Integral involving the trigamma function ψ(1)(x)
Today we show the proof of this result proposed by @Ali39342137 ∫10ln(x)xn+xn−1+⋯+1dx=1(n+1)2[ψ(1)(2n+1)+ψ(1)(1n+1)]
Proof:
Note that 1+x+⋯+xn−1+xn=n∑j=0xj=1−xn+11−x
Hence
I=∫10ln(x)xn+xn−1+⋯+1dx=∫10(1−x)ln(x)1−xn+1dx=∫10ln(x)1−xn+1dx⏟J−∫10xln(x)1−xn+1dx⏟K
Since 0<x<1
J=∫10ln(x)1−xn+1dx=∫10∞∑j=0x(n+1)jln(x)dx=∞∑j=0∫10x(n+1)j(ddt|t=0+xt)dx=ddt|t=0+∞∑j=0∫10x(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)1−xn+1dx=∫10∞∑j=0x(n+1)j+1ln(x)dx=∞∑j=0∫10x(n+1)j+1(ddt|t=0+xt)dx=ddt|t=0+∞∑j=0∫10x(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+xn−1+⋯+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+xn−1+⋯+1dx=∞∑j=01(n+1)2(j+2n+1)2−∞∑j=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+xn−1+⋯+1dx=1(n+1)2[ψ(1)(2n+1)+ψ(1)(1n+1)]
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