\[\sum_{j=0}^{n}\genfrac{[}{]}{0.0pt}{}{n}{j}_{q}(-z)^{j}q^{\genfrac{(}{)}{0.0pt%
}{}{j}{2}}=\left(z;q\right)_{n}={(1-z)(1-zq)\cdots(1-zq^{n-1})}.\]