1,460 research outputs found
Shapes of polyhedra and triangulations of the sphere
The space of shapes of a polyhedron with given total angles less than 2\pi at
each of its n vertices has a Kaehler metric, locally isometric to complex
hyperbolic space CH^{n-3}. The metric is not complete: collisions between
vertices take place a finite distance from a nonsingular point. The metric
completion is a complex hyperbolic cone-manifold. In some interesting special
cases, the metric completion is an orbifold. The concrete description of these
spaces of shapes gives information about the combinatorial classification of
triangulations of the sphere with no more than 6 triangles at a vertex.Comment: 39 pages. Published copy, also available at
http://www.maths.warwick.ac.uk/gt/GTMon1/paper25.abs.htm
On Fenchel-Nielsen coordinates on Teichm\"uller spaces of surfaces of infinite type
We introduce Fenchel-Nielsen coordinates on Teicm\"uller spaces of surfaces
of infinite type. The definition is relative to a given pair of pants
decomposition of the surface. We start by establishing conditions under which
any pair of pants decomposition on a hyperbolic surface of infinite type can be
turned into a geometric decomposition, that is, a decomposition into hyperbolic
pairs of pants. This is expressed in terms of a condition we introduce and
which we call Nielsen convexity. This condition is related to Nielsen cores of
Fuchsian groups. We use this to define the Fenchel-Nielsen Teichm\"uller space
associated to a geometric pair of pants decomposition. We study a metric on
such a Teichm\"uller space, and we compare it to the quasiconformal
Teichm\"uller space, equipped with the Teichm\"uller metric. We study
conditions under which there is an equality between these Teichm\"uller spaces
and we study topological and metric properties of the identity map when this
map exists
Shape and symmetry determine two-dimensional melting transitions of hard regular polygons
The melting transition of two-dimensional (2D) systems is a fundamental
problem in condensed matter and statistical physics that has advanced
significantly through the application of computational resources and
algorithms. 2D systems present the opportunity for novel phases and phase
transition scenarios not observed in 3D systems, but these phases depend
sensitively on the system and thus predicting how any given 2D system will
behave remains a challenge. Here we report a comprehensive simulation study of
the phase behavior near the melting transition of all hard regular polygons
with vertices using massively parallel Monte Carlo simulations
of up to one million particles. By investigating this family of shapes, we show
that the melting transition depends upon both particle shape and symmetry
considerations, which together can predict which of three different melting
scenarios will occur for a given . We show that systems of polygons with as
few as seven edges behave like hard disks; they melt continuously from a solid
to a hexatic fluid and then undergo a first-order transition from the hexatic
phase to the fluid phase. We show that this behavior, which holds for all
, arises from weak entropic forces among the particles. Strong
directional entropic forces align polygons with fewer than seven edges and
impose local order in the fluid. These forces can enhance or suppress the
discontinuous character of the transition depending on whether the local order
in the fluid is compatible with the local order in the solid. As a result,
systems of triangles, squares, and hexagons exhibit a KTHNY-type continuous
transition between fluid and hexatic, tetratic, and hexatic phases,
respectively, and a continuous transition from the appropriate "x"-atic to the
solid. [abstract truncated due to arxiv length limitations]
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