Basic hypergeometric phi

The basic hypergeometric series ${}_j\phi{}_{\ell}$ is defined by $${}_j\phi_{\ell}(a_1,\ldots,a_j;b_1,\ldots,b_{\ell};q,z)={}_j \phi_{\ell} \left[ \begin{array}{llllll} a_1 & a_2 & \ldots & a_j \\

   &     &        &     & ; q,z \\

b_1 & b_2 & \ldots & b_{\ell} \end{array}\right]=\displaystyle\sum_{k=0}^{\infty} \dfrac{(a_1;q)_k \ldots (a_j;q)_k}{(b_1;q)_k \ldots (b_{\ell};q)_k} \left((-1)^kq^{k \choose 2} \right)^{1+\ell-j}z^k=\displaystyle\sum_{k=0}^{\infty} \dfrac{(a_1;q)_k \ldots (a_j;q)_k}{(b_1;q)_k \ldots (b_{\ell};q)_k} \left(-q^{\frac{k-1}{2}} \right)^{k(1+\ell-j)}z^k.$$

Properties

Theorem: The following formula holds: $$\displaystyle\lim_{q \rightarrow 1^-} {}_j \phi_{\ell} \left[ \begin{array}{l|l} q^{a_1}, \ldots, q^{a_j} \\ q^{b_1}, \ldots, q^{b_{\ell}} \end{array} \Bigg| q,z(1-q)^{1+\ell-j} \right]={}_j F_{\ell}\left(a_1,\ldots,a_j;b_1,\ldots,b_{\ell};(-1)^{1+\ell-j}z \right)$$

Proof: █

Theorem: ($q$-Pfaff-Saalschütz) The following formula holds: $${}_3\phi_2(q^{-n},a,b;c,d;q,q) = \dfrac{\left(\frac{d}{a};q \right)_n \left( \frac{d}{b};q \right)_n}{\left(d;q \right)_n \left(\frac{d}{ab};q \right)_n},$$ with $cd=abq^{1-n}$.

Proof: █

Theorem: ($q$-Chu-Vandermonde sum) The following formula holds: $${}_2\phi_1(q^{-n},a;c;q,q) = \dfrac{\left( \frac{c}{a};q \right)_n}{(c;q)_n} a^n.$$

Proof: █

Theorem: ($q$-Gauss theorem) The following formula holds: $${}_2\phi_1 \left( a,b;c;q,\dfrac{c}{ab} \right) = \dfrac{\left(\frac{c}{a};q \right)_{\infty} \left( \frac{c}{b};q \right)_{\infty}}{\left(c;q\right)_{\infty} \left(\frac{c}{ab};q \right)_{\infty}}; \left| \dfrac{c}{ab} \right|<1.$$

Proof: █