Prove that the surface of revolution
$$
\mathbf{x}=(u \cos \theta) \mathbf{e}_1+(u \sin \theta) \mathbf{e}_2+f(v) \mathbf{e}_3
$$
where $u=C_1 \cos (v / a)+C_2 \sin (v / a)$ and $f(v)=\int \sqrt{1-(d u / d v)^2} d v$ is a surface of constant positive Gaussian curvature $K=1 / a^2$ for all $C_1, C_2$. For what values of $C_1$ and $C_2$ is the surface a sphere?