Q-Binomial coefficient
From specialfunctionswiki
The $q$-Binomial coefficient is $$\left[ \begin{array}{ll} n \\ k \end{array} \right]_q = \dfrac{(q;q)_n}{(q;q)_k(q;q)_{n-k}},$$ where $(q;q)_{\xi}$ denotes the q-Pochhammer symbol.
