Φ(2)⁡(a;b,b′;c,c′;x,y)=(b,a⁢x;q)∞(c,x;q)∞⁢∑n,r≧0(a,b′;q)n⁢(c/b,x;q)r⁢br⁢yn(q,c′;q)n⁢(q)r⁢(a⁢x;q)n+r,