Sunday, 18 December 2016

calculus - Prove intinfty0!fracmathbbdx1+xn=fracpinsinfracpin using real analysis techniques only



I have found a proof using complex analysis techniques (contour integral, residue theorem, etc.) that shows 0dx1+xn=πnsinπn for nN+{1}



I wonder if it is possible by using only real analysis to demonstrate this "innocent" result?



Edit
A more general result showing that
$$\int\limits_{0}^{\infty} \frac{x^{a-1}}{1+x^{b}} \ \text{dx} = \frac{\pi}{b \sin(\pi{a}/b)}, \qquad 0 < a
can be found in another math.SE post



Answer



011+xn dx=00e(1+xn)t dt dx



=00etetxn dx dt=1n00eteu(ut)1n11t du dt



=1n0t1net0u1n1eu du dt=1n0t1net Γ(1n) dt



=1n Γ(11n)Γ(1n)=πncsc(πn)


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