Euler phi
From specialfunctionswiki
The Euler phi function (not to be confused with the Euler totient) is defined for $q \in \mathbb{C}$ with $|q|<1$ by $$\phi(q) = \displaystyle\prod_{k=1}^{\infty} 1-q^k.$$
Graph of $\phi$.
Domain coloring $\phi$.
Properties
Relationship between Euler phi and q-Pochhammer
