Evaluate this infinite product

sequences-and-series

Any tips on how to evaluate this infinite product?
$$\pi(F,m)=\prod_{k=m}^{\infty}(1-F^k) $$
Where $0<F<1$. I would like to get an analytic form for the function $\pi(F,m)$. I figure it has to converge because the sequence of partial products is monotonically decreasing but is bounded below by $0$, but don't know how to get much farther.

The only thing I've tried that seemed promising was looking at the product
$$\sigma(F,m) = \prod_{k=1}^{m}(1-F^k)$$
with the idea that $\pi(F,m)=\sigma(F,\infty)/\sigma(F,m-1)$. From here then
$$\ln(\sigma(F,m))=\sum_{k=1}^{m}\ln(1-F^k)$$
Substituting the Taylor series for $\ln(1-x)=-\sum x^n/n$,
$$=-\sum_{k=1}^{m}\sum_{n=1}^{\infty}\frac{F^{kn}}{n}$$
I have no idea if I'm allowed to change the order of the sums here, but if so then
$$=-\sum_{n=1}^{\infty}\frac{1}{n}\sum_{k=1}^{m}F^{kn}$$
$$=-\sum_{n=1}^{\infty}\frac{1}{n}\frac{F^n(1-F^{nm})}{1-F^n}$$
At this point I don't know how to move forward with this series. Looking for help with either this series, or the original product!

Best Answer

As far as I know, the only formal result is $$\pi(F,m)=\prod_{k=m}^{\infty}(1-F^k)=\left(F^m;F\right){}_{\infty }$$ where appears the q-Pochhammer symbol

$$\sigma(F,m) = \prod_{k=1}^{m}(1-F^k)=(F;F)_m$$

You will find this function in Mathematica, Wolfram Alpha, Maple, Sage and most programming environments.