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Assume that $\Sigma$ is a stricly convex surface in $\mathbb{E}^3$ homeomorphic to a sphere. Further, assume that $p_0,\ p_1\in \Sigma$ are intersection points with planes $z=0,\ z=1$ and the surface $\Sigma$ is between the two planes. Then $p_1$ is a cut point of $p_0$ ? (I do not know whether or not this is true)

Definition : Consider a intrinsic metric on $\Sigma$, a length of simple path. Then $p_1$ is not a cut point of $p_0$ if there is unique shortest path from $p_0$ to any point sufficiently close to $p_1$.

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  • $\begingroup$ Could you please define "cut point"? Thanks. $\endgroup$ Commented Apr 24, 2020 at 21:00
  • $\begingroup$ Thanks for the definition. $\endgroup$ Commented Apr 24, 2020 at 22:33
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    $\begingroup$ Counterexample : Consider a chart $T(u,v)=(0,u,0)+f(u)(\cos\ v,0,\sin\ v),\ f(u)=1-u^4,\ -1\leq u\leq 1,\ v\in [-\frac{\pi}{2}-\epsilon,\frac{\pi}{2}+\epsilon]$ for the surface. Here $T(0,v)$ is a geodesic and sectional curvature is zero along the geodesic so that Jacobi field is $t(0,1,0)$ along $T(0,t+\frac{\pi}{2})$ $\endgroup$ Commented Apr 25, 2020 at 1:17

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No.

It is clear that this is true for a sphere, but cutting of a spherical cap from the sphere leaves the intersection points with the planes the same, but now there is a unique shortest path through this new flat area.

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Of course you can make this object strictly convex, differentiable etc. by smoothing out the cut.

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Intuitively, it seems that there is no reason that $p_1$ in a tilted ellipsoid is a cut point of $p_0$. Because the orientation of the ellipsoid is a global issue (i.e., the location of $p_0$ and $p_1$ change with the tilt), whereas cut-point-ness is intrinsic.


Ellipsoid
In this symmetric example, perhaps there are a pair of equal-length geodesics connecting $p_0$ to $p_1$, but if I arranged $\Sigma$ to be asymmetric with respect to the coordinate axes, there would be a unique shortest geodesic connecting those two $z=0,1$ tangency points.

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