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Monday, December 27, 2021

Integral of the day XVII

Another integral posted on Twitter

Integral involving the complete Gamma function Γ


Today we evaluate this integral proposed by @BenriBot_ccbs: 10ln(1x1+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(1x1+x)x(1+x2)dx=210ln(w)1w4dw(w1x1+x)=210(ddt|t=0+wt)1w4dw=ddt|t=0+210wt1w4dw=ddt|t=0+210wt(1w4)12dw=ddt|t=0+2410st34(1s)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)=limt0+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(1x)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(1x1+x)x(1+x2)dx=210ln(w)1w4dw(w1x1+x)=210(ddt|t=0+wt)1w4dw=ddt|t=0+210wt1w4dw=ddt|t=0+210n=0(12n)(1)nw4nwtdw=ddt|t=0+2n=0(12n)(1)n10w4n+tdw=ddt|t=0+2n=0(12n)(1)n4n+t+1=2n=0(12n)[ddt|t=0+(1)n4n+t+1]=2n=0(12n)(1)n(4n+1)2 Using the formula that relate the binomial coefficient to the Pochhammer polynomial (rising factorial): (vm)=(vm+1)mm! and the reflection formula (x)n=(1)n(xn+1)n Therefore I=2n=0(12n)(1)n(4n+1)2=2n=0(12n+1)nn!(1)n(4n+1)2 from (1)=2n=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=2n=0(12)nn!1(4n+1)2=216n=0(12)nn!1(n+14)2=2n=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,cab+1,ac+1;1]=Γ(12a+1)Γ(ab+1)Γ(ac+1)Γ(12abc+1)Γ(a+1)Γ(12ab+1)Γ(12ac+1)Γ(abc+1)(a2b2c)>2 Putting a=12,b=14,c=14: I=23F2[12,14,1454,54;1]=2Γ(12a+1)Γ(ab+1)Γ(ac+1)Γ(12abc+1)Γ(a+1)Γ(12ab+1)Γ(12ac+1)Γ(abc+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(1x1+x)x(1+x2)dx=πΓ2(54) which is basically the same result as before

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