\( \boldsymbol{x}(u, v) = u \hskip 0.1em \begin{pmatrix} \cos v \\ \sin v \\ 0 \end{pmatrix} + \begin{pmatrix} 0 \\ 0 \\ a v \end{pmatrix} \)

\( \left( - \dfrac{\pi}{2} \leq u \leq \dfrac{\pi}{2}, \, - 3 \pi \leq v \leq 3 \pi \right) \)

\( a = \) 更新

\( (0.1 \leq a \leq 10) \)