$\eta = \pm 1$ の稜線

$${}^{t} \{ n \} = \{ n \} ( \! ( \{ \xi \} ) \! )$$

$$= \frac{ \frac{ \partial {}^{t} \{ x \} }{ \partial \xi } }{ \left\vert \frac{ \partial {}^{t} \{ x \} }{ \partial \xi } \right\vert }$$ (5.40)