Three Colorability of an Arrangement Graph of Great Circles

Morse-Bott 函数的小圆排列

Arrangements of small circles for Morse-Bott functions

本文提出一种通过归纳添加小圆构造圆排列的系统方法，将其与Morse-Bott函数及Reeb图联系起来，研究排列中区域坍缩图的局部变化。

Arrangements of Pseudocircles: On Circularizability

An arrangement of pseudocircles is a collection of simple closed curves on the sphere or in the plane such that any two of the curves are either disjoint or intersect in exactly two crossing points. We call an arrangement intersecting if every pair of pseudocircles intersects twice. An arrangement is circularizable if there is a combinatorially equivalent arrangement of circles. In this paper we present the results of the first thorough study of circularizability. We show that there are exactly four non-circularizable arrangements of 5 pseudocircles (one of them was known before). In the set o…

Optimal packings of congruent circles on a square flat torus

We consider packings of congruent circles on a square flat torus, i.e., periodic (w.r.t. a square lattice) planar circle packings, with the maximal circle radius. This problem is interesting due to a practical reason - the problem of "super resolution of images." We have found optimal arrangements for N=6, 7 and 8 circles. Surprisingly, for the case N=7 there are three different optimal arrangements. Our proof is based on a computer enumeration of toroidal irreducible contact graphs.

圆的排列、由其围成的区域及标号的 Poincaré-Reeb 图

Arrangements of circles, the regions surrounded by them and labeled Poincaré-Reeb graphs

本文研究圆的排列及其围成的区域，通过在 Poincaré-Reeb 图的顶点和边上引入标号来编码圆的信息，并讨论了添加圆引起的新类型图变化，发现该区域是自然实代数映射的像。

On the Average Complexity of the $k$-Level

Let ${\cal L}$ be an arrangement of $n$ lines in the Euclidean plane. The \emph{$k$-level} of ${\cal L}$ consists of all vertices $v$ of the arrangement which have exactly $k$ lines of ${\cal L}$ passing below $v$. The complexity (the maximum size) of the $k$-level in a line arrangement has been widely studied. In 1998 Dey proved an upper bound of $O(n\cdot (k+1)^{1/3})$. Due to the correspondence between lines in the plane and great-circles on the sphere, the asymptotic bounds carry over to arrangements of great-circles on the sphere, where the $k$-level denotes the vertices at distance at mo…

Well-distributed great circles on S^2

Let $C_1, \dots, C_n$ denote the $1/n-$neighborhood of $n$ great circles on $\mathbb{S}^2$. We are interested in how much these areas have to overlap and prove the sharp bounds $$ \sum_{i, j = 1 \atop i \neq j}^{n}{|C_i \cap C_j|^s} \gtrsim_s \begin{cases} n^{2 - 2s} \qquad &\mbox{if}~0 \leq s < 2 \\ n^{-2} \log{n} \qquad &\mbox{if}~s = 2\\ n^{1- 3s/2} \qquad &\mbox{if}~s > 2. \end{cases} .$$ For $s=1$ there are arrangements for which the sum of mutual overlap is uniformly bounded (for the analogous problem in $\mathbb{R}^2$ the lower bound is $\gtrsim \log{n}$) and there are strong connection…

Extending drawings of complete graphs into arrangements of pseudocircles

Motivated by the successful application of geometry to proving the Harary-Hill Conjecture for "pseudolinear" drawings of $K_n$, we introduce "pseudospherical" drawings of graphs. A spherical drawing of a graph $G$ is a drawing in the unit sphere $\mathbb{S}^2$ in which the vertices of $G$ are represented as points -- no three on a great circle -- and the edges of $G$ are shortest-arcs in $\mathbb{S}^2$ connecting pairs of vertices. Such a drawing has three properties: (1) every edge $e$ is contained in a simple closed curve $γ_e$ such that the only vertices in $γ_e$ are the ends of $e$; (2) if…

伪线与伪圆排列的翻转图连通性

Flip Graph Connectivity for Arrangements of Pseudolines and Pseudocircles

本文研究伪线和伪圆排列的翻转图连通性，证明伪线翻转图连通性等于最小度n-2，伪圆翻转图在相交排列及柱面相交排列上连通，并证明两类翻转图直径均为Θ(n^3)。