Mellin transform of sinx and cosx
We show the proof of the following result posted by @infseriesbot ∫∞0(sinx−cosx)lnx√xdx=π32√2
Proof
First we can start differentiating under the integral sign: ∫∞0(sinx−cosx)lnx√xdx=∫∞0(sinx−cosx)(ddt|t=0+xt)√xdx=ddt|t=0+∫∞0xt+12−1(sinx−cosx)dx The right hand side is the derivative of the Mellin transform of the function (sinx−cosx) at t+12: We can split the Mellin into two, the Mellin of sinx and cosx: {Msinx}(t+12)=∫∞0xt+12−1sinxdx {Mcosx}(t+12)=∫∞0xt+12−1cosxdx 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)=eix−e−ix2i=∞∑n=0(ix)n2in!−∞∑n=0(−ix)n2in!=∞∑n=0(−x)n[(−i)n−(i)n]2in! By de Moivre's theorem (−i)n=−(e−iπ2)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+12−1sinxdx=Γ(t+12)sin(tπ2+π4) ∫∞0xt+12−1cosxdx=Γ(t+12)cos(tπ2+π4) Therefore: ddt|t=0+∫∞0xt+12−1sinxdx=ddt|t=0+Γ(t+12)sin(tπ2+π4)=limt→0+12−Γ(t+12)[πsin(πt2+π4)+2cos(πt2+π4)ψ(0)(t+12)]=π322√2+√π2ψ(0)(12) ddt|t=0+∫∞0xt+12−1cosxdx=ddt|t=0+Γ(t+12)cos(tπ2−π4)=limt→0+12Γ(t+12)[2cos(πt2+π4)ψ(0)(t+12)−πsin(πt2+π4)]=√π2ψ(0)(12)−π322√2 Hence, we can conclude: ∫∞0(sinx−cosx)lnx√xdx=ddt|t=0+∫∞0xt+12−1(sinx−cosx)dx=π32√2 ∫∞0(sinx−cosx)lnx√xdx=π32√2
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