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A branch of geometry dealing with convex sets and functions. Polytopes, convex bodies, discrete geometry, linear programming, antimatroids, ...
17
votes
Is the diameter of a centrally symmetric convex body realized by a pair of antipodal points?
For $n=3$ this question was
asked in 1996 by
James Propp,
conjecturing that the answer is Yes.
(I got the reference from this
page, which concerns the very special case of a rectangular box
in ${\bf …
4
votes
Intersections and curvature in the plane
David Speyer writes "the only way this strategy could work
is if we improve the $2$ in the previous paragraph to $\pi$."
That improvement follows.
Suppose $f < 0$ and $f + f'' \leq 0$ on $(0,c)$, with …
3
votes
Accepted
On special points within convex solids with all planar sections passing through them having ...
There are plenty of $(C,P)$ such that all sections through $P$
have the same area. The construction readily adapts to higher moments
about $C$ but not to perimeter. The existence of a single convex …
8
votes
intersection of the unit cube and a hyperplane containing the main diagonal
Counterexample: for $(m,n)=(3,4)$ we can get a regular octahedron as
the intersection of the tesseract $\Delta_4$ with the hyperplane
$x_1+x_2 = x_3+x_4$. [It's easier to think of the equivalent
but …
5
votes
Accepted
Listing lattice points in a simplex
Hermite normal form (HNF) should work much as you suggest.
Translate $v_0$ to 0. Let $L = \bigoplus_{i=1}^n {\bf Z} v_i$. HNF gives an explicit decomposition of $G := {\bf Z}^n / L$ as a direct sum …
2
votes
Distributing points with respect to a concave function
It seems that for large $n$ there will be an upper bound $c_n \alpha$ with $c_n$ asymptotic to $\frac{2}{9n}$ (and thus a bit better than the $\frac{1}{6n}$ suggested by the value of $c_1$). The asym …
6
votes
Accepted
What does the image of the integer lattice under a norm look like?
I doubt that there is a simple characterization.
In any case conditions (1,2,3) are not sufficient.
For example, if $n=2$ then $S$ cannot be ${\bf Z}_{\geq 100}$,
and there are similar counterexamples …
6
votes
Estimating shortest paths in planar drawings of graphs
Looks like Saúl RM has already achieved an arbitrarily high distance.
Here's a simpler construction that at least gets distance $> 1$.
The square has vertices $(\pm1/2, \pm1/2)$.
The vertices near th …