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Question Number 28169 by abdo imad last updated on 21/Jan/18

prove that  ∫_0 ^1    (((lnx)^k )/(1−x))dx=(−1)^k  (k!)ξ(k+1)  .

$${prove}\:{that}\:\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\:\frac{\left({lnx}\right)^{{k}} }{\mathrm{1}−{x}}{dx}=\left(−\mathrm{1}\right)^{{k}} \:\left({k}!\right)\xi\left({k}+\mathrm{1}\right)\:\:. \\ $$

Commented by abdo imad last updated on 23/Jan/18

we have  ∫_0 ^1  (((ln(x))^k )/(1−x))dx= ∫_0 ^1 (lnx)^k (Σ_(p=0) ^(+∞)  x^p )dx  = Σ_(p=0) ^(+∞)   ∫_0 ^1  x^p  (lnx)^k dx  let put   I_(p,k) =∫_0 ^1  x^p (lnx)^k dx  by parts we get  I_(p,k)   = [ (1/(p+1)) x^(p+1)  (lnx)^k ]_0 ^1  − ∫_0 ^1  (1/(p+1)) x^(p+1)  (k/x) (lnx)^(k−1)  dx  =((−k)/(p+1)) ∫_0 ^1   x^p  (lnx)^(k−1) dx= ((−k)/(p+1)) I_(p,k−1 )  =(((−1)^2 k(k−1))/((p+1)^2 )) I_(p,k−2)   =(((−1)^k  k!)/((p+1)^k )) I_(p,0) =(((−1)^k  k!)/((p+1)^(k+1) ))  so  ∫_0 ^1     (((lnx)^k )/(1−x))dx  = Σ_(p=0) ^(+∞)   (((−1)^k  k!)/((p+1)^(k+1) )) = (−1)^k  k! Σ_(p=0) ^(+∞)    (1/((p+1)^(k+1) ))  =(−1)^k  k! Σ_(p=1) ^(+∞)   (1/p^(k+1) )= (−1)^k  k! ξ(k+1)  .

$${we}\:{have}\:\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\frac{\left({ln}\left({x}\right)\right)^{{k}} }{\mathrm{1}−{x}}{dx}=\:\int_{\mathrm{0}} ^{\mathrm{1}} \left({lnx}\right)^{{k}} \left(\sum_{{p}=\mathrm{0}} ^{+\infty} \:{x}^{{p}} \right){dx} \\ $$$$=\:\sum_{{p}=\mathrm{0}} ^{+\infty} \:\:\int_{\mathrm{0}} ^{\mathrm{1}} \:{x}^{{p}} \:\left({lnx}\right)^{{k}} {dx}\:\:{let}\:{put}\: \\ $$$${I}_{{p},{k}} =\int_{\mathrm{0}} ^{\mathrm{1}} \:{x}^{{p}} \left({lnx}\right)^{{k}} {dx}\:\:{by}\:{parts}\:{we}\:{get} \\ $$$${I}_{{p},{k}} \:\:=\:\left[\:\frac{\mathrm{1}}{{p}+\mathrm{1}}\:{x}^{{p}+\mathrm{1}} \:\left({lnx}\right)^{{k}} \right]_{\mathrm{0}} ^{\mathrm{1}} \:−\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\frac{\mathrm{1}}{{p}+\mathrm{1}}\:{x}^{{p}+\mathrm{1}} \:\frac{{k}}{{x}}\:\left({lnx}\right)^{{k}−\mathrm{1}} \:{dx} \\ $$$$=\frac{−{k}}{{p}+\mathrm{1}}\:\int_{\mathrm{0}} ^{\mathrm{1}} \:\:{x}^{{p}} \:\left({lnx}\right)^{{k}−\mathrm{1}} {dx}=\:\frac{−{k}}{{p}+\mathrm{1}}\:{I}_{{p},{k}−\mathrm{1}\:} \:=\frac{\left(−\mathrm{1}\right)^{\mathrm{2}} {k}\left({k}−\mathrm{1}\right)}{\left({p}+\mathrm{1}\right)^{\mathrm{2}} }\:{I}_{{p},{k}−\mathrm{2}} \\ $$$$=\frac{\left(−\mathrm{1}\right)^{{k}} \:{k}!}{\left({p}+\mathrm{1}\right)^{{k}} }\:{I}_{{p},\mathrm{0}} =\frac{\left(−\mathrm{1}\right)^{{k}} \:{k}!}{\left({p}+\mathrm{1}\right)^{{k}+\mathrm{1}} }\:\:{so} \\ $$$$\int_{\mathrm{0}} ^{\mathrm{1}} \:\:\:\:\frac{\left({lnx}\right)^{{k}} }{\mathrm{1}−{x}}{dx}\:\:=\:\sum_{{p}=\mathrm{0}} ^{+\infty} \:\:\frac{\left(−\mathrm{1}\right)^{{k}} \:{k}!}{\left({p}+\mathrm{1}\right)^{{k}+\mathrm{1}} }\:=\:\left(−\mathrm{1}\right)^{{k}} \:{k}!\:\sum_{{p}=\mathrm{0}} ^{+\infty} \:\:\:\frac{\mathrm{1}}{\left({p}+\mathrm{1}\right)^{{k}+\mathrm{1}} } \\ $$$$=\left(−\mathrm{1}\right)^{{k}} \:{k}!\:\sum_{{p}=\mathrm{1}} ^{+\infty} \:\:\frac{\mathrm{1}}{{p}^{{k}+\mathrm{1}} }=\:\left(−\mathrm{1}\right)^{{k}} \:{k}!\:\xi\left({k}+\mathrm{1}\right)\:\:. \\ $$

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