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Given $m$ points $P=\{p_0, p_1, ..., p_m\}$ in high dimensions (e.g. 100), it is known that computing (or even representing) their convex hull $\text{conv}(P)$ is generally intractable due to the exponential dependence on dimensions. In this case, I wonder:

  1. Is there an efficient algorithm to check whether some other given point $\tilde{p}$ is an interior point in the convex hull (i.e. there is a small ball centered at $\tilde{p}$ and lies in $\text{conv}(P)$)? I know we can construct a linear programming problem to check if a point lies inside the convex hull, but my question here is to further check if the convex hull has "volume" and if $\tilde{p}$ lies in its interior.
  2. Following 1, can we compute or efficiently lower-bounding the largest enclosed (inscribed) ball centered at $\tilde{p}$ and lies in $\text{conv}(P)$?

I'm an amateur in geometry and I appreciate any potential solutions or suggestions. Thank you for your time!

The same question posted on MathSE.

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  • $\begingroup$ Also posted to Math.StackExchange here. $\endgroup$ Commented Jan 21, 2021 at 15:49
  • $\begingroup$ @RavenclawPrefect Thanks for adding the link! $\endgroup$
    – Dazheng
    Commented Jan 21, 2021 at 16:14
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    $\begingroup$ I’m voting to close this question solely because it was asked simultaneously on MSE and seems to be getting engagement and answers there (i.e. closing here just to avoid duplication). But as a non-expert, my impression is that the question would be OK in either site individually. $\endgroup$
    – Yemon Choi
    Commented Jan 22, 2021 at 3:04
  • $\begingroup$ I’m voting to close this question because it is getting answers in MSE $\endgroup$
    – Leo Alonso
    Commented Jan 26, 2021 at 20:05

1 Answer 1

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x is not in the interior of conv(P) iff there is an affine function $u(y) = a+ b\cdot y$ such that $u(x)\le 0$ and $u(p_i)\ge0$ for $i=0,\ldots,m$. If this problem has no feasible solution, then x is in the interior.

Alternatively, x is in the relative interior of conv(P) iff for some t > 0, $x = \sum_{i=0}^m a_i p_i$, where $\sum_i a_i = 1$, and $t\le a_i \le 1$. Maximimzing t subject to these constraints is a linear program.

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  • $\begingroup$ Thank you very much for your answer, @Yoav! I'm digesting this. In the meantime, I've edited the question a bit to clarify my additional question -- if we don't know whether the conv(P) has "volume" (non-empty interior, not the relative interior) or not, do we have some way to check it? Thank you for your time! $\endgroup$
    – Dazheng
    Commented Jan 21, 2021 at 16:32
  • $\begingroup$ conv(P) has nonempty interior iff the matrix [p1-p0, p2-p0,...pm-p0] is full rank $\endgroup$ Commented Jan 21, 2021 at 16:47

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