The integral is
∫0−∞x1/3x5−1dx
I have tried taking the typical half circle contour and finding the enclosed residues:
Res(f,1)=15Res(f,e2πi5)=e8πi155Res(f,e4πi5)=e−2πi155
By Jordan's lemma, the arc contributes nothing. However, I have a problem with the non integrable singularity at 1 on R+.
Can I deviate to avoid it while also keeping track of the contribution from the positive real axis? There doesn't seem to be great symmetry at 1 thanks to the numerator. Is my contour not a good choice?
edit: Also tried changing variables to evaluate on the positive real axis, but this again introduces the bad singularity.
Answer
Assuming that x1/3 is defined as −(−x)1/3 on R−, the change of variable x=−z3 bring the given integral in the form
∫+∞03z3z15+1dz
and by setting 11+z15=u the previous integral can be computed through Euler's Beta function. By exploiting the reflection formula for the Γ function, Γ(z)Γ(1−z)=πsin(πz), such integral simplifies to
π5sin4π15=π5cos7π30=4π5√7−√5+√6(5−√5).
No comments:
Post a Comment