straight line from 0 to A followed by a circular arc from A back to 0. That's not going to work, because there's no reason the intergal over the semicircle should tend to 0 as A→∞.
How would you use RT to find ∫∞0dt/(1+t2)? You'd start by noting that ∫∞0dt1+t2=12∫∞−∞dt1+t2,and apply RT to the second integral.
You can't do exactly that here, because the function 1/(1+t3) is not even. But there's an analogous trick available.
Hint: Let f(z)=11+z3.If ω=e2πi/3 then f(ωz)=f(z).(Now you're going to apply RT to the boundary of a certain sector of opening 2π/3... be careful about the "dz"...)
No comments:
Post a Comment