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Wednesday, December 8, 2021

Integral of the day XIV

Useful integral

Integral involving the trigamma function ψ(1)(x)


Today we show the proof of this result proposed by @Ali39342137 10ln(x)xn+xn1++1dx=1(n+1)2[ψ(1)(2n+1)+ψ(1)(1n+1)]


Proof:

Note that 1+x++xn1+xn=nj=0xj=1xn+11x
Hence I=10ln(x)xn+xn1++1dx=10(1x)ln(x)1xn+1dx=10ln(x)1xn+1dxJ10xln(x)1xn+1dxK
Since 0<x<1 J=10ln(x)1xn+1dx=10j=0x(n+1)jln(x)dx=j=010x(n+1)j(ddt|t=0+xt)dx=ddt|t=0+j=010x(n+1)j+tdx=ddt|t=0+j=01(n+1)j+t+1=j=0[ddt|t=0+1(n+1)j+t+1]=j=01((n+1)j+1)2
K=10xln(x)1xn+1dx=10j=0x(n+1)j+1ln(x)dx=j=010x(n+1)j+1(ddt|t=0+xt)dx=ddt|t=0+j=010x(n+1)j+1+tdx=ddt|t=0+j=01(n+1)j+2+t=j=0[ddt|t=0+1(n+1)j+2+t]=j=01((n+1)j+2)2
Hence 10ln(x)xn+xn1++1dx=j=01((n+1)j+1)2+j=01((n+1)j+2)2=j=01(n+1)2(j+1n+1)2+j=01(n+1)2(j+2n+1)2
Recall that he polygamma function has the series representation ψ(m)(z)=(1)m+1m!k=01(z+k)m+1
Therefore 10ln(x)xn+xn1++1dx=j=01(n+1)2(j+2n+1)2j=01(n+1)2(j+1n+1)2=1(n+1)2[ψ(1)(2n+1)+ψ(1)(1n+1)]
Hence, we can conclude 10ln(x)xn+xn1++1dx=1(n+1)2[ψ(1)(2n+1)+ψ(1)(1n+1)]

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