\[\left(aq^{-n};q\right)_{n-k}=\frac{\left(q/a;q\right)_{n}}{\left(q/a;q\right)_%
{k}}\left(-\frac{a}{q}\right)^{n-k}q^{\genfrac{(}{)}{0.0pt}{}{k}{2}-\genfrac{(%
}{)}{0.0pt}{}{n}{2}},\]