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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 …
Noam D. Elkies's user avatar
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 …
Noam D. Elkies's user avatar
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 …
Noam D. Elkies's user avatar
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 …
Noam D. Elkies's user avatar
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 …
Noam D. Elkies's user avatar
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 …
Noam D. Elkies's user avatar
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 …
Noam D. Elkies's user avatar
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 …
Noam D. Elkies's user avatar