TITLE: An integration of product $(1-x^n)$
QUESTION [22 upvotes]: Prove $$\int_0^1\prod_{n=1}^\infty(1-x^n)dx=\frac{4\pi\sqrt3}{\sqrt{23}}\frac{\sinh\frac{\pi\sqrt{23}}3}{\cosh\frac{\pi\sqrt{23}}2}.$$
In fact, product $(1-x^n)$ is difficult to compute, I hope you can show me some ideas, thank you!

REPLY [9 votes]: I didn't manage to find Marco's answer I seemed to recall, so I will just re-do the proof.
By Euler pentagonal number theorem we have
$$ \prod_{k=1}^{+\infty}(1-x^k) = \sum_{m=-\infty}^{+\infty}(-1)^m x^{\frac{3m^2-m}{2}} \tag{1}$$
hence by termwise integration the initial integral equals
$$ \sum_{m=-\infty}^{+\infty}\frac{2(-1)^m}{3m^2-m+2} \tag{2}$$
and the last series can be computed through the following identity, that comes from the logarithmic derivative of the Weierstrass product for the $\cosh$ function:
$$ \tanh(x) = \sum_{n\geq 0}\frac{8x}{4x^2+(2n+1)^2\pi^2} \tag{3}$$
or through
$$ \sum_{n\geq 0}\frac{1}{(n+a)(n+b)}=\frac{\psi(a)-\psi(b)}{a-b} \tag{4}$$
and the dilogarithm reflection and duplication formulas. Putting all together,
$$ \boxed{\int_{0}^{1}\prod_{k\geq 1}(1-x^k)\,dx = \color{red}{\frac{4\pi\sqrt{3}\sinh{\frac{\pi\sqrt{23}}{3}}}{{\sqrt{23}\cosh{\frac{\pi\sqrt{23}}{2}}}} }}$$
follows.