Prove that for any small enough translation of a simplex either the new simplex has a vertex of the old simplex or viceversa

45 Views Asked by At

Consider a $n$ dimensional simplex. Prove (or disprove) that a small enough translation of it in any direction produces a simplex which either has a vertex of the old simplex, either the new simplex has a vertex in the old simplex.


This property seems interesting and is obvious for the two dimensional case, as seen in the picture: enter image description here

Here, in any direction one would move the triangle, one of the vertices would be in the interior of the new triangle, or the new triangle would have a vertex in the old one. How can this be properly proved in the general case?


To formalize the claim, let $V_k$ be a vertex of the simplex $\mathcal{S}$ and denote $\Omega_k = \mathcal{B}(V_k,\epsilon) \cap \mathcal{S}$.

Then we basically claim that $$ \left( \bigcup_{k=1}^{n+1} \{x - V_k | x \in \Omega_k\} \right) \cup \left( \bigcup_{k=1}^{n+1} \{-x + V_k | x \in \Omega_k\} \right) = \mathcal{B}(0,\epsilon)$$


Edit

It seems that in general

$$ \left( \bigcup_{k=1}^{n+1} \{x - V_k | x \in \Omega_k\} \right) \cup \left( \bigcup_{k=1}^{n+1} \{-x + V_k | x \in \Omega_k\} \right) \subseteq \mathcal{B}(0,\epsilon)$$ Let

$$ \Lambda = \left( \bigcup_{k=1}^{n+1} \{x - V_k | x \in \Omega_k\} \right) \cup \left( \bigcup_{k=1}^{n+1} \{-x + V_k | x \in \Omega_k\} \right) $$ then can we estimate $$ \frac{\text{vol}(\Lambda)}{\text{vol}(\mathcal{B}(0,\epsilon))}$$

1

There are 1 best solutions below

3
On BEST ANSWER

It's not true in three dimensions. Take a regular tetrahedron with unit sides, resting on the $(x,y)-$plane with one vertex on the positive $x$-axis and one vertex on the positive $z$-axis. Now raise it a distance $\epsilon$, and translate it along the $x$-axis by a distance $\epsilon$.