Hot spots conjecture for a class of acute triangles

We show that the hot spots conjecture of J. Rauch holds for acute triangles if one of the angles is not larger than \(\pi /6\). More precisely, we show that the second Neumann eigenfunction on those acute triangles has no maximum or minimum inside the domain. We first simplify the problem by showing that absence of critical points on two sides implies no critical points inside a triangle. This result applies to any acute triangle and might help prove the conjecture for arbitrary acute triangles. Then we show that there are no critical points on two sides assuming one small angle. We also establish simplicity for the smallest positive Neumann eigenvalue for all non-equilateral acute triangles. This result was already known for obtuse triangles, and it fails for the equilateral case.     

Acute triangulations of trapezoids

In this paper we discuss acute triangulations of trapezoids. It is known that every rectangle can be triangulated into eight acute triangles, and that this is best possible. In this paper we prove that all other trapezoids can be triangulated into at most seven acute triangles.     

n维Mbius变换的正则性

本文利用高维Ｍｂｉｕｓ变换的 Ｃｌｉｆｆｏｒｄ矩阵表示,讨论了高维 Ｍｂｉｕｓ变换的正则性;证明了三维抛物Ｍｂｉｕｓ变换一定是正则的,得到了三维非抛物Ｍｂｉｕｓ变换是正则的一条充要条件;同时举例说明上述充要条件在ＳＬ（２,　ｎ）（ｎ＝２ｋ,ｋ≥２）中不成立．     

n维Mbius变换的正则性

本文利用高维Ｍｂｉｕｓ变换的 Ｃｌｉｆｆｏｒｄ矩阵表示,讨论了高维 Ｍｂｉｕｓ变换的正则性;证明了三维抛物Ｍｂｉｕｓ变换一定是正则的,得到了三维非抛物Ｍｂｉｕｓ变换是正则的一条充要条件;同时举例说明上述充要条件在ＳＬ（２,　ｎ）（ｎ＝２ｋ,ｋ≥２）中不成立．     

Characterizations of lattice surfaces

We answer a question of Vorobets by showing that the lattice property for flat surfaces is equivalent to the existence of a positive lower bound for the areas of affine triangles. We show that the set of affine equivalence classes of lattice surfaces with a fixed positive lower bound for the areas of triangles is finite and we obtain explicit bounds on its cardinality. We deduce several other characterizations of the lattice property.     

Triangulation refinement by using edge subdivision

It is proved that any triangulation of a flat polygonal region can be refined by using repeated subdivisions of an edge so that: (1) the maximum diameter of the triangles would be less than any pre-assigned positive number, and (2) the minimum interior angle of the triangles of the triangulation obtained would be not less than the minimum interior angle of the triangles of the original triangulation divided by 9. The required triangulation refinement is constructed in two steps: first, the triangulation is refined so that the triangles of the triangulation obtained can be combined into pairs, and only boundary triangles may be left unpaired; at this step each triangle is split into at most 4 parts. Then the triangulation obtained is refined once again in order that the diameter of each triangle be less then a prescribed ?. At each of the steps, the minimum interior angle of triangles is reduced by at most 3 times. This is guaranteed by the lemma saying that the interior angles of the triangles into which the original triangle is divided by a median are at least as great as one-third of the minimum interior angle of the original triangle.     

Lines Tangent to Four Triangles in Three-Dimensional Space

We investigate the lines tangent to four triangles in R³. By a construction, there can be as many as 62 tangents. We show that there are at most 162 connected components of tangents, and at most 156 if the triangles are disjoint. In addition, if the triangles are in (algebraic) general position, then the number of tangents is finite and it is always even.     

单位球面S~6中的Blaschke等参超曲面

单位球面中的一个无脐点浸入子流形称为Blaschke等参子流形如果它的Mbius形式恒为零并且所有的Blaschke特征值均为常数.维数m4的Blaschke等参超曲面已经有了完全的分类.截止目前,Mbius等参超曲面的所有已知例子都是Blaschke等参的.另一方面,确实存在许多不是Mbius等参的Blaschke等参超曲面,它们都具有不超过两个的不同Blaschke特征值.在已有分类定理的基础上,本文对于5维Blaschke等参超曲面进行了完全的分类.特别地,我们证明了S6中具有多于两个不同Blaschke特征值的Blaschke等参超曲面一定是Mbius等参的,给出了此前一个问题的部分解答.     

General Parity Result and Cycle‐Plus‐Triangles Graphs

We generalize a parity result of Fleishner and Stiebitz that being combined with Alon–Tarsi polynomial method allowed them to prove that a 4‐regular graph formed by a Hamiltonian cycle and several disjoint triangles is always 3‐choosable. Also we show how a version of polynomial method gives slightly more combinatorial information about colorings than direct application of Alon's Combinatorial Nullstellensatz.     

An attempt to differential Galois theory of second order polynomial system and solvable subgroup of Mbius transformations

By introducing the conception "relativistic differential Galois group" for the second order polynomial systems, we establish the relation between the conformal relativistic differential Galois group and the subgroup of Mobius transformations, and prove that the system is integrable in the sense of Liouville if its conformal relativistic differential Galois group is solvable with a derived length at most 2. Some omissions on the structures of solvable subgroups of Mobius transformations at the first author's article published in this journal in 1996 are refreshed in this paper.     

Semiclassical Topological MöBius Planes

A new rather large family of locally compact 2-dimensional topological Mobius planes is introduced here. This family consists exactly of those Mobius planes which can be obtained by pasting together two halves of the classical real Mobius plane suitably. Isomorphism classes, automorphisms, and the Hering type of these planes are determined.     

本原Heronian三角形的一个注记

得到了无穷多个锐角本原Heronian三角形,其内切圆及三个旁切圆半径均为整数,并得到了无穷多个本原Heronian三角形,其内切圆及三个旁切圆半径均不为整数.     

Tilings of Convex Polygons with Congruent Triangles

By the spectrum of a polygon A we mean the set of triples (??,??,??) such that A can be dissected into congruent triangles of angles ??,??,??. We propose a technique for finding the spectrum of every convex polygon. Our method is based on the following classification. A tiling is called regular if there are two angles of the triangles, ?? and ?? such that at every vertex of the tiling the number of triangles having angle ?? equals the number of triangles having angle ??. Otherwise the tiling is irregular. We list all pairs (A,T) such that A is a convex polygon and T is a triangle that tiles A regularly. The list of triangles tiling A irregularly is always finite, and can be obtained, at least in principle, by considering the system of equations satisfied by the angles, examining the conjugate tilings, and comparing the sides and the area of the triangles to those of A. Using this method we characterize the convex polygons with infinite spectrum, and determine the spectrum of the regular triangle, the square, all rectangles, and the regular N-gons with N large enough.     

具有无界度的无限圆填充的刚性(英文)

刚性是圆填充理论的一个重要的性质.已经知道,平面上无限的有界度的圆填充的刚性可以用环绕数的方法来证明.本文应用环绕数和指标的技术,结合有限覆盖定理证明了几乎填满整个黎曼球面具有相同复形的无限无界度圆填充对M(o|¨)bius变换来说是等价的,也就是,一个圆填充是另一个圆填充在M(o|¨)bius变换下的像.这给出了无限无界度圆填充的刚性的一种新的证明.     

Decomposition of closed orientable geometric surfaces into acute geodesic triangles

We prove that for every \(g\ge 2\), a differentiable closed orientable geometric surface of genus g may be decomposed into \(16g-16\) acute geodesic triangles. We also determine the number of acute geodesic triangles needed for the sphere and the torus.     

On an algorithm for calculating diffraction integrals

An algorithm for calculating integrals of rapidly oscillating functions given on a smooth two-dimensional surface is proposed. The surface is approximated by a collection of flat triangles with the values of the integrand known at their vertices. These values are used as reference ones to extend the function to other points of a triangle. The integral of the extended function over the surface of a triangle is calculated exactly. The desired value of the full diffraction integral is determined as the sum of the integrals calculated over the surfaces of all triangles. The resulting formulas for integral calculation involve singularities (indeterminate forms). Much attention is given to representations of these formulas in such a way that the indeterminate forms are automatically evaluated. Numerical results are presented.     

SUBMANIFOLDS IN S~(n+p) WITH PARALLEL MBIUS FORM

§ 1　IntroductionLet x:M→ Sn+p be an n-dimensional submanifold in the unit sphere without umbilicpoints.Let { ei} be a local orthonormal basis with respectto the firstfundamental form I=dx·dx with dual basis{ θi} .Let II =∑ijαhαijθiθjeαbe the second fundamental form of xand H =∑αHαeαbe the mean curvature vector of x,where we use the range of indices:1≤ i,j,k,l≤ n,　　 n + 1≤α,β,γ≤ n + p,Hα=1n∑ihαij,{ eα} is a local orthonormal basis forthe normal bundle ofx.We defi…     

Minimizing Neumann fundamental tones of triangles: An optimal Poincaré inequality

The first nonzero eigenvalue of the Neumann Laplacian is shown to be minimal for the degenerate acute isosceles triangle, among all triangles of given diameter. Hence an optimal Poincaré inequality for triangles is derived.The proof relies on symmetry of the Neumann fundamental mode for isosceles triangles with aperture less than π/3. Antisymmetry is proved for apertures greater than π/3.     

Flatness of Gaussian curvature and area of ideal triangles

Let M be aC ^k ,k 4, compact surface of genus greater than two whose curvature is negative in all points but along a simple closed geodesic (t) where the curvature is zero at every point. We show that the area of ideal triangles having a lifting of as an edge is infinite. This provides a family of surfaces having ideal triangles of infinite area whose geodesic flows are equivalent to Anosov flows, in contrast with the well-known examples of surfaces with flat strips which also have ideal triangles of infinite area. By the CAT-comparison theory we can deduce, using these surfaces as models, that aC compact surface of non-positive curvature having one geodesic along which the curvature is zero has ideal triangles of infinite area.Partially supported by CNPq of Brazilian Government     

Distance Types in Banach Spaces

We introduce and study the notion of a distance type, on a Banach space, defined by a nested sequence of convex sets. Among other things, we show that there always exist distance types that are not types in the classical sense. Then, we recover the notion of the flat nested sequence of Milman and Milman and show that distance types defined by flat nested sequences coincide with the bidual types of Farmaki. These results are applied to show that a flat nested sequence of convex sets is Wijsman convergent to the intersection of their weak*-closures in bidual space.