𝒟qn⁡((z;q)∞(a⁢b⁢z/c;q)∞⁢ϕ12⁡(a,bc;q,z))=(c/a,c/b;q)n(c;q)n⁢(1-q)n⁢(a⁢bc)n⁢(z⁢qn;q)∞(a⁢b⁢z/c;q)∞⁢ϕ12⁡(a,bc⁢qn;q,z⁢qn).