Sklar's Theorem in Finite Settings

This paper deals with the well-known Sklar's theorem, which shows how joint distribution functions are related to their marginals by means of copulas. The main goal is to prove a discrete version of this theorem involving copula-like operators defined on a finite chain, that will be called discrete copulas. First, the idea of subcopulas in this finite setting is introduced and the problem of extending a subcopula to a copula is solved. This is precisely the key point which allows to state and prove the discrete version of Sklar's theorem.     

On a class of operators for expert systems

We use the concept of directed algebra (closely related to De Morgan triplets) to modelize connectives in expert systems when linguistic terms are introduced. Mainly this article describes all directed algebra structures on a totally ordered finite set. © 1993 John Wiley & Sons, Inc.     

Phosphatidylethanolamine is the donor of the terminal phosphoethanolamine group in trypanosome glycosylphosphatidylinositols

A variety of eukaryotic cell surface proteins, including the variant surface glycoproteins of African trypanosomes, rely on a covalently attached lipid, glycosylphosphatidylinositol (GPI), for membrane attachment. GPI anchors are synthesized in the endoplasmic reticulum by stepwise glycosylation of phosphatidylinositol (via UDP-GlcNAc and dolichol-P-mannose) followed by the addition of phosphoethanolamine. The experiments described in this paper are aimed at identifying the biosynthetic origin of the terminal phosphoethanolamine group. We show that trypanosome GPIs can be labelled via CDP-[3H]ethanolamine or [beta-32P]CDP-ethanolamine in a cell-free system, indicating that phosphoethanolamine is acquired en bloc. In pulse-chase experiments with CDP-[3H]ethanolamine we show that the GPI phosphoethanolamine is not derived directly from CDP-ethanolamine, but instead from a relatively stable metabolite, such as phosphatidylethanolamine (PE), generated from CDP-ethanolamine in the cell-free system. To test the possibility that PE is the immediate donor of the GPI phosphoethanolamine moiety, we describe metabolic labelling experiments with [3H]serine and show that GPIs can be labelled in the absence of detectable radiolabelled CDP-ethanolamine, presumably via [3H]PE generated from [3H]phosphatidylserine (PS). The data support the proposal that the terminal phosphoethanolamine group in trypanosome GPIs is derived from PE.     

Physicochemical properties of calcium phosphate coatings produced by pulsed laser deposition at different water vapour pressures

Conformational peculiarities of DNA complexes with histones of the H1 family have been studied by the method of circular dichroism (CD). The H1 histones were isolated from spermatozoa of the sea urchin Strongylocentrotus intermedius, starfish Aphelasterias japonica, and bivalve mollusc Chlamis islandicus and also from the rat thymus. It is shown that these sperm-specific histones do not compact DNA in low ionic strength solution. At physiologic conditions H1 from the sea urchin and starfish sperms compact DNA more intensively than other histones. The H1 from rat thymus has a minimum ability to compact DNA. This histone does not change the structure of DNA double helix. It was supposed that it could be associated with interactions of this histone with DNA in the major groove of its helix. At the same time sperm-specific H1 can interact with DNA not only in the major groove but also in the minor groove, and this induces changes in DNA structure. This DNA-protein interaction is specific for the sperm chromatin and may support the supercompact organization of the sperm chromatin.     

Neural crest formation in Xenopus laevis: mechanisms of Xslug induction

A study of the induction of the prospective neural crest in Xenopus laevis embryos has been carried out, using the expression of Xslug as a specific marker for the neural crest. We have analyzed the competence and the specification of the neural crest. The competence to express Xslug was analyzed using two different approaches: (1) in vitro culture of conjugates of dorsal mesoderm and ectoderm taken from embryos at different ages and (2) grafts of equivalent pieces of ectoderm in the neural fold region of a gastrula or a neurula. Similar results were obtained with both methods: the ectoderm loses the competence to respond to neural fold induction at the end of gastrulation. Neural crest specification was analyzed by culturing a region of the ectoderm that contained the prospective neural crest and analyzing Xslug expression. Our results show that neural folds are specified autonomously to express Xslug by the end of gastrulation. By grafting labeled neural plate into lateral epidermis we have shown that neural crest can be induced by an interaction between neural plate and epidermis. Furthermore, neural crest cells come from both tissues. We have discarded the possibility that these neural crest cells are induced by a signal coming from the underlying lateral plate, by a homeogenetic signal coming from the host neural plate, or by regeneration of crest cells from the dissected neural plate. We propose a model to explain how the neural crest cells are induced at the border of the neural plate in X. laevis.     

Characterisation of rare earth selective emitters for thermophotovoltaic applications

We investigate selective radiation emitters made from rare earth oxides suitable for thermophotovoltaic (TPV) systems. Yb₂O₃ and Er₂O₃ emitters were fabricated and their radiation power, temperature and emissivity were measured in the entire relevant spectral range. We found temperatures of 1735 K for the Yb₂O₃ emitter and of 1680 K for the Er₂O₃ emitter both heated with a 1.35 kW butane burner. The maximum emissivities of the selective peaks were 0.85 at 1.27 eV, and 0.82 at 0.80 eV for the Yb₂O₃ and the Er₂O₃ emitter, respectively. The emission spectra show gas emission lines originating from the combustion process in addition to the selective emission bands. An estimation based on a simplified combustion model show that a TPV system with a system efficiency of about 10% can be realised using an Yb₂O₃ emitter, silicon photocells and a perfect selective filter.     

A scoping study of the application of neutral beam heating on the TCV tokamak

The TCV tokamak contributes to the physics understanding of fusion plasmas, broadening the parameter range of reactor relevant regimes, by investigations based on an extensive use of the existing main experimental tools: flexible shaping and high power real time-controllable electron cyclotron heating (ECH) and current drive (ECCD) systems. A proposed implementation of direct ion heating on the TCV by the installation of a 20–35 keV neutral beam injection (NBI) with a total power of 1–3 MW would permit an extension of the accessible range of ion to electron temperatures (T_i/T_e ～ 0.1–0.8) to well beyond unity, depending on the NBI/ECH mix and the plasma density. A NBI system would provide TCV with a tool for plasma study at reactor relevant T_i/T_e ratios ～1 and in investigating fast ion and MHD physics together with the effects of plasma rotation and high plasma β scenarios. The feasibility studies for a NBI heating on TCV presented in this paper were undertaken to construct a specification for the neutral beam injectors together with an experimental geometry for possible operational scenarios.     

GPI-anchored proteins are organized in submicron domains at the cell surface

Lateral heterogeneities in the classical fluid-mosaic model of cell membranes are now envisaged as domains or 'rafts' that are enriched in (glyco)sphingolipids, cholesterol, specific membrane proteins and glycosylphosphatidylinositol (GPI)-anchored proteins. These rafts dictate the sorting of associated proteins and/or provide sites for assembling cytoplasmic signalling molecules. However, there is no direct evidence that rafts exist in living cells. We have now measured the extent of energy transfer between isoforms of the folate receptor bound to a fluorescent analogue of folic acid, in terms of the dependence of fluorescence polarization on fluorophore densities in membranes. We find that the extent of energy transfer for the GPI-anchored folate-receptor isoform is density-independent, which is characteristic of organization in sub-pixel-sized domains at the surface of living cells; however, the extent of energy transfer for the transmembrane-anchored folate-receptor isoform was density-dependent, which is consistent with a random distribution. These domains are likely to be less than 70 nm in diameter and are disrupted by removal of cellular cholesterol. These results indicate that lipid-linked proteins are organized in cholesterol-dependent submicron-sized domains. Our methodology offers a new way of monitoring nanometre-scale association between molecules in living cells.     

De sitter relativity: a natural scenario for an evolving Λ

In de Sitter special relativity, spacetime translations are replaced by a combination of translations and proper conformal transformations. As a consequence, the energy-momentum current is replaced by a combination of ordinary energy-momentum and proper conformal currents. Whereas the ordinary energy-momentumtensor remains to be a dynamic source of the spacetime curvature, the proper conformal current appears as a kinematic source of ??. The de Sitter special relativity, therefore, allows for a new interpretation of dark energy as an entity encoded in the kinematic group of spacetime. Furthermore, since ordinary energy is allowed to transform into dark energy and vice versa, it provides a natural scenario for an evolving cosmological term. A qualitative discussion on how a ??-evolving universe would behave is presented.