Euler phi
From specialfunctionswiki
The Euler phi function is defined for $q \in \mathbb{C}$ with $|q|<1$ by $$\phi(q) = \displaystyle\prod_{k=1}^{\infty} 1-q^k.$$
Graph of $\phi$.
- Complex qpochhammer (q,q) infty.png
Domain coloring of analytic continuation of $(q,q)_{\infty}$ to the unit disk.
