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Saturday, August 14, 2021

Feynman's trick IX

Feynman's trick IX and Mellin transform

Mellin transform of sinx and cosx



We show the proof of the following result posted by @infseriesbot 0(sinxcosx)lnxxdx=π322

Proof

First we can start differentiating under the integral sign: 0(sinxcosx)lnxxdx=0(sinxcosx)(ddt|t=0+xt)xdx=ddt|t=0+0xt+121(sinxcosx)dx The right hand side is the derivative of the Mellin transform of the function (sinxcosx) at t+12: We can split the Mellin into two, the Mellin of sinx and cosx: {Msinx}(t+12)=0xt+121sinxdx {Mcosx}(t+12)=0xt+121cosxdx Now we can expand sinx and cosx using the Euler's formula and the de Moivre theorem in order to apply the Ramanujan's master theorem: sin(w)=eixeix2i=n=0(ix)n2in!n=0(ix)n2in!=n=0(x)n[(i)n(i)n]2in! By de Moivre's theorem (i)n=(eiπ2)n=cos(nπ2)+isin(nπ2)=cos(nπ2)isin(nπ2) in=(eiπ2)n=cos(nπ2)+isin(nπ2) Then [(i)n(i)n]2i=2isin(nπ2)2i=sin(nπ2) Therefore sinx=n=0(x)n[sin(nπ2)]n!dw In a similar way we can show: cosx=n=0(x)n[cos(nπ2)]n!dw Applying the Ramanujan's master theorem and using the fact that sin is odd and cos is even 0xt+121sinxdx=Γ(t+12)sin(tπ2+π4) 0xt+121cosxdx=Γ(t+12)cos(tπ2+π4) Therefore: ddt|t=0+0xt+121sinxdx=ddt|t=0+Γ(t+12)sin(tπ2+π4)=limt0+12Γ(t+12)[πsin(πt2+π4)+2cos(πt2+π4)ψ(0)(t+12)]=π3222+π2ψ(0)(12) ddt|t=0+0xt+121cosxdx=ddt|t=0+Γ(t+12)cos(tπ2π4)=limt0+12Γ(t+12)[2cos(πt2+π4)ψ(0)(t+12)πsin(πt2+π4)]=π2ψ(0)(12)π3222 Hence, we can conclude: 0(sinxcosx)lnxxdx=ddt|t=0+0xt+121(sinxcosx)dx=π322 0(sinxcosx)lnxxdx=π322

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