# How can one compute this simple infinite product? [duplicate]

Possible Duplicate:
Limit of a particular variety of infinite product/series

Define $$F(x) = \prod_{n=1}^\infty(1-x^n)$$ where $|x|<1$.

How can one compute $F(1/2)$? (Without an obvious polynomial expansion or brute-force calculation.)

This is sometimes called Euler's function.

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## marked as duplicate by Raymond Manzoni, Norbert, Henry T. Horton, TMM, Austin MohrDec 13 '12 at 19:54

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Some related information (no 'closed form'). – Raymond Manzoni Dec 13 '12 at 18:02
Quick simulation stops around 0.288788... – gt6989b Dec 13 '12 at 18:07
@RaymondManzoni that's a great source. Nearly as good as a closed form. – Pricklebush Tickletush Dec 13 '12 at 18:10
Glad it helped @AlecS but I fear we will have to close this as duplicate! – Raymond Manzoni Dec 13 '12 at 18:16

## 1 Answer

That product is not so simple as you think. Euler proved that $$F(x) = \prod_{n=1}^\infty(1-x^n)=\sum_{-\infty}^{+\infty}(-1)^kx^{\frac{k(3k+1)}{2}}$$ This problem arises in partition number theory and is called Euler's pentagonal number theorem.

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Thanks a lot. I hadn't seen this identity. – Pricklebush Tickletush Dec 13 '12 at 18:25
@AlecS: This paper from Bell may help you too. – Raymond Manzoni Dec 13 '12 at 18:29
@RaymondManzoni I had begun reading it already. :) – Pricklebush Tickletush Dec 13 '12 at 18:36
@AlecS: a rather fascinating subject I'll admit... – Raymond Manzoni Dec 13 '12 at 18:45