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Search Results: 1 - 10 of 32747 matches for " Daniel Weibel "
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Homotopy Ends and Thomason Model Categories
Charles Weibel
Mathematics , 2001,
Abstract: In the last year of his life, Bob Thomason reworked the notion of a model category, used to adapt homotopy theory to algebra, and used homotopy ends to affirmatively solve a problem raised by Grothendieck: find a notion of model structure which is inherited by functor categories. In this paper we explain and prove Thomason's results, based on his private notebooks. The first half presents Thomason's ideas about homotopy ends and its generalizations. This material may be of independent interest. Then we define Thomason model categories and give some examples. The usual proof shows that the homotopy category exists. In the last two sections we prove the main theorem: functor categories inherit a Thomason model structure, at least when the original category is enriched over simplicial sets and fibrations are preserved by limits.
Maximal f-vectors of Minkowski sums of large numbers of polytopes
Christophe Weibel
Computer Science , 2010,
Abstract: It is known that in the Minkowski sum of $r$ polytopes in dimension $d$, with $r
Photon stimulated ion desorption of condensed CO2 at ~ 85 K studied by synchrotron radiation
Mota, Gunar V. S.;Weibel, Daniel E.;Araújo, Geovane S.;Rocco, Maria Luiza M.;
Brazilian Journal of Physics , 2006, DOI: 10.1590/S0103-97332006000600047
Abstract: photon stimulated ion desorption (psid) from condensed carbon dioxide has been studied for photon excitation energies ranging from 93 to 193 ev. psid studies have been performed at the brazilian synchrotron light source (lnls), campinas, during a multi-bunch operation mode of the storage ring. the results showed that after photon excitation several ions desorbed from the co2 films: c+ , o+ , co+ and o2+. psid experiments showed that ion desorption was enhanced only at the si resonance excitations. when the thickness of the co2 was ~ 500 l or higher, almost no desorption yield was observed. the study of the dependence of the relative partial ion yield on the photon excitation showed that the x-ray induced electron stimulated desorption (xesd) mechanism has to be invoked to explain the origin of the desorbed ions in the energy region studied.
Random demographic household surveys in highly mobile pastoral communities in Chad
Weibel,Daniel; Béchir,Mahamat; Hattendorf,Jan; Bonfoh,Bassirou; Zinsstag,Jakob; Schelling,Esther;
Bulletin of the World Health Organization , 2011, DOI: 10.2471/BLT.10.077206
Abstract: problem: reliable demographic data is a central requirement for health planning and management, and for the implementation of adequate interventions. this study addresses the lack of demographic data on mobile pastoral communities in the sahel. approach: a total of 1081 arab, fulani and gorane women and 2541 children (1336 boys and 1205 girls) were interviewed and registered by a biometric fingerprint scanner in five repeated random transect demographic and health surveys conducted from march 2007 to january 2008 in the lake chad region in chad. local setting: important determinants for the planning and implementation of household surveys among mobile pastoral communities include: environmental factors; availability of women for interviews; difficulties in defining "own" children; the need for informationeducation-communication campaigns; and informed consent of husbands in typically patriarchal societies. relevant changes: due to their high mobility, only 5% (56/1081) of registered women were encountered twice. therefore, it was not possible to establish a demographic and health cohort. lessons learnt: prospective demographic and health cohorts are the most accurate method to assess child mortality and other demographic indices. however, their feasibility in a highly mobile pastoral setting remains to be shown. future interdisciplinary scientific efforts need to target innovative methods, tools and approaches to include marginalized communities in operational health and demographic surveillance systems.
Review of Survey activities 2007: Correlation of carbon isotope events in the Danish Upper Cretaceous chalk
Weibel, Rikke,Keulen, Nynke
Geological Survey of Denmark and Greenland Bulletin , 2008,
Abstract:
On f-vectors of Minkowski additions of convex polytopes
Komei Fukuda,Christophe Weibel
Mathematics , 2005, DOI: 10.1007/s00454-007-1310-2
Abstract: The objective of this paper is to present two types of results on Minkowski sums of convex polytopes. The first is about a special class of polytopes we call perfectly centered and the combinatorial properties of the Minkowski sum with their own dual. In particular, we have a characterization of face lattice of the sum in terms of the face lattice of a given perfectly centered polytope. Exact face counting formulas are then obtained for perfectly centered simplices and hypercubes. The second type of results concerns tight upper bounds for the f-vectors of Minkowski sums of several polytopes.
A linear equation for Minkowski sums of polytopes relatively in general position
Komei Fukuda,Christophe Weibel
Mathematics , 2007,
Abstract: The objective of this paper is to study a special family of Minkowski sums, that is of polytopes relatively in general position. We show that the maximum number of faces in the sum can be attained by this family. We present a new linear equation that is satisfied by f-vectors of the sum and the summands. We study some of the implications of this equation.
Schur-finiteness in $λ$-rings
C. Mazza,C. Weibel
Mathematics , 2010,
Abstract: We introduce the notion of a Schur-finite element in a $\lambda$-ring.
Unstable operations in étale and motivic cohomology
Bertrand Guillou,Charles Weibel
Mathematics , 2013,
Abstract: We classify all \'etale cohomology operations on $H_\et^n(-,\muell{i})$, showing that they were all constructed by Epstein. We also construct operations $P^a$ on the mod-$\ell$ motivic cohomology groups $H^{p,q}$, differing from Voevodsky's operations; we use them to classify all motivic cohomology operations on $H^{p,1}$ and $H^{1,q}$ and suggest a general classification.
Some surfaces of general type for which Bloch's conjecture holds
Claudio Pedrini,Charles Weibel
Mathematics , 2013,
Abstract: We give many examples of surfaces of general type with $p_g=0$ for which Bloch's conjecture holds, for all values of $K^2$ except 9. Our surfaces are equipped with an involution.
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