CAOP

\[ \begin{align} p_n(x;a,b|q) &= \sum_{k=0}^\infty \frac{(q^{-n};q)_k (abq^{n+1};q)_k}{(aq;q)_k (q;q)_k} \left(qx\right)^k\\ &= {}_{2}\phi_{1}\!\left(\left. {q^{-n}, abq^{n+1} \atop aq} \; \right| q ; qx \right) \end{align} \]