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Saturday, August 7, 2021

Catalan constant

Integral related to Catalan constan

Integral involving Catalan constant


Today we show the proof of the following result found on Twitter: π2001yxπ+1dxdy=2β(2)

Proof

I=π2001yxπ+1dxdy=π20y2π0w2yπ1w2+1dwdy(wxπ2y) Recall the integral representation fo sec(x) (proof in the Appendix): sec(x)=2π0t2xπt2+1dt Hence 2π0w2yπ1w2+1dw=sec(yπ2)=csc(y)=1sin(y) Therefore I=π20ysin(y)dy=20arccot(ew)dw(wln(cot(y2))) Recall the series expansion for arccot(x) n=0(1)n(2n+1)x2n1 Hence I=20arccot(ew)dw=20n=0(1)n(2n+1)ew(2n+1)dw=2n=0(1)n(2n+1)0ew(2n+1)dw=2n=0(1)n(2n+1)2=2β(2) π2001yxπ+1dxdy=2β(2)

Appendix: Integral representation of secx

How do we know that secx=2π0y2xπ1+y2dy?

The proof can be obtained trough contour integration in a branch cut but here we will use a more straigthforward approach with the help of the Ramanujan's master theorem

Let |a|<1
Recall 11+y2=10ty2dt Therefore 0ya1+y2dy=010yaty2dtdy=100yaty2dydt=12100wa212twdwdt(wy2) The integral 0wa212twdw is the Mellin transform of tw at a+12
Recall that tw=ewln(t)=n=0lnn(t)wnn!=n=0(ln(t))n(wn)n! Therefore, by Ramanujan's master theorem 0wa212twdw=Γ(a+12)[ln(t)]a+12 Therefore 12100wa212twdwdt=1210Γ(a+12)[ln(t)]a+12dt=12Γ(a+12)10[ln(t)]a+12dt=12Γ(a+12)10va+12evdv(vln(t))=12Γ(a+12)Γ(1a2) Finally, recall the Euler's reflection formula Γ(z)Γ(1z)=πsinπz=πcscπz Therefore 0ya1+y2dy=12Γ(a+12)Γ(1a2)=π2csc(π1+a2)=π2sec(πa2) If we let x=πa2 then a=2xπ. Then 2π0y2xπ1+y2dy=secx

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