741 research outputs found
Bucolic Complexes
We introduce and investigate bucolic complexes, a common generalization of
systolic complexes and of CAT(0) cubical complexes. They are defined as simply
connected prism complexes satisfying some local combinatorial conditions. We
study various approaches to bucolic complexes: from graph-theoretic and
topological perspective, as well as from the point of view of geometric group
theory. In particular, we characterize bucolic complexes by some properties of
their 2-skeleta and 1-skeleta (that we call bucolic graphs), by which several
known results are generalized. We also show that locally-finite bucolic
complexes are contractible, and satisfy some nonpositive-curvature-like
properties.Comment: 45 pages, 4 figure
Hypercellular graphs: partial cubes without as partial cube minor
We investigate the structure of isometric subgraphs of hypercubes (i.e.,
partial cubes) which do not contain finite convex subgraphs contractible to the
3-cube minus one vertex (here contraction means contracting the edges
corresponding to the same coordinate of the hypercube). Extending similar
results for median and cellular graphs, we show that the convex hull of an
isometric cycle of such a graph is gated and isomorphic to the Cartesian
product of edges and even cycles. Furthermore, we show that our graphs are
exactly the class of partial cubes in which any finite convex subgraph can be
obtained from the Cartesian products of edges and even cycles via successive
gated amalgams. This decomposition result enables us to establish a variety of
results. In particular, it yields that our class of graphs generalizes median
and cellular graphs, which motivates naming our graphs hypercellular.
Furthermore, we show that hypercellular graphs are tope graphs of zonotopal
complexes of oriented matroids. Finally, we characterize hypercellular graphs
as being median-cell -- a property naturally generalizing the notion of median
graphs.Comment: 35 pages, 6 figures, added example answering Question 1 from earlier
draft (Figure 6.
Ramified rectilinear polygons: coordinatization by dendrons
Simple rectilinear polygons (i.e. rectilinear polygons without holes or
cutpoints) can be regarded as finite rectangular cell complexes coordinatized
by two finite dendrons. The intrinsic -metric is thus inherited from the
product of the two finite dendrons via an isometric embedding. The rectangular
cell complexes that share this same embedding property are called ramified
rectilinear polygons. The links of vertices in these cell complexes may be
arbitrary bipartite graphs, in contrast to simple rectilinear polygons where
the links of points are either 4-cycles or paths of length at most 3. Ramified
rectilinear polygons are particular instances of rectangular complexes obtained
from cube-free median graphs, or equivalently simply connected rectangular
complexes with triangle-free links. The underlying graphs of finite ramified
rectilinear polygons can be recognized among graphs in linear time by a
Lexicographic Breadth-First-Search. Whereas the symmetry of a simple
rectilinear polygon is very restricted (with automorphism group being a
subgroup of the dihedral group ), ramified rectilinear polygons are
universal: every finite group is the automorphism group of some ramified
rectilinear polygon.Comment: 27 pages, 6 figure
A new real-time high speed ultrasonic imaging system
An ultrasonic imaging technique was to be developed for sizing and
characterization of defects in thick sections of steel. The problems
generally found with the existing techniques in such applications are
low speed, inadequate image quality, large size and high cost. [Continues.
Helly groups
Helly graphs are graphs in which every family of pairwise intersecting balls
has a non-empty intersection. This is a classical and widely studied class of
graphs. In this article we focus on groups acting geometrically on Helly graphs
-- Helly groups. We provide numerous examples of such groups: all (Gromov)
hyperbolic, CAT(0) cubical, finitely presented graphical C(4)T(4) small
cancellation groups, and type-preserving uniform lattices in Euclidean
buildings of type are Helly; free products of Helly groups with
amalgamation over finite subgroups, graph products of Helly groups, some
diagram products of Helly groups, some right-angled graphs of Helly groups, and
quotients of Helly groups by finite normal subgroups are Helly. We show many
properties of Helly groups: biautomaticity, existence of finite dimensional
models for classifying spaces for proper actions, contractibility of asymptotic
cones, existence of EZ-boundaries, satisfiability of the Farrell-Jones
conjecture and of the coarse Baum-Connes conjecture. This leads to new results
for some classical families of groups (e.g. for FC-type Artin groups) and to a
unified approach to results obtained earlier
Over-expression of UDP-glucose pyrophosphorylase in hybrid poplar affects carbon allocation
The effects of the over-expression of the Acetobacter xylinum UDP-glucose pyrophosphorylase (UGPase) under the control of the tandem repeat Cauliflower Mosaic Virus promoter (2335S) on plant metabolism and growth were investigated in hybrid poplar (Populus alba3grandidentata). Transcript levels, enzyme activity, growth parameters, leaf morphology, structural and soluble carbohydrates, and soluble metabolite levels were quantified in both transgenic and wild-type trees. Transgenic 2335S::UGPase poplar showed impaired growth rates, displaying reduced height growth and stem diameter. Morphologically, 2335S::UGPase trees had elongated axial shoots, and leaves that were substantially smaller in size when compared with wild-type trees at equivalent developmental stages. Biochemical analysis revealed significant increases in soluble sugar, starch, and cellulose contents, and concurrent decreases in lignin content. Lignin monomer composition was altered in favour of syringyl moieties. Detailed soluble metabolite analysis revealed that 2335S::UGPase trees had as much as a 270-fold increase in the salicylic acid 2-O-b-Dglucoside (SAG), a compound typically associated with the stress response. These data suggest that while it is possible to alter the allocation of carbon in favour of cellulose biosynthesis, whole plant changes result in unexpected decreases in growth and an increase in defence metabolites
Over-expression of UDP-glucose pyrophosphorylase in hybrid poplar affects carbon allocation
The effects of the over-expression of the Acetobacter xylinum UDP-glucose pyrophosphorylase (UGPase) under the control of the tandem repeat Cauliflower Mosaic Virus promoter (2335S) on plant metabolism and growth were investigated in hybrid poplar (Populus alba3grandidentata). Transcript levels, enzyme activity, growth parameters, leaf morphology, structural and soluble carbohydrates, and soluble metabolite levels were quantified in both transgenic and wild-type trees. Transgenic 2335S::UGPase poplar showed impaired growth rates, displaying reduced height growth and stem diameter. Morphologically, 2335S::UGPase trees had elongated axial shoots, and leaves that were substantially smaller in size when compared with wild-type trees at equivalent developmental stages. Biochemical analysis revealed significant increases in soluble sugar, starch, and cellulose contents, and concurrent decreases in lignin content. Lignin monomer composition was altered in favour of syringyl moieties. Detailed soluble metabolite analysis revealed that 2335S::UGPase trees had as much as a 270-fold increase in the salicylic acid 2-O-b-Dglucoside (SAG), a compound typically associated with the stress response. These data suggest that while it is possible to alter the allocation of carbon in favour of cellulose biosynthesis, whole plant changes result in unexpected decreases in growth and an increase in defence metabolites
- …