Saturday, 10 August 2019

integration - Contour integral: int0inftyfracx1/3x51mathrmdx



The integral is
0x1/3x51dx



I have tried taking the typical half circle contour and finding the enclosed residues:
Res(f,1)=15Res(f,e2πi5)=e8πi155Res(f,e4πi5)=e2πi155
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(1z)=πsin(πz), such integral simplifies to
π5sin4π15=π5cos7π30=4π575+6(55).


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