CAOP

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