CAOP

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