165 research outputs found
Equivelar and d-Covered Triangulations of Surfaces. I
We survey basic properties and bounds for -equivelar and -covered
triangulations of closed surfaces. Included in the survey is a list of the
known sources for -equivelar and -covered triangulations. We identify all
orientable and non-orientable surfaces of Euler characteristic
which admit non-neighborly -equivelar triangulations
with equality in the upper bound
. These
examples give rise to -covered triangulations with equality in the upper
bound . A
generalization of Ringel's cyclic series of neighborly
orientable triangulations to a two-parameter family of cyclic orientable
triangulations , , , is the main result of this
paper. In particular, the two infinite subseries and
, , provide non-neighborly examples with equality for
the upper bound for as well as derived examples with equality for the upper
bound for .Comment: 21 pages, 4 figure
The homotopy theory of polyhedral products associated with flag complexes
If is a simplicial complex on vertices the flagification of is
the minimal flag complex on the same vertex set that contains .
Letting be the set of vertices, there is a sequence of simplicial
inclusions . This induces a sequence of maps of polyhedral
products .
We show that and have right homotopy
inverses and draw consequences. For a flag complex the polyhedral product
of the form is a co--space if and only if
the -skeleton of is a chordal graph, and we deduce that the maps and
have right homotopy inverses in this case.Comment: 25 page
Métriques polyèdrales sur les bords de variétés hyperboliques convexes et flexibilité des polyèdres hyperboliques
Cette thèse de doctorat contient trois résultats principaux. Dans le premier chapitre on construit un polyèdre infinitésimalement flexible dans l'espace hyperbolique à trois dimensions dont le volume n'est pas stationnaire sous sa flexion infinitésimale. Dans le deuxième chapitre on obtient une condition nécessaire de flexibilité des suspensions dans l'espace hyperbolique à trois dimensions. Dans les deux derniers chapitres on démontre l'existence d'une variété quasi-Fuchsienne convexe compacte dont la métrique induite de son bord est une métrique polyèdrale hyperbolique prescrite.This thesis contains three main results. In the first chapter we construct an infinitesimally flexible polyhedron in hyperbolic 3-space such that its volume is not stationary under the infinitesimal flex. In the second chapter we obtain a necessary condition for flexibility of suspensions in hyperbolic 3-space. In the last two chapters we show the existence of a quasi-Fuchsian convex compact manifold such that the induced metric on its boundary coincides with a prescribed hyperbolic polyhedral metric
Three cocluded insect viruses : a biophysical and biological study of the nuclear-polyhedrosis virus of Colias electo, the granulosis virus of Heliothis armigera and the nuclear-polyhedrosis virus of Heliothis zea
An investigation was undertaken in some detail of three virus strains of insect pests of agricultural importance, viz. a nuclear-polyhedrosis virus of the lucerne caterpillar, Colias electo, and a granulosis virus of the bollworm, Heliothis armigera, both found in South Africa, and a nuclear-polyhedrosis virus of the bollworm, Heliothis zea, isolated in America, with a view to ascertaining a knowledge of some of the fundamental properties and basic biology of these infective agents. On the basis of the information gained the viruses could be differentiated and their broad classification was established. The morphology of the polyhedra, capsules and virus particles observed by light and electron microscopy has been completed and measurements of the viral elements have been made. Some biophysical properties of the virus particles and their inclusion bodies were recorded, i.e. their resistance to chemical and physical treatments and their relative mobility in an electric field under standard conditions. Observations were made on procedures which brought about varying degrees of purification and concentration of the virus particles from putrefying larvae and the most successful of these were found to be reproducible. They involved the purification of the inclusion bodies and their digestion by weak alkali to release the virus particles. Both preparations of the viral elements were further purified by zone electrophoresis in sucrose density gradients. Some information was gathered on the mode of transmission of the infection from insect to insect by contact or cannibalism, from one generation to the next through the eggs, and particularly from one area to another by virus survival in avian faeces. The incidence and rate of the infection in the larvae was increased by environmental changes such as raising the temperature and also to some extent by spraying with a suspension of endospores of Bacillus thuringiensis. Exposure to other stress conditions was not successful in initiating a fatal infection in the insects. Of particular interest, however, was the observation that by injecting a 'foreign' virus a fatal infection was induced by activation of a native occult virus in the larvae of the silkworm, Bornbyx mori. In the context of the possible application of these infective agents to future methods of biological control of economically disastrous pests, these preliminary experiments were not unrewarding
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