Equation 12.19.9BE92C16DAFFD

\begin{equation} \iota _{\tau ,u,a,b}[z_0:z_1] =[z_0:z_1:\mathrm{e}^{\tau }(z_0+uz_1): \mathrm{e}^{\tau }(az_1+bz_0)]\in X. \end{equation}