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I am trying to calculate this integral, but I find it is very challenging $$\int_{0}^{1}\frac{1-x}{1+x}\cdot \frac{x^n}{\sqrt{x^4-2x^2+1}}\mathrm dx$$

but somehow I have managed to local its closed form to be $$(-1)^n\left(\frac{1}{2}-n\ln 2+\sum_{j=0}^{n-1}H^{*}_j\right)$$

where $n\ge 0$, $H^{*}_0=0$ and $H^{*}_k=\sum_{j=1}^{k}\frac{(-1)^{j+1}}{j}$

I have try

$$\frac{1-x}{1+x}\cdot \frac{x^n}{(x^2-1)^{1/2}}$$

$$-x^n\sqrt{\frac{x-1}{(x+1)^2}}$$

$$-\int_{0}^{1}x^n\sqrt{\frac{x-1}{(x+1)^3}}\mathrm dx$$

from this point I tried to use the binomial to expand $$\sqrt{\frac{x-1}{(x+1)^3}}$$ but it seem not possible

user26857
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Sibawayh
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3 Answers3

4

In $0\le x<1$

$$\frac{1-x}{1+x}\cdot \frac{x^n}{\sqrt{x^4-2x^2+1}}=\dfrac{x^n}{(1+x)^2}$$

If $\displaystyle I_n=\int_{0}^{1}\dfrac{1-x}{1+x}\cdot \frac{x^n}{\sqrt{x^4-2x^2+1}}\mathrm dx$

$$I_n+2I_{n-1}+I_{n-2}=\int_0^1x^{n-2} dx$$

Now $I_0=?,I_1=?$

4

I will show that $$\int_0^1 \frac{1 - x}{1 + x} \frac{x^n}{\sqrt{x^4 - 2x^2 + 1}} \, dx = -\frac{1}{2} + n (-1)^{n + 1} \left (\ln 2 + \sum_{k = 1}^{n - 1} \frac{(-1)^k}{k} \right ), n \geqslant 1.$$ Note here we interpret the empty sum as being equal to zero (the empty sum is the case when $n = 1$ in the finite sum) and I assume $n \in \mathbb{N}$.

For the case when $n = 0$, a direct evaluation yields: $I_0 = \frac{1}{2}$.

As already noted, since $$\frac{1-x}{1+x}\cdot \frac{x^n}{\sqrt{x^4-2x^2+1}}=\dfrac{x^n}{(1+x)^2},$$ the integral becomes $$I_n = \int_0^1 \frac{x^n}{(1 + x)^2} \, dx.$$ For $n \in \mathbb{N}$, integrating by parts we have \begin{align} I_n &= \left [-\frac{x^n}{1 + x} \right ]_0^1 + n\int_0^1 \frac{x^{n - 1}}{1 + x} \, dx\\ &= -\frac{1}{2} + n \int_0^1 \frac{x^{n - 1}}{1 + x} \, dx\\ &= -\frac{1}{2} + n \sum_{k = 0}^\infty (-1)^k \int_0^1 x^{n + k - 1} \, dx\\ &= -\frac{1}{2} + n \sum_{k = 0}^\infty \frac{(-1)^k}{n + k}. \end{align} Note here the geometric sum for $1/(1 + x)$ of $\sum_{k = 0}^\infty (-1)^k x^k$ has been used. Reindexing the sum $k \mapsto k - n$ gives \begin{align} I_n &= -\frac{1}{2} + n (-1)^n \sum_{k = n}^\infty \frac{(-1)^k}{k}\\ &= -\frac{1}{2} + n(-1)^n \left (\sum_{k = 1}^\infty \frac{(-1)^k}{k} - \sum_{k = 1}^{n - 1} \frac{(-1)^k}{k} \right )\\ &= -\frac{1}{2} + n(-1)^{n + 1} \left (\ln 2 + \sum_{k = 1}^{n - 1} \frac{(-1)^k}{k} \right ), \end{align} where I have made use of the well-known result of $\ln 2 = -\sum_{k = 1}^\infty (-1)^k/k$.

In terms of your $H^*_n$ notation for the finite sum $\sum_{k = 1}^n \frac{(-1)^{k + 1}}{k}$, this result can be re-expressed as $$I_n = -\frac{1}{2} + n (-1)^{n + 1} (\ln 2 - H^*_{n - 1}).$$

To show my result is equivalent to the result you quote, one would need to show, after playing around with finite sums, that $$\sum_{k = 1}^{n - 1} H^*_k = -\frac{1}{2} (1 + (-1)^n) + n H^*_{n - 1}.$$

omegadot
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My solution is similar to that of @omegadot but I think it may be a little cleaner. For $n\in\mathbb{Z}^+$ let $$C_n=(-1)^n\int_0^1\frac{x^{n-1}}{x+1}dx$$ Then $$C_1=-\int_0^1\frac{dx}{x+1}=\left.-\ln(x+1)\right|_0^1=-\ln2$$ $$C_{n+1}-C_n=(-1)^{n+1}\int_0^1\frac{x^n+x^{n-1}}{x+1}dx=(-1)^{n+1}\int_0^1x^{n-1}dx=\frac{(-1)^{n+1}}n$$ So we can sum a telescoping series to get $$C_n-C_1=\sum_{k=1}^{n-1}\left(C_{k+1}-C_k\right)=\sum_{k=1}^{n-1}\frac{(-1)^{k+1}}k$$ Then since $$\sqrt{x^4-2x^2+1}=\sqrt{\left(1-x^2\right)^2}=\left|1-x^2\right|=1-x^2=(1+x)(1-x)$$ for $0\le x\le 1$, we have for $n\in\mathbb{Z}^+$ $$\begin{align}\int_0^1\frac{1-x}{1+x}\cdot\frac{x^n}{\sqrt{x^4-2x^2+1}}dx&=\int_0^1\frac{x^n}{(x+1)^2}dx=\left.-\frac1{(x+1)}x^n\right|_0^1+n\int_0^1\frac{x^{n-1}}{x+1}dx\\ &=-\frac12+(-1)^nnC_n\\ &=-\frac12+(-1)^nn\left[-\ln 2-\sum_{k=1}^{n-1}\frac{(-1)^k}k\right]\end{align}$$ This is using a recurrence relation rather than an infinite series. Of course for $n=0$ the integral works out to $$\int_0^1\frac{dx}{(x+1)^2}=\left.-\frac1{(x+1)}\right|_0^1=-\frac12+1=\frac12$$

user5713492
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