# Questions tagged [convex-geometry]

A branch of geometry dealing with convex sets and functions. Polytopes, convex bodies, discrete geometry, linear programming, antimatroids, ...

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### Can we always shift two disjoint convex bodies a little bit to decrease the volume of their convex hull?

Let $K,L\subset\mathbb R^d$ be two disjoint compact convex sets with non-empty interiors. Can $x=0$ be a point of local minimum for the function $F(x)=\text{vol}_d(\text{conv(K,L+x))}$?
I was asked ...

**1**

vote

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37 views

### Volume of caps of a polytope

Let $K$ be a polytope in $\mathbb R^d$, blow it up by a factor $\lambda>0$. For a unit vector $u \in \mathbb S^{d-1}$, $\lambda K$ has 2 support hyperplanes $H_1$ and $H_2$ with corresponding ...

**4**

votes

**1**answer

75 views

### Regularity of John's ellipsoid

Consider a finite dimensional real Banach space $E$, with norm say $|\cdot|$. Let $N$ denote the set of all norms on $E$. Suppose that $\varphi_1, \varphi_2 \in N$ have unit balls $B_1$ and $B_2$, ...

**2**

votes

**1**answer

79 views

### Reference for Minkowski functional when 0 is not in the interior

The Minkowski functional on a normed linear space $E$ is usually defined for convex (or sometimes even non convex) subsets $C$ of $E$ such that $0 \in \operatorname{int}(C)$. Is there any standard ...

**0**

votes

**1**answer

70 views

### Is an ambiguity set with Wasserstein distance of order 1 is convex?

I have a question about the convexity of an Wasserstein ambiguity set.
Let $W_1(\mu, \nu)$ be the Wasserstein distance of order 1 between $\mu$ and $\nu$, defined as
$$W_1(\mu, \nu) := \min\limits_{\...

**0**

votes

**1**answer

69 views

### Closed form solutions for maximal subsets of convex polytopes

I'm looking for any known exact results about inscribing simple convex bodies inside a convex polytope. The most famous is the Löwner-John ellipsoid, but as far as I understood in general there is no ...

**2**

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53 views

### Smooth dependence of convex functions on Monge-Ampère measure

Let $\Omega\subset\mathbb{R}^2$ be a bounded convex domain, $f$ be a positive smooth function on $\Omega$ and $\phi:\partial\Omega\rightarrow\mathbb{R}$ be a continuous function. It is known that as ...

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33 views

### An implementation of Minkowski reconstruction in 3 dimensions

By a theorem of Minkowski from 1903, an $n$-dimensional polytope $P\subset \mathbb R^n$ is determined up to translation by its unit face normal $u_1,\dots,u_k\in S^{n-1}$ and the corresponding $(n-1)$ ...

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33 views

### When is the set of faces of a convex polytope algebraically independent?

This is related to another question of mine
Let $V=\Bbb R^n$. Morelli defined the (commutative unital) ring $L(V)$ to be the additive group generated by the indicator functions of convex polytopes in ...

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67 views

### The ring generated by a convex polytope and its faces

Let $V=\Bbb R^n$. Morelli defined the (commutative unital) ring $L(V)$ to be the additive group generated by the indicator functions of convex polytopes in $V$ with multiplication induced by Minkowski ...

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61 views

### Inclusion of convex polytopes and embedding from $\ell_2$ to $\ell_\infty$

I would like to dig deeper into the problem posted Probability that a convex shape contains the unit ball:
If you pick n points uniformly at random from the surface of a d
dimensional sphere of ...

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vote

**1**answer

169 views

### Boundary regularity for the Monge-Ampère equation $\det D^2u=1$

Let $\Delta$ be an open triangle in $\mathbb{R}^2$ and $u\in C^0(\overline{\Delta})\cap C^\infty(\Delta)$ be the convex function satisfying
$$
\det D^2u=1,\quad u|_{\partial\Delta}=0.
$$
Classical ...

**1**

vote

**1**answer

60 views

### Linear relations between volume of a polytope and its faces

Let $P$ be a polytope. Is anything known about the set of linear relations that hold between the volumes of the (not-necessarily proper) faces of $P$ as $P$ “varies slightly”? By varies slightly I ...

**14**

votes

**3**answers

311 views

### What is known about sufficient conditions for the rigidity of a convex surface?

A convex surface is a connected open subset of the boundary of a convex body in $\mathbb{R}^3$.
An "infinitesimal bending" of a convex surface $S$ is a deformation of $S$ given by a velocity field $v:...

**2**

votes

**1**answer

101 views

### Polygon of convex arcs

Convex polygons in the plane $R^2$ arise in linear programming where the constraints are linear. The objective linear function attains its maximum at a vertex of the feasible region(if exists).
Assume ...

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votes

**1**answer

129 views

### Minkowski sum of polytopes from their facet normals and volumes

By Minkowski's work in the early 1900s, every polytope $P\subset\mathbb R^n$ is determined up to translation by its unit facet normals $u_1,\dots,u_k$ and facet volumes $\alpha_1,\dots,\alpha_k$. ...

**7**

votes

**1**answer

214 views

### Log-concavity of areas of level sets

Suppose $f: \mathbb{R}^d \to \mathbb{R}$ is a smooth convex function.
Consider the level sets of the function, namely $M_s = \{x: f(x) = s\}$.
Is it true/known that the surface areas of $M_s$ are ...

**1**

vote

**0**answers

56 views

### On Topological Tverberg Theorem

Let $r$ be a prime power. The Topological Tverberg Theorem says that any continuous map from a $(r-1)(d+1)$-simplex to $\mathbb{R}^d$ identifies points from $r$ disjoint faces. It is not hard to see ...

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**0**answers

22 views

### Projecting a polyhedral cone onto its intersection with the infinity-norm ball

For a point in a convex polyhedral cone, $x\in \mathcal{C} = \{\sum_{i=1}^m \alpha_i r_i \vert \alpha_i \geq 0, r_i \in \mathbb{R}^n \}$, is there an efficient algorithm to project $x$ onto the ...

**2**

votes

**0**answers

98 views

### Can computers take uniform samples from a polytope?

For each $r \in \mathbb N $ write $\mathbb Z/ 10^r = \{a/10^r: a \in \mathbb Z\}$ and $P(r)$ for the lattice $(\mathbb Z/10^r)^N \subset \mathbb R^N$.
Suppose the plane $P \subset \mathbb R^N$ is ...

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35 views

### Level sets of strongly convex and smooth functions

Let $f: \mathbb{R}^N \to \mathbb{R}$ be a $\alpha$-strongly convex and $\beta$-strongly smooth function, i.e.,
$$ f(x) + \langle\nabla f(x), y- x\rangle + \frac{\alpha}{2}\|y-x\|^2
\leq f(y) \leq f(x) ...

**4**

votes

**1**answer

98 views

### a compact set with nonempty convex sections

Let $X = [0,1]^d$ be the unit cube in the $d$-dimensional Euclidean space.
For every $x \in X$ and every coordinate $i=1,2,\ldots,d$ denote by $x_{-i} := (x_j)_{j \neq i}$.
Given a set $Y \subseteq X$ ...

**3**

votes

**1**answer

63 views

### Number of Inner Diagonals of Convex Hulls of $n+2$ Points in Convex Configuration in $E^n$

Question:
Is it true that $E^2$ is the only Euclidean space, in which the convex hull
of $n+2$ points in convex configuration has two inner diagonals and in all other cases there is only one ...

**0**

votes

**3**answers

93 views

### On the properness of the graph of a convex function

Let $f : \Omega \subseteq \mathbb{R}^n \to \mathbb{R}$ be a smooth and convex function. Let us assume that $\Gamma_f = \mathrm{graph}(f) $ is a complete hypersurface of $\mathbb{R}^{n+1}$. Then I know ...

**5**

votes

**2**answers

234 views

### Is every polytope combinatorially equivalent to the intersection of a simplex and a linear subspace?

I wonder whether such a result is known, and if so, whether the proof is trivial.
By polytope I mean the convex hull of finitely many points in $\Bbb R^n$. Assume the simplex to be symmetric and ...

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votes

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73 views

### Geometric meaning of the chi-square “measure of association”

In Statistics, there's a standard test of independence of two random variables taking values in finite sets $I,J$. It relies on the computation of $\chi$-square statistics,
$$
\chi^2:=\sum_{(i,j)\in ...

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votes

**1**answer

77 views

### Why the convexity condition on the definition of a face of a convex set?

A face of a closed convex set $X\subseteq\mathbb{R}^n$ is defined to be a set $F\subseteq X$ such that:
$F$ is convex.
Every line segment from $X$ whose interior meets $F$ is contained in $F$.
Is ...

**2**

votes

**0**answers

32 views

### Extreme points in $d$-dimensional quadrant hull of $n$ random points in a halfplane

Finding the asymptotic growth of the expected number of extreme points of the quadrant hull in $d$ dimensions seems to be a well studied problem. Interestingly, the solution appears to vary widely by ...

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58 views

### Applying a piecewise linear function to vertices of a polytope while remaining in facet representation

Let $P \subseteq \mathbb{R}^d$ be a polytope with vertices $V$, and let $f : \mathbb{R}^d \to \mathbb{R}$ be a function. Let $P' \subseteq \mathbb{R}^{d+1}$ be the polytope with vertices $\{(v, f(v)) \...

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58 views

### Convex hull of piece-wise linear functions

Let $K>1$ be a positive integer. Consider a function class $$\mathcal{F}_K:=\Big\{\max_{1\leq k\leq K} a_k^\top x + b_k:\ ||a_k||\leq 1, |b_k|\leq 1, \forall 1\leq k\leq K \Big\}$$ on some compact ...

**14**

votes

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489 views

### Structures of the space of neural networks

A neural network can be considered as a function
$$\mathbf{R}^m\to\mathbf{R}^n\quad
\text{by}\quad x\mapsto w_N\sigma(h_{N-1}+w_{N-1}\sigma(\dotso h_2+w_2\sigma(h_1+w_1 x)\dotso)),$$
where the $w_i$ ...

**4**

votes

**1**answer

142 views

### Is the preimage of a face under an affine map a face?

Let $X\subseteq\mathbb{R}^n$ be a convex set. Let $\pi{:}\ X\to\mathbb{R}^m$ be a linear map, with $m<n$ (for example, a projection). Let $\pi^{-1}(y)=\{x\in X\mid\pi(x)=y\}$ denote the inverse of $...

**7**

votes

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239 views

### Visual proof of convergence for Steiner's symmetrization

I want to find a visual proof of the following fact:
For any convex figure in the plane there is a sequence of Steiner's symmetrizations that makes it arbitrary close to a circular disc.
All ...

**8**

votes

**2**answers

239 views

### Is it possible to continuously select a probability distribution over fixed points in Brouwer's fixed point theorem?

According to Brouwer's fixed point theorem, for compact convex $K\subset\mathbb{R}^n$, every continuous map $K\rightarrow K$ has a fixed point.
However, these fixed points cannot be chosen ...

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votes

**2**answers

102 views

### Does this formula for caliper diameter hold for concave polyhedra?

I recently asked on MathOverflow and also asked several people I know to prove the following:
How do I prove that the average caliper diameter of the polyhedron across all possible rotations is ...

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votes

**3**answers

170 views

### Average caliper diameter (mean width) of a polyhedron

Define the caliper diameter of a polyhedron as follows:
Let $P_1$ and $P_2$ be two planes both of which are parallel to the x axis such that the perpendicular distance between $P_1$ and $P_2$ is the ...

**7**

votes

**0**answers

88 views

### Ellipsoid minimizing Banach-Mazur distance to convex body

Given a (symmetric) convex body $K \subset \mathbb{R}^n$ (equivalently, given a norm on $\mathbb{R}^n$), there is a unique ellipsoid of maximal volume in $K$, called the John ellipsoid. The John ...

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46 views

### Choquet Theorem for the cone of non-negative operators

Let $\mathcal B_+$ be the convex cone of bounded non-negative self-adjoint operators on $L^2(\mathbb R)$. With the perspective of applying Choquet's Theorem, I would like to know if the extreme points ...

**4**

votes

**1**answer

103 views

### Cutting a convex body into two congruent pieces

This question is related to How to make a sandwich from just one piece of bread?, asked on Feb 23 '17 by erz, and it goes as follows:
Question. If a convex closed and bounded region $C$ in the ...

**11**

votes

**3**answers

532 views

### Steiner's inequality reference request

I remember seeing somewhere that for every connected compact set $\Omega$ in $\mathbb{R}^2$ with piecewise $C^1$ boundary we have
$$A(\Omega_r)\leq A(\Omega)+L(\partial \Omega)r+ \pi r^2,$$
where
$$\...

**0**

votes

**0**answers

30 views

### Realisation of a Polytope as a convex set [duplicate]

Suppose I have ALL the combinatorial data of an abstract Polytope: a list of all facets and incidence relations.
Is there a way to produce linear functions, in a suitable $R^d$, so that the region ...

**0**

votes

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138 views

### Probability that the perturbed convex hull is larger than the original one

I am wondering if any convex geometers/probabilists have looked at the following question:
Given $n$ randomly distributed (not sure what assumption to put there) points in $\mathbb{R}^d$, for each ...

**3**

votes

**1**answer

93 views

### Is the level set of a product of affine linear functions comprised of convex curves?

Internet searches haven't helped. Can you?
Let $\, f = \prod_{i=1}^n (a_i x + b_i y + c_i).$
Is each component of $\, f^{-1}(1)$ a convex curve?
I expect so, and can prove it for $n=2,$ but I'm ...

**0**

votes

**0**answers

73 views

### Volume of parametric integral of convex set

Given $t,\mu>0$. I am interested in computing the volume of the $n$-dimesnional set
$$\int_{0}^{t}e^{A(t-\tau)}\begin{pmatrix}0\\ 0\\ \vdots\\ 0\\1\end{pmatrix}U\:{\rm{d}}\tau,$$
where the set $U$ ...

**2**

votes

**1**answer

114 views

### Convex misfit of finite-dimensional convex bodies

The finite-dimensional convex bodies topic belongs to the combinatorial geometry. The direction below has a bit more of an algebraic flavor. Let me/us know if some or all of the notions below are ...

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vote

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48 views

### Relative interior of a normal cone at a face of a convex polytope?

Suppose $A$ is a nonempty convex polytope in $\mathbb{R}^n$. Suppose $F$ is a face of $A$.
Consider the normal cone of $A$ at $F$:
$C_A(F)=\{v\in\mathbb{R}^n:v\cdot x\geq v\cdot y\ \forall\ x\in ...

**3**

votes

**1**answer

108 views

### Size of a minimal non-negative conic basis

Suppose $v_1,\dots,v_n \in \mathbb{R}^k$ are entry-wise non-negative (column) vectors with $k<n$. Let $r \leq k$ be the non-negative rank of the matrix $V = [v_1 v_2 \cdots v_n]$ (i.e., the ...

**9**

votes

**2**answers

458 views

### Parallelepiped is defined by the volumes of its faces

Let $v_1,...,v_n\in \mathbb{R}^n$ be linearly independent. The parallelepiped defined by these vectors is $P(v_1,...,v_n)=\{\sum_{i=1}^{n}\alpha_i v_i|~0\le\alpha_i\le 1\}$. Observe that while the ...

**6**

votes

**1**answer

164 views

### Triangulations of convex surfaces

Let $M$ be a smooth closed positively curved surface in Euclidean 3-space, $T$ be a geodesic triangulation of $M$, and $E$ be the edge graph of the convex hull of vertices of $T$.
It is easy to see ...

**4**

votes

**0**answers

152 views

### Existence of geodesic convex functions

By a result of Shing-Tung Yau [1974, Mathematische Annalen 207: 269-270], there are no non-trivial continuous geodesic convex functions on complete manifolds with finite volume.
What happened if we ...