# Questions tagged [convexity]

The convexity tag has no usage guidance.

463
questions

**-1**

votes

**0**answers

57 views

### $ h $ is an affine function?

Let $X$ be a separable Banach space, the associated dual space is denoted by $X^*$ and the usual duality between $X$ and $X^*$ by $\langle , \rangle$. For $C$ nonempty weakly compact convex subsets of ...

**2**

votes

**1**answer

55 views

### Naming convention: looking for better terminology for “centrally symmetric smooth strictly convex bodies”

I have recently found myself researching a certain type of convex body in $\mathbb{R}^2$, namely centrally symmetric smooth strictly convex bodies.
Instead of repeating such a sentence repetitively I ...

**5**

votes

**1**answer

172 views

### a square root inequality for symmetric matrices?

In this post all my matrices will be $\mathbb R^{N\times N}$ symmetric positive semi-definite (psd), but I am also interested in the Hermitian case. In particular the square root $A^{\frac 12}$ of a ...

**4**

votes

**1**answer

114 views

### Is there a non-convex function with non-decreasing average rate of change?

$\newcommand{\R}{\mathbb R}$
Let $f$ be a function from $\R$ to $\R$. It is said that $f$ is midpoint-convex if for any real $x$ and $y$ we have $f(x-y)+f(x+y)\ge2f(x)$ or, equivalently, $f(x+y)-f(x)\...

**0**

votes

**0**answers

16 views

### Order-relational conditions for 4 points being in convex configuration

In the euclidean plane a simple sufficient condition for 4 points being in convex configuration is as follows:
if the points are $\lbrace A,\,B,\,C,\,D\rbrace$ of which $\lbrace A,\,B,\,C\rbrace$ are ...

**2**

votes

**0**answers

36 views

### A complete metric space with some convex-type property

Let $(X,d)$ be a complete metric space with this property:
for each $x∈X$, $r>0$ and $y∈X$ with $d(x,y)<r$, there exists $z∈X$ such that $d(x,y)+d(y,z)=d(x,z)=r$.
I want to know if the family ...

**1**

vote

**1**answer

75 views

### Gradient flows: convex potential vs. contractive flow?

Take a $\mathcal C^2$ potential $V:\mathbb R^d\to \mathbb R$, and assume that it is bounded from below (say $\min V=0$ for simplicity, so that $V\geq 0$).
Consider the autonomous gradient-flow
$$
\dot ...

**3**

votes

**0**answers

57 views

### Effective radius of section of a convex set compared to that of the convex itself

The effective radius $er(A)$ of a $n$-solid $A$, is defined by Schramm (see the question by Gil Kalai
Volumes of Sets of Constant Width in High Dimensions)
to be the radius of the $n$-ball that has ...

**2**

votes

**1**answer

276 views

### real analytic function with given shape

I am looking for a 5 parameter family of analytic functions $f:[0,1]\to R$ such that
(0) $f$ has zeros at $0,p,1$.
(1) $f$ is convex in $[0,p]$ and concave in $[p,1]$.
(2) The five parameters, $p$ ...

**1**

vote

**1**answer

70 views

### Superdifferentiable and subdifferentiable at $x$ implies differentiable at $x$

Let $(M,g)$ be a compact connected smooth Riemannian manifold without boundary and let $\phi: M \rightarrow \mathbb{R}$ be a function on $M$. We say that $\phi$ is superdifferiantiable at $x$ with ...

**1**

vote

**1**answer

163 views

### Conditions such that norm of matrix vector can be written as the derivative of the norm of the vector for some convex fonction

Problem statement:
Let $A$ be a matrix $\mathbb{R}^{d \times d}$, I want to find some conditions on $A$ such that there exists a differentiable convex function $f: \mathbb{R_{+}} \rightarrow \mathbb{...

**2**

votes

**1**answer

119 views

### Is the optimum of this problem convex in the constraint parameter?

Let $f:\mathbb R^+ \to \mathbb R$ be a smooth function, satisfying $f(1)=0$, and suppose that
$|f|$ grows with the distance from $1$: $|f(x)|$ is strictly increasing when $x \ge 1$, and strictly ...

**4**

votes

**4**answers

222 views

### algorithm for convex $C^2$ interpolation

Let $x_0<x_1<\ldots<x_n$ and $f_0,f_1,\ldots,f_n$ be real numbers and
$$s_i=(f_i-f_{i-1})/(x_i-x_{i-1}),~~~c_i=(s_{i+1}-s_i)/(x_{i+1}-x_{i-1}).$$
If $f$ is a convex function defined on $[x_0,...

**0**

votes

**0**answers

27 views

### Extended-value subgradients

(I am not an expert in convex analysis, as may become clear.)
Let $R$ be the extended reals, $R = \mathbb{R} \cup \{\pm \infty\}$.
Standard texts define the subgradient of a convex function $f: \...

**3**

votes

**1**answer

132 views

### Geodesic convexity and the Geometric Hessian

This is an elementary question in differential geometry. We know that for a smooth real-valued function $f$ defined on an open geodesically convex set of a Riemannian manifold $ \mathcal{X} \subset \...

**0**

votes

**0**answers

77 views

### Finding a specific solution to $X^T\Sigma X = D$

I'm looking to solve for a specific $X$ in the following equation:
$$X^T\Sigma X = D,$$
where $\Sigma \succ 0$, $D$ is a diagonal matrix with strictly positive entries, and all matrices are square. It ...

**11**

votes

**1**answer

331 views

### Aleksandrov's proof of the second order differentiability of convex functions

Aleksandrov [A], proved a remarkable property of convex functions.
Theorem. If $f:\mathbb{R}^n\to\mathbb{R}$ is convex, then for almost every $x\in\mathbb{R}^n$ there is $Df(x)\in\mathbb{R}^n$ and ...

**6**

votes

**1**answer

249 views

### A conjecture (or theorem?) on unit vectors in a Euclidean space

I have heard (if I am not mistaken) that there exists the following conjecture (or theorem?).
Let $u_1,\dots,u_n$ be unit vectors in an $n$-dimensional Euclidean vector space. Then there exists ...

**16**

votes

**2**answers

700 views

### Reference to a conjecture on unit vectors in Euclidean space

I have heard that there exists the following conjecture (if I am not mistaken).
Let $u_1,\dots,u_n$ be unit vectors in an $n$-dimensional Euclidean vector space. Then there exists another unit vector ...

**2**

votes

**0**answers

105 views

### A property of $f(x)=x^{4(1-x)^2}+(1-x)^{4x^2}$

Let $0<x<0.5$ or $0.5<x<1$ defines the following function :
$$f(x)=x^{4(1-x)^2}+(1-x)^{4x^2}$$
Claim :
The function $g(x)=\exp(|1-f(x)|)-1$ is a logarithmically concave function
Or :...

**2**

votes

**0**answers

77 views

### Isometries between two convex bodies [closed]

Let $A,B$ be two convex compact subsets with non-empty interior in a Euclidean space $\mathbb{R}^n$. Let $f\colon A\to B$ be a bijective isometry between them.
Does there exist an isometry $F\colon ...

**0**

votes

**0**answers

58 views

### Linearly independent support vectors of a convex set

Let $\Omega\subset\mathbb{R}^n$ a compact strictly convex set containing $0$ in its interior and let $k\leq n$.
Given a vector $x\neq 0$ in $\mathbb{R}^n$ a supporting vector $\xi_x$ in the ...

**4**

votes

**0**answers

105 views

### On intrinsic volumes

Let $\Gamma$ be a convex polytope in $\mathbb R^n$. The $k$-th intrinsic volume of $\Gamma$ is the number
$$
\text{v}_k(\Gamma)=\sum_{\Delta\in{\mathcal B}(\Gamma,k)}\text{vol}_k(\Delta)\psi_\Gamma(\...

**5**

votes

**0**answers

220 views

### Open convex hull of a closed set

Let $X$ be a closed set in a Euclidean space of finite dimension and suppose that its convex hull $H$ is open. I can prove that, in this case, $H$ is a Cartesian product of a line with an open convex ...

**7**

votes

**1**answer

282 views

### Does midpoint-convex imply rationally convex?

Say that a function $f$ defined on $\mathbb{Q}^n$ is midpoint convex if $f((x+y)/2) \le (f(x) + f(y))/2$. Say that it is rationally convex if, for $\lambda \in \mathbb{Q} \cap [0,1]$ and $\bar \lambda ...

**0**

votes

**0**answers

53 views

### Proximity operator composition

Suppose that $f,g$ are convex and lower-semi continuous functions from $\mathbb{R}^d$ to $\mathbb{R}$ such that $f\circ g$ is lsc and convex. I know that the proximity operators $\operatorname{Prox}...

**2**

votes

**1**answer

137 views

### Are closed convex subsets of a Banach space weakly closed without the axiom of choice?

It is a well-known fact that closed convex sets in Banach spaces are weakly closed. The common proof is based on the Hahn-Banach theorem that uses the axiom of choice. Is there any proof of this fact ...

**0**

votes

**1**answer

218 views

### Expectations, double integrals and Jensen's inequality

$\def\anonfunc#1{#1(\cdot)}$Consider two random variables distributed $v\sim \anonfunc G$ and
$c \sim \anonfunc F$ with pdfs $\anonfunc g$ and $\anonfunc f$. Let the supports of $c$ and
$v$ be $[x,y]$....

**3**

votes

**0**answers

66 views

### Sufficient condition for convex conjugate to be second-order differentiable

Let $f:\mathbb{R}^n\to (-\infty,\infty]$ be a convex lower-semicontinuous function, we then define its conjugate by
$$
f^*(y)=\sup_{x\in \mathbb{R}^n}\{x^Ty-f(x)\}.
$$
Then there exist well-known ...

**3**

votes

**1**answer

153 views

### Quotient space of a locally uniformly rotund space

If $X$ is a uniformly rotund space , then for any closed subspace $M$ of $X$, $X/M$ is uniformly rotund. Does this hold for a locally uniformly rotund space? That is if $X$ is locally uniformly ...

**0**

votes

**2**answers

193 views

### Goldowsky-Tonelli theorem for upper semi continuous function

Let $f:(0, \infty) \rightarrow \mathbb{R}$ be a convex and continuous function. We know that $\partial^{e} f(. )$(either right or left derivative) is non-decreasing and upper semi-continuous function. ...

**3**

votes

**1**answer

137 views

### Is there a definition for “convexity” of spatial (non-planar) polygons? [closed]

I was thinking that there should exist a definition for "convexity" of spatial polygons.
A planar convex quadrilateral that has one vertex moved (perpendicularly) out of the plane should continue to ...

**4**

votes

**1**answer

92 views

### What is the probability of an empty convex $k$-gon among many given points?

Given a finite number of points in the plane in general position, call a convex subset empty if its hull doesn't contain any other of the points.
For a big number $n$ of randomly distributed ...

**1**

vote

**0**answers

60 views

### Convexity of conditional relative entropy for Markov distributions

Consider two Markov processes $p$ and $q$. The conditional relative entropy between them is
\begin{align}
D(p\parallel q)& =\sum_a p(a)\sum_b p(b\mid a)\log\frac{p(b\mid a)}{q(b\mid a)}\\
& =\...

**0**

votes

**0**answers

37 views

### Example of a sequence of logarithmically convex functions on $\mathbb{R}$ and for all $n\in\mathbb{N}$ in the spirit of one evoked in an article

To ask this question I was inspired in some words, if I understand well, from the authors of a preprint on arXiv in section 4.1, that I believe that is [1], to ask next question.
We consider the ...

**0**

votes

**1**answer

89 views

### A property of convex cones in Euclidean spaces

EDIT: Let $K$ be a closed convex cone in a Euclidean space of finite dimension. Assume it is non-zero and not the whole space.
Does there exist a non-zero point $x\in K$ such that
$$(x,y)\geq 0 ...

**2**

votes

**1**answer

130 views

### Monotone function which is separately convex but not convex

I am looking for a function $f\,:\,\mathbb{R}^n\to\mathbb{R}$ which is separately convex and increasing but not convex.
That is to say, the function is convex and increasing in each coordinate while ...

**2**

votes

**1**answer

98 views

### Closest points of curves on convex surfaces

Let $\Sigma$ be a convex surface with strictly positive curvature in $\mathbb{R}^3$ and $\Gamma \subset \Sigma$ be a closed simple space curve with parameterization $\gamma : \mathbb{S}^1 \rightarrow \...

**1**

vote

**1**answer

53 views

### Subadditive function with special growth

Related to one of my previous question (for which I have received an answer, thanks) I have the following new one. Maybe I am describing the empty set but not being a specialist at all of the domain I ...

**6**

votes

**1**answer

181 views

### How to calculate the volume of a section of a convex body?

The following is essentially a partial case for my previous question.
Let $B\subset\mathbb{R}^m$ be the unit ball with respect to a concrete norm on $\mathbb{R}^m$, say $l^p$-norm, $p\in (1,\infty)$....

**4**

votes

**1**answer

271 views

### Simple proof of Prékopa's Theorem: log-concavity is preserved by marginalization

The following result is well-known:
Suppose that $H(x,y)$ is a log-concave distribution for $(x,y) \in \mathbb R^{m \times n}$ so that by definition we have
$$H \left( (1 - \lambda)(x_1,y_1) + \...

**1**

vote

**0**answers

31 views

### Modelling exact unions of polytopes in homogeneous case?

We can model disjunctions (note I am not looking for convex hull) of $t$ unbounded convex polyhedra given by $A^{(1)}x^{(1)}\leq b^{(1)}$,$\dots$,$A^{(t)}x^{(t)}\leq b^{(t)}$ exactly with a mixed ...

**1**

vote

**0**answers

69 views

### John's ellipsoid of a polytope

Suppose that $X$ is $\mathbb R^n$ with some polyhedral norm, that is, the unit ball of $X$ is an $n$-dimensional polytope. Assume that the John ellipsoid of $X$ is an Euclidean ball that touches every ...

**2**

votes

**0**answers

76 views

### Neumann problem on a convex domain

Let $\Omega$ be a convex open bounded subset of $\mathbb R^n$ and let $u$ be the solution of
$$
\begin{cases}
∆ u=1\quad\text{in $\Omega$,}
\\
\frac{\partial u}{\partial \nu}=\frac{\vert \Omega\vert_n}...

**4**

votes

**1**answer

104 views

### Mapping inclusion theorem for the numerical range

We denote the numerical range of a complex square matrix $A \in \mathbb{C}^{n\times n}$ by $W(A)$.
Let $A \in \mathbb{C}^{n\times n}$ and let $f: \mathbb{C} \to \mathbb{C}$ be, say, an entire ...

**4**

votes

**1**answer

257 views

### On convex functions which are non constant on every segment

I have been studying for the last few weeks the paper Dirichlet problem for demi-coercive functionals by Anzellotti, Buttazzo and Dal Maso. In this work the authors introduced and studied the ...

**3**

votes

**1**answer

218 views

### Is there exists (strictly) convex function on hemisphere?

Given $\mathbb{S}^n_+:=\{x\in \mathbb{R}^{n+1}: |x|=1,x_{n+1}>0\}$ be the open domain in $\mathbb{S}^n$, or be viewed as the geodesic ball centered at the pole with radius $\frac{\pi}{2}$ in $\...

**0**

votes

**0**answers

49 views

### linear functions/hyperplanes vs. convex functions/convex sets in Hilbert space

The simplest Hahn-Banach extension theorem in Hilbert space $X$ avoids the use of the axiom of choice by virtue of the Riesz representation theorem. But what about the version of the theorem where the ...

**0**

votes

**1**answer

86 views

### Inequality involving product-of-minus vs minus-of-product for positive integers

I'm encountering this inequality for dimensionality reduction problem. The simplified form looks as follows:
Consider positive integers $a_1$, $a_2$, $b_1$ and $b_2$ where $a_1>b_1$ and $a_2>...

**1**

vote

**1**answer

157 views

### Is there a way to turn a non convex set to a convex one? [closed]

Perhaps the question is rather vague, but if we are given a non-convex set S, can we construct some invertible mapping f so that f(x) becomes a convex set?
In my problem , I am given a set of matrix ...