Q-shifted factorial
From specialfunctionswiki
The $q$-shifted factorial $(a;q)_n$ is defined by the formula $$(a;q)_n=\displaystyle\prod_{k=0}^{n-1} 1-aq^{k-1}=(1-a)(1-aq)(1-aq^2)\ldots(1-aq^{n-1}).$$
The $q$-shifted factorial $(a;q)_n$ is defined by the formula $$(a;q)_n=\displaystyle\prod_{k=0}^{n-1} 1-aq^{k-1}=(1-a)(1-aq)(1-aq^2)\ldots(1-aq^{n-1}).$$
