# Very curious sequence of integrals $I_n=\int_0^1 \frac {(x(1-x))^{4n}} {1+x^2}\mathrm dx$

I was studying the behaviour of very curious sequence of integrals $$I_n=\int_0^1 \frac {(x(1-x))^{4n}} {1+x^2} \,\mathrm dx$$ which gives a very beautiful result for $n=1$; I tried to calculate for different values of $n$ but every time what I get is a $4^{n-1}$ times $\pi$ along with a fraction that in the denominator has almost a product a consecutive primes, Can we generalize this pattern? Any help would be appreciated!

Here are few calculations:

$$I_1=22/7-\pi$$

$$I_2=-\frac {2^2 \cdot 43\cdot 1097} {3\cdot 5\cdot 7\cdot 11 \cdot 13} +4\pi$$

$$I_3=\frac {13\cdot 31\cdot 13912991} {3\cdot 5\cdot 7\cdot 11\cdot 13\cdot 17\cdot 19\cdot 23}-16\pi$$

## 2 Answers

This problem is extensively studied in the paper Integral approximations of $\pi$ with non-negative integrands. by S. K. Lucas. See page 5 for explicit formula.

• Very neat. Stuff like this makes math so exciting. – Cameron Williams Nov 26 '13 at 18:36
• Interesting! However, the question about the primes in the denominators still stands... – Bruno Joyal Nov 27 '13 at 3:11

Well, I guess I can't compete with this research cited by Norbert, but maybe this will be of any help. One can try to expand $(1-x)^n = \sum_{k=0}^n {n \choose k}(-x)^k$, so $$\int_0^1 \frac {(x(1-x))^{4n}} {1+x^2} \,\mathrm dx= \sum_{k=0}^n {n \choose k}(-1)^k\int_0^1 \frac {x^{4n+k}} {1+x^2} \,\mathrm dx$$ Then one can change variable so $$\int_0^1 \frac {x^{4n+k}} {1+x^2} \,\mathrm dx=\frac{1}{2}\int_0^1 \frac {t^{2n+\frac{k-1}{2}}} {1+t} \,\mathrm dt$$ The last integral (in the indefinite form) is: $$\int_0^1 \frac {t^{2n+\frac{k-1}{2}}} {1+t} \,\mathrm dt=\frac{2 t^{\frac{1}{2} (k+4 n+1)} \, _2F_1\left(1,\frac{1}{2} (k+4 n+1);\frac{1}{2} (k+4 n+3);-t\right)}{k+4 n+1}$$ Plugging the limits will give: $$\int_0^1 \frac {(x(1-x))^{4n}} {1+x^2} \,\mathrm dx=\frac{1}{4}\sum_{k=0}^n {n \choose k}(-1)^k\left(\psi ^{(0)}\left(\frac{k}{4}+n+\frac{3}{4}\right)-\psi ^{(0)}\left(\frac{k}{4}+n+\frac{1}{4}\right)\right)$$ where $\psi ^{(0)}\left(\frac{k}{4}+n+\frac{1}{4}\right)$ is the $0$-derivative of the digamma function $\psi(z)$.