arXiv subjects
Publications and source records attributed to Yury Kochetkov.
We demonstrate in an elementary way how to construct a frieze pattern of width $m-3$ from a partition of a convex $m$-gon by not intersecting diagonals.
We give a new proof of the following statement: the Catalan number $C_n$ is divisible by $n+2$, if $n$ is odd and $n\not\equiv 1\text{ mod }3$.
In this experimental work we study billiard trajectories in triangular pyramids and try to establish conditions that guarantee the existence (or absence) of 4-cycles (there can be not more, than three of them). We formulate conjectures and prove some statements. For example, if a pyramid has two orthogonal faces, then it has not more than two 4-cycles. Also we study 4-cycles of the "physical" billiard in pyramids, i.e. in the presence of gravity. Here we present our observations for a generic case.
We consider the space $P$ of generic complex 5-degree polynomials. Critical values of such polynomial, i.e. four points in the complex plane, either are vertices of a convex quadrangle $Q$, or vertices of a triangle $T$ with one point inside $T$. The inverse image of $Q$ is a tree-like connected structure of five ovals (a cactus). The inverse image of $T$ is also a cactus, but of four ovals. Transformations of cacti of the first type into cacti of the second type and vice versa allow one to represent the space $P$ as a ribbon bipartite graph of genus 3.
A cubic Galois polynomial is a cubic polynomial with rational coefficients that defines a cubic Galois field. Its discriminant is a full square and its roots $x_1,x_2,x_3$ (enumerated in some order) are real. There exists (and only one) quadratic polynomial $q$ with rational coefficients such that $q(x_1)=x_2, q(x_2)=x_3, q(x_3)=x_1$. The polynomial $r=q(q)\text{ mod } p$ cyclically permutes roots of $p$ in the opposite order: $r(x_1)=x_3, r(x_3)=x_2, r(x_2)=x_1$. We prove that there exist a unique Galois polynomial $p_1$ and a unique Galois polynomial $p_2$ such that the polynomial $q$ cyclically permutes roots of $p_1$ and the polynomial $r$ do the same with roots of $p_2$. Polynomials $p$ and $p_1$ (and also $p$ and $p_2$) will be called \emph{coupled}. Two polynomials are \emph{linear equivalent}, if one of them is obtained from another by a linear change of variable. By $C(p)$ we denote the class of polynomials, linear equivalent to $p$. The coupling realizes a bijection between classes $C(p)$ and $C(p_1)$ (and between classes $C(p)$ and $C(p_2)$). Classes $C(p)$ and $C(p_1)$ (and classes $C(p)$ and $C(p_2)$) will be called \emph{adjacent}. We consider a graph: its vertices -- are classes of the linear equivalency and two vertices are connected by an edge, if the corresponded classes are adjacent. Connected components of this graph will be called \emph{superclasses}. In this work we give a description of superclasses.
In this purely experimental work we try to represent the set of plane maps with 3 vertices and 3 faces as a bipartite ribbon graph. In particular, this construction allows one to estimate the genus of the initial set.
For a two dimensional vector $\bar v=(α,β)$, where $α>0, β>0$ are irrational numbers independent over $\mathbb{Q}$, we consider the set $D_n=\{(iα\,{\rm mod}\,1,iβ\,{\rm mod}\,1),i=1,\ldots,n\}$ in a two dimensional torus and the partition of this torus into Voronoi cells. Areas and forms of these cells are the subject of this experimental work.
A Latin square of order $n$ with symbols $a_1,\ldots,a_n$ can be considered as a multiplication table for binary operation in the set $A=\{a_1,\ldots,a_n\}$. We prove that, if this operation is associative, then $A$ is a group.
A polynomial $p\in \mathbb{C}[z]$ with three finite values is called the Zolotarev polynomial. For a class of such polynomials with the given degree, given passport and simple critical points we define a \emph{combinatorial moduli space}. A combinatorial moduli space have the same essential properties as analytical moduli space, but much easier to construct. We study these objects for Zolotarev polynomials of degree 5, 6 and 7.
Let us consider a family $F(α,β,γ,δ)$ of convex quadrangles in the plane with given angles $\{α,β,γ,δ\}$ and with the perimeter $2π$. Such quadrangle $Q\in F(α,β,γ,δ)$ can be considered as a point $(x_1,x_2,x_3,x_4)\in\mathbb{R}^4$, where $\{x_1,x_2,x_3,x_4\}$ are lengths of edges. Then to $F$ a finite open segment $I\subset\mathbb{R}^4$ is corresponded. A quadrangle in $F$, that corresponds to the midpoint of $I$ is called a \emph{balanced quadrangle}. Let $M$ be the set of balanced quadrangles. The function $f:M\to M$ is defined in the following way: angles of the balanced quadrangle $Q'$, $Q'=f(Q)$, are numerically equal to edges of $Q$. The map $f$ defines a dynamical system in the space of balanced quadrangles. In this work we study properties of this system.
Let $M$ be the space of triangles, defined up to shifts, rotations and dilations. We define two maps $f:M\to M$ and $g:M\to M$. The map $f$ corresponds to a triangle of perimeter $π$ the triangle with angles numerically equal to edges of the initial triangle. The map $g$ corresponds to a triangle of perimeter $2π$ the triangle with \emph{exterior} angles numerically equal to edges of the initial triangle. For $p\in M$ the sequence $\{p,f(p),f(f(p)),\ldots\}$ converges to the equilateral triangle and the sequence $\{p,g(p),g(g(p)),\ldots\}$ converges to the "degenerate triangle" with angles $(0,0,π)$. In Supplement an analogous problem about inscribed-circumscribed quadrangles is discussed.
A convex quadrangular pyramid $ABCDE$, where $ABCD$ is the base and $E$ -- the apex, is called \emph{strongly flexible}, if it belongs to a continuous family of pairwise non-congruent quadrangular pyramids that have the same lengths of corresponding edges. $ABCDE$ is called \emph{strongly rigid}, if such family does not exist. We prove the strong rigidity of convex quadrangular pyramids and prove that strong rigidity fails in the self-intersecting case. Let $L=\{l_1,\ldots,l_8\}$ be a set of positive numbers, then a \emph{realization} of $L$ is a convex quadrangular pyramid $ABCDE$ such, that $|AB|=l_1$, $|BC|=l_2$, $|CD|=l_3$, $|DA|=l_4$, $|EA|=l_5$, $|EB|=l_6$, $|EC|=l_7$, $|ED|=l_8$. We prove that the number of pairwise non-congruent realizations is $\leqslant 4$ and give an example of a set $L$ with three pairwise non-congruent realizations.
Let $P=A_1\ldots A_n$ be a generic polygon in three-dimensional space and let $v_1,v_2,\ldots,v_n$ be vectors $\overline{A_1A_2},\overline{A_2A_3},\ldots,\overline{A_nA_1}$, respectively. $P$ will be called \emph{regular}, if there exist vectors $u_1,\ldots,u_n$ such that cross products $[u_1,u_2],[u_2,u_3],\ldots,[u_n,u_1]$ are equal to vectors $v_2,v_3,\ldots,v_1$, respectively. In this case the polygon $P'$, defined be vectors $u_2-u_1,u_3-u_2,\ldots,u_1-u_n$ will be called the \emph{derived polygon} or the \emph{derivative} of the polygon $P$. In this work we formulate conditions for regularity and discuss geometric properties of derived polygons for $n=4,5,6$.
We consider quadrangles of perimeter $2$ in the plane with marked directed edge. To such quadrangle $Q$ a two-dimensional plane $Π\in\mathbb{R}^4$ with orthonormal base is corresponded. Orthogonal plane $Π^\bot$ defines a plane quadrangle $Q^\circ$ of perimeter $2$ and with marked directed edge. This quadrangle is defined uniquely (up to rotation and symmetry). Quadrangles $Q$ and $Q^\circ$ will be called dual to each other. The following properties of duality are proved: a) duality preserves convexity, non convexity and self-intersection; b) duality preserves the length of diagonals; c) the sum of lengths of corresponding edges in $Q$ and $Q^\circ$ is $1$.
A real polynomial $p$ of degree $n$ is called a Morse polynomial if its derivative has $n-1$ pairwise differentreal roots and values of $p$ in these roots (critical values) are also pairwise different. The plot of such polynomial is called a "snake". By enumerating critical points and critical values in the increasing order we construct a permutation $a_1,\ldots,a_{n-1}$, where $a_i$ is the number of polynomial's value in $i$-th critical point. This permutation is called the \emph{passport} of the snake (polynomial). In this work for Morse polynomials of degrees 5 and 6 we describe the partition of the coefficient space into domains of constant passport.
In this paper we at first consider plane trees with the root vertex and a marked directed edge, outgoing from the root vertex. For such trees we introduce a new characteristic --- the \emph{parity}, using the bracket code. It turns out that the parity depends only on the root vertex (not on the marked edge). And in the case of an even number of vertices the parity does not depend on the root vertex also. Then we consider rotation groups of bipartite trees, study their properties and prove that in the case of even number of vertices rotation groups of even and odd trees are different.
We present an easy proof of Polya's theorem on random walks: with the probability one a random walk on the two-dimensional lattice returns to the starting point.
In [3] L.Zapponi studied the arithmetic of plane bipartite trees with prime number of edges. He obtained a lower bound on the degree of tree's definition field. Here we obtain a similar lower bound in the following case. There exists a prime number $p$ such, that: a) the number of edges is divisible by $p$, but not by $p^2$; b) for any proper subset of the set of white (or black) vertices the sum of their degrees is not divisible by this $p$.