Integral involving the complete Gamma function Γ
Today we evaluate this integral proposed by @BenriBot_ccbs: ∫10ln(1−x1+x)√x(1+x2)dx=−√πΓ2(54) Update: I added another solution which involves the complete Beta function and seems to be far easier.
Proof
Solution 1 (New). Using the complete beta function B(x,y): I=∫10ln(1−x1+x)√x(1+x2)dx=√2∫10ln(w)√1−w4dw(w↦1−x1+x)=√2∫10(ddt|t=0+wt)√1−w4dw=ddt|t=0+√2∫10wt√1−w4dw=ddt|t=0+√2∫10wt(1−w4)−12dw=ddt|t=0+√24∫10st−34(1−s)−12ds=ddt|t=0+√24B(t+14,12) Recall the derivative of the complete beta function: ∂∂xB(x,y)=B(x,y)(ψ(x)−ψ(x+y)) Hence I=ddt|t=0+√24B(t+14,12)=limt→0+√216B(t+14,12)[ψ(t+14)−ψ(t+34)]=√216B(14,12)[ψ(14)−ψ(34)]=√216Γ(14)Γ(12)Γ(34)[ψ(14)−ψ(34)] From the reflection formula ψ(1−x)−ψ(x)=πcot(πx) we have I=√216Γ(14)Γ(12)Γ(34)[ψ(14)−ψ(34)]=−√2π3416Γ(14)Γ(34) which is bascially the same as −√πΓ2(54).
Solution 2. Using 3F2: I=∫10ln(1−x1+x)√x(1+x2)dx=√2∫10ln(w)√1−w4dw(w↦1−x1+x)=√2∫10(ddt|t=0+wt)√1−w4dw=ddt|t=0+√2∫10wt√1−w4dw=ddt|t=0+√2∫10∞∑n=0(−12n)(−1)nw4nwtdw=ddt|t=0+√2∞∑n=0(−12n)(−1)n∫10w4n+tdw=ddt|t=0+√2∞∑n=0(−12n)(−1)n4n+t+1=√2∞∑n=0(−12n)[ddt|t=0+(−1)n4n+t+1]=−√2∞∑n=0(−12n)(−1)n(4n+1)2 Using the formula that relate the binomial coefficient to the Pochhammer polynomial (rising factorial): (vm)=(v−m+1)mm! and the reflection formula (−x)n=(−1)n(x−n+1)n Therefore I=−√2∞∑n=0(−12n)(−1)n(4n+1)2=−√2∞∑n=0(−12−n+1)nn!(−1)n(4n+1)2 from (1)=−√2∞∑n=0(12)nn!1(4n+1)2 from (2) Recall the recursion formula for the argument of the rising factorial: n+x=x(x+1)n(x)n Hence I=−√2∞∑n=0(12)nn!1(4n+1)2=−√216∞∑n=0(12)nn!1(n+14)2=−√2∞∑n=0(12)n(14)n(14)n(54)n(54)n1n! from (3)=−√23F2[12,14,1454,54;1] This is the Generalized hypergeometric function 3F2(a,b,c;d,e;z). This function satisfies the Dixon's well-poised sum: 3F2[a,b,ca−b+1,a−c+1;1]=Γ(12a+1)Γ(a−b+1)Γ(a−c+1)Γ(12a−b−c+1)Γ(a+1)Γ(12a−b+1)Γ(12a−c+1)Γ(a−b−c+1)ℜ(a−2b−2c)>−2 Putting a=12,b=14,c=14: I=−√23F2[12,14,1454,54;1]=−√2Γ(12a+1)Γ(a−b+1)Γ(a−c+1)Γ(12a−b−c+1)Γ(a+1)Γ(12a−b+1)Γ(12a−c+1)Γ(a−b−c+1)=−√2Γ(54)Γ(54)Γ(54)Γ(34)Γ(32)Γ(1)Γ(1)Γ(1)=−√2Γ3(54)Γ(34)Γ(32)=−√πΓ2(54) Therefore, we can conclude ∫10ln(1−x1+x)√x(1+x2)dx=−√πΓ2(54) which is basically the same result as before
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