arXiv subjects

Gennadiy Averkov

Publications and source records attributed to Gennadiy Averkov.

61 records · Page 4Linked to original sources

arXiv2010

Description of polygonal regions by polynomials of bounded degree

We show that every (possibly unbounded) convex polygon $P$ in $R^2$ with $m$ edges can be represented by inequalities $p_1 \ge 0,...,p_n \ge 0,$ where the $p_i$'s are products of at most $k$ affine functions each vanishing on an edge of $P$ and $n=n(m,k)$ satisfies $s(m,k) \le n(m,k) \le (1+ε_m) s(m,k)$ with $s(m,k):=\max \{m/k,\log_2 m\}$ and $ε_m \to 0$ as $m \to \infty$. This choice of $n$ is asymptotically best possible. An analogous result on representing the interior of $P$ in the form $p_1 > 0,..., p_n > 0$ is also given. For $k \le m/\log_2 m$ these statements remain valid for representations with arbitrary polynomials of degree not exceeding $k$.

math.MG↗

arXiv2008

Three-dimensional polyhedra can be described by three polynomial inequalities

Bosse et al. conjectured that for every natural number $d \ge 2$ and every $d$-dimensional polytope $P$ in $\real^d$ there exist $d$ polynomials $p_0(x),...,p_{d-1}(x)$ satisfying $P=\{x \in \mathbb{R}^d : p_0(x) \ge 0, >..., p_{d-1}(x) \ge 0 \}.$ We show that for dimensions $d \le 3$ even every $d$-dimensional polyhedron can be described by $d$ polynomial inequalities. The proof of our result is constructive.

math.MG↗

arXiv2008

Notes on algebra and geometry of polynomial representations

Consider a semi-algebraic set A in R^d constructed from the sets which are determined by inequalities p_i(x)>0, p_i(x)\ge 0, or p_i(x)=0 for a given list of polynomials p_1,...,p_m. We prove several statements that fit into the following template. Assume that in a neighborhood of a boundary point the semi-algebraic set A can be described by an irreducible polynomial f. Then f is a factor of a certain multiplicity of some of the polynomials p_1,...,p_m. Special cases when A is elementary closed, elementary open, a polygon, or a polytope are considered separately.

math.AG↗

arXiv2008

Representing elementary semi-algebraic sets by a few polynomial inequalities: A constructive approach

Let P be an elementary closed semi-algebraic set in R^d, i.e., there exist real polynomials p_1,...,p_s such that P= \{x \in R^d : p_1(x) \ge 0, >..., p_s(x) \ge 0 \}; in this case p_1,...,p_s are said to represent P. Denote by $n$ the maximal number of the polynomials from \{p_1,...,p_s\} that vanish in a point of P. If P is non-empty and bounded, we show that it is possible to construct n+1 polynomials representing P. Furthermore, the number n+1 can be reduced to n in the case when the set of points of P in which n polynomials from \{p_1,...,p_s\} vanish is finite. Analogous statements are also obtained for elementary open semi-algebraic sets.

math.AG↗

arXiv2007

Confirmation of Matheron's conjecture on the covariogram of a planar convex body

The covariogram g_K of a convex body K in E^d is the function which associates to each x in E^d the volume of the intersection of K with K+x. In 1986 G. Matheron conjectured that for d=2 the covariogram g_K determines K within the class of all planar convex bodies, up to translations and reflections in a point. This problem is equivalent to some problems in stochastic geometry and probability as well as to a particular case of the phase retrieval problem in Fourier analysis. It is also relevant for the inverse problem of determining the atomic structure of a quasicrystal from its X-ray diffraction image. In this paper we confirm Matheron's conjecture completely.

math.MG↗

arXiv2007

Representing simple d-dimensional polytopes by d polynomials

A polynomial representation of a convex d-polytope P is a finite set \{p_1(x),...,p_n(x)\} of polynomials over E^d such that P=\setcond{x \in \E^d}{p_1(x) \ge 0 {for every} 1 \le i \le n}. By s(d,P) we denote the least possible number of polynomials in a polynomial representation of P. It is known that d \le s(d,P) \le 2d-1. Moreover, it is conjectured that s(d,P)=d for all convex d-polytopes P. We confirm this conjecture for simple d-polytopes by providing an explicit construction of d polynomials that represent a given simple d-polytope P.

math.MG↗

arXiv2007

Retrieving convex bodies from restricted covariogram functions

The covariogram g_K(x) of a convex body K \subseteq E^d is the function which associates to each x \in E^d the volume of the intersection of K with K+x. Matheron asked whether g_K determines K, up to translations and reflections in a point. Positive answers to Matheron's question have been obtained for large classes of planar convex bodies, while for d\geq 3 there are both positive and negative results. One of the purposes of this paper is to sharpen some of the known results on Matheron's conjecture indicating how much of the covariogram information is needed to get the uniqueness of determination. We indicate some subsets of the support of the covariogram, with arbitrarily small Lebesgue measure, such that the covariogram, restricted to those subsets, identifies certain geometric properties of the body. These results are more precise in the planar case, but some of them, both positive and negative ones, are proved for bodies of any dimension. Moreover some results regard most convex bodies, in the Baire category sense. Another purpose is to extend the class of convex bodies for which Matheron's conjecture is confirmed by including all planar convex bodies possessing two non-degenerate boundary arcs being reflections of each other.

math.MG↗