arXiv subjects

E. Morales-Amaya

Publications and source records attributed to E. Morales-Amaya.

5 recordsLinked to original sources

arXiv2026

Convex bodies with centrally symmetric sections

Let $K\subset \mathbb{R}^n$ be a convex body, $n\geq 3$. We say that $K$ satisfies the Barker-Larman condition if there exists a ball $B$ in the interior of $K$ such that for every suppor hyperplane $Π$ of $B$, the section $Π\cap K$ is a centrally symmetric set. Barker and Larman conjectured that the Barker-Larman condition characterizes the ellipsoid. In this work we prove an special case of such conjecture, in particular, we assume that the convex body $K$ is centrally symmetric. Our main result is the following: Let $K$ be a centrally symmetric and strictly convex body, with center at $O$, and let $B$ be a ball in the interior of $K$ and not containing $O$: If $K$ satisfies the Barker-Larman condition with respect to $B$ and $B$ is suitable for $K$ (intuitively, $B$ is suitable for $K$ if the boundary of $B$ is not very close to the boundary of $K$), then $K$ is an ellipsoid.

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arXiv2025

Characterizations of the sphere by means of point-projections

In this work we prove the following: let $K$ be a convex body in the Euclidean space $\mathbb{R}^n$, $n\geq 3$, contained in the interior of the unit ball of $\mathbb{R}^n$, and let $p\in \mathbb{R}^n$ be a point such that, from each point of $\mathbb{S}^{n-1}$, $K$ looks centrally symmetric and $p$ appears as the center, then $K$ is a ball.

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arXiv2025

Characterizations of ellipsoids by means of the strong intersection property

Let $E_1,E_2\subset \mathbb{R}^n$ be two homothetic solid ellipsoids, $n\geq 3$, with center at the origin $O$ of a system coordinates of $\mathbb{R}^n$, and $E_1\subset E_2$. Then there exists a $O$-symmetric ellipsoid $E_3$ such that $E_3$ is homothetic to $E_1$ and, for all $x\in \partial E_2$, there exists an hyperplano $Π(x)$, $O\in Π(x)$, such that the relation \begin{eqnarray} S(E_1,x)\cap S(E_1,-x)= Π(x) \cap E_3. \end{eqnarray} holds, where $S(E_1,x)$ and $S(E_1,-x)$ are the supporting cones of $E_1$ with apex $x$ and $-x$, respectively. In this work we prove that aforesaid condition characterizes the ellipsoid. In fact, we prove that if $K,S, G\subset \mathbb{R}^n$ are three convex bodies, $n\geq 3$, $O\in K$, $K\subset G\subset S$ and $G$ strictly convex and, for all $x\in \partial S$, there exists $y\in \partial S$, $O$ in the line defined by $x,y$, an hyperplane $Π(x)$, $O\in Π(x)$, such that the relation \begin{eqnarray} S(K,x)\cap S(K,y)= Π(x) \cap \partial G. \end{eqnarray} holds, where $S(K,x)$ and $S(K,y)$ are the supporting cones of $K$ with apex $x$ and $y$, respectively, then $G,K$ and $S$ are $O$-symmetric homothetic ellipsoids. In this case, we say that the convex body $K$ has the 'strong intersection property' relative to $O$ and $S$ and with 'associated' body $G$. Thus our main result affirm that if the convex body $K$ has the strong intersection property relative to $O$ and $S$ and with associated strictly convex body $G$, then $K,S$ and $G$ are concentric homothetic ellipsoids.

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arXiv2021

A characterization of centrally symmetric convex bodies in terms of visual cones

In this work we prove the following result: Let $K$ be a strictly convex body in the Euclidean space $\mathbb{R}^n, n\geq 3$, and let $L$ be a hypersurface, which is the image of an embedding of the sphere $\mathbb{S}^{n-1}$, such that $K$ is contained in the interior of $L$. Suppose that, for every $x\in L$, there exists $y\in L$ such that the support double-cones of $K$ with apexes at $x$ and $y$, differ by a translation. Then $K$ and $L$ are centrally symmetric and concentric.

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arXiv2021

Sections and projections of nested convex bodies

One of the most important problems in Geometric Tomography is to establish properties of a given convex body if we know some properties over its sections or its projections. There are many interesting and deep results that provide characterizations of the sphere and the ellipsoid in terms of the properties of its sections or projections. Another kind of characterizations of the ellipsoid is when we consider properties of the support cones. However, in almost all the known characterizations, we have only a convex body and the sections, projections, or support cones, are considered for this given body. In this article we proved some results that characterizes the Euclidean ball or the ellipsoid when the sections or projections are taken for a pair of nested convex bodies, i.e., two convex bodies $K$, $L$ such that $L\subset\text{int}\, K.$ We impose some relations between the corresponding sections or projections and some apparently new characterizations of the ball or the ellipsoid appear. We also deal with properties of support cones or point source shadow boundaries when the apexes are taken in the boundary of $K$.

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