Combinatorial Yamabe flow on hyperbolic surfaces with boundary

This paper studies the combinatorial Yamabe flow on hyperbolic surfaces with boundary. It is proved by applying a variational principle that the length of boundary components is uniquely determined by the combinatorial conformal factor. The combinatorial Yamabe flow is a gradient flow of a concave function. The long time behavior of the flow and the geometric meaning is investigated.Comment: 10 page

A characterization of hyperbolic geometry among Hilbert geometry

In this paper we characterize hyperbolic geometry among Hilbert geometry by the property that three medians of any hyperbolic triangle all pass through one point.Comment: 5 pages, 4 figure

Extremum problems for eigenvalues of discrete Laplace operators

The discrete Laplace operator on a triangulated polyhedral surface is related to geometric properties of the surface. This paper studies extremum problems for eigenvalues of the discrete Laplace operators. Among all triangles, an equilateral triangle has the maximal first positive eigenvalue. Among all cyclic quadrilateral, a square has the maximal first positive eigenvalue. Among all cyclic $n$-gons, a regular one has the minimal value of the sum of all nontrivial eigenvalues and the minimal value of the product of all nontrivial eigenvalues.Comment: 12 page

Geometric angle structures on triangulated surfaces

In this paper we characterize a function defined on the set of edges of a triangulated surface such that there is a spherical angle structure having the function as the edge invariant (or Delaunay invariant). We also characterize a function such that there is a hyperbolic angle structure having the function as the edge invariant.Comment: 10 page

A note on circle patterns on surfaces

In this paper we give two different proofs of Bobenko and Springborn's theorem of circle pattern: there exists a hyperbolic (or Euclidean) circle pattern with proscribed intersection angles and cone angles on a cellular decomposed surface up to isometry (or similarity).Comment: 16 pages, 2 figure

On parameterizations of Teichm\"uller spaces of surfaces with boundary

In \cite{rigidity}, Luo introduced a $\psi_{\lambda}$ edge invariant which turns out to be a coordinate of the Teichm\"uller space of a surface with boundary. And he proved that for $\lambda \geq 0$, the image of the Teichm\"uller space under $\psi_{\lambda}$ edge invariant coordinate is an open cell. In this paper we verify his conjecture that for $\lambda<0$, the image of the Teichm\"uller space is a bounded convex polytope.Comment: 12 pages, 4 figure

Canonical equations of Hamilton for the nonlinear Schr\"{o}dinger equation

We define two different systems of mathematical physics: the second-order differential system (SODS) and the first-order differential system (FODS). The Newton's second law of motion and the nonlinear Schr\"{o}dinger equation (NLSE) are the exemplary SODS and FODS, respectively. We obtain a new kind of canonical equations of Hamilton (CEH), which are of some kind of symmetry in form and are formally different with the conventional CEH without symmetry [H. Goldstein, C. Poole, J. Safko, Classical Mechanics, third ed., Addison-Wesley, 2001]. We also prove that the number of the CEHs is equal to the number of the generalized coordinates for the FODS, but twice the number of the generalized coordinates for the SODS. We show that the FODS can only be expressed by the new CEH, but do not by the conventional CEH, while the SODS can be done by both the new and the conventional CEHs. As an example, we prove that the nonlinear Schr\"{o}dinger equation can be expressed with the new CEH in a consistent way.Comment: 12 pages, no figures. arXiv admin note: substantial text overlap with arXiv:1212.195

Cyclic polygons in classical geometry

Formulas about the side lengths, diagonal lengths or radius of the circumcircle of a cyclic polygon in Euclidean geometry, hyperbolic geometry or spherical geometry can be unified.Comment: 8 pages, 3 figure

On a Conjecture of Milnor about Volume of Simplexes

We establish the second part of Milnor's conjecture on the volume of simplexes in hyperbolic and spherical spaces. A characterization of the closure of the space of the angle Gram matrices of simplexes is also obtained.Comment: 15 pages. Version for publicatio

Cell decompositions of Teichm\"uller spaces of surfaces with boundary

A family of coordinates $\psi_h$ for the Teichm\"uller space of a compact surface with boundary was introduced in \cite{l2}. In the work \cite{m1}, Mondello showed that the coordinate $\psi_0$ can be used to produce a natural cell decomposition of the Teichm\"uller space invariant under the action of the mapping class group. In this paper, we show that the similar result also works for all other coordinate $\psi_h$ for any $h \geq 0$.Comment: 11 pages, 6 figure