Integral involving Catalan constant
Today we show the proof of the following result found on Twitter: ∫π20∫∞01y√xπ+1dxdy=2β(2)
Proof
I=∫π20∫∞01y√xπ+1dxdy=∫π20y2π∫∞0w2yπ−1w2+1dwdy(w↦xπ2y) Recall the integral representation fo sec(x) (proof in the Appendix): sec(x)=2π∫∞0t2xπt2+1dt Hence 2π∫∞0w2yπ−1w2+1dw=sec(y−π2)=csc(y)=1sin(y) Therefore I=∫π20ysin(y)dy=2∫∞0arccot(ew)dw(w↦ln(cot(y2))) Recall the series expansion for arccot(x) ∞∑n=0(−1)n(2n+1)x−2n−1 Hence I=2∫∞0arccot(ew)dw=2∫∞0∞∑n=0(−1)n(2n+1)e−w(2n+1)dw=2∞∑n=0(−1)n(2n+1)∫∞0e−w(2n+1)dw=2∞∑n=0(−1)n(2n+1)2=2β(2) ∴∫π20∫∞01y√xπ+1dxdy=2β(2)
Appendix: Integral representation of secx
How do we know that secx=2π∫∞0y2xπ1+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=∫∞0∫10yaty2dtdy=∫10∫∞0yaty2dydt=12∫10∫∞0wa2−12twdwdt(w↦y2) The integral ∫∞0wa2−12twdw 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 ∫∞0wa2−12twdw=Γ(a+12)[−ln(t)]−a+12 Therefore 12∫10∫∞0wa2−12twdwdt=12∫10Γ(a+12)[−ln(t)]−a+12dt=12Γ(a+12)∫10[−ln(t)]−a+12dt=12Γ(a+12)∫10v−a+12e−vdv(v↦−ln(t))=12Γ(a+12)Γ(1−a2) Finally, recall the Euler's reflection formula Γ(z)Γ(1−z)=πsinπz=πcscπz Therefore ∫∞0ya1+y2dy=12Γ(a+12)Γ(1−a2)=π2csc(π1+a2)=π2sec(πa2) If we let x=πa2 then a=2xπ. Then 2π∫∞0y2xπ1+y2dy=secx
No comments:
Post a Comment