θ2⁡(z|τ)=θ2⁡(0|τ)⁢cos⁡z⁢∏n=1∞cos⁡(n⁢π⁢τ+z)⁢cos⁡(n⁢π⁢τ-z)cos2⁡(n⁢π⁢τ),