2,130 research outputs found
Asymptotic rank of spaces with bicombings
The question, under what geometric assumptions on a space X an n-quasiflat in
X implies the existence of an n-flat therein, has been investigated for a long
time. It was settled in the affirmative for Busemann spaces by Kleiner, and for
manifolds of non-positive curvature it dates back to Anderson and Schroeder. We
generalize the theorem of Kleiner to spaces with bicombings. This structure is
a weak notion of non-positive curvature, not requiring the space to be uniquely
geodesic. Beside a metric differentiation argument, we employ an elegant
barycenter construction due to Es-Sahib and Heinich by means of which we define
a Riemannian integral serving us in a sort of convolution operation.Comment: 16 pages, 1 figur
Convex geodesic bicombings and hyperbolicity
A geodesic bicombing on a metric space selects for every pair of points a
geodesic connecting them. We prove existence and uniqueness results for
geodesic bicombings satisfying different convexity conditions. In combination
with recent work by the second author on injective hulls, this shows that every
word hyperbolic group acts geometrically on a proper, finite dimensional space
X with a unique (hence equivariant) convex geodesic bicombing of the strongest
type. Furthermore, the Gromov boundary of X is a Z-set in the closure of X, and
the latter is a metrizable absolute retract, in analogy with the Bestvina--Mess
theorem on the Rips complex.Comment: 22 page
Flats in spaces with convex geodesic bicombings
In spaces of nonpositive curvature the existence of isometrically embedded
flat (hyper)planes is often granted by apparently weaker conditions on large
scales. We show that some such results remain valid for metric spaces with
non-unique geodesic segments under suitable convexity assumptions on the
distance function along distinguished geodesics. The discussion includes, among
other things, the Flat Torus Theorem and Gromov's hyperbolicity criterion
referring to embedded planes. This generalizes results of Bowditch for Busemann
spaces.Comment: Final version, to appear in Analysis and Geometry in Metric Spaces
(AGMS
Gap Filling of 3-D Microvascular Networks by Tensor Voting
We present a new algorithm which merges discontinuities in 3-D images of tubular structures presenting undesirable gaps. The application of the proposed method is mainly associated to large 3-D images of microvascular networks. In order to recover the real network topology, we need to ïŹll the gaps between the closest discontinuous vessels. The algorithm presented in this paper aims at achieving this goal. This algorithm is based on the skeletonization of the segmented network followed by a tensor voting method. It permits to merge the most common kinds of discontinuities found in microvascular networks. It is robust, easy to use, and relatively fast. The microvascular network images were obtained using synchrotron tomography imaging at the European Synchrotron Radiation Facility. These images exhibit samples of intracortical networks. Representative results are illustrated
L'Identification des Idées
L'intĂ©rĂȘt considĂ©rable de la thĂ©orie de la culture dĂ©fendue par Dan Sperber, dans son livre La contagion des idĂ©es, est d'expliciter plus fermement que de coutume les prĂ©supposĂ©s ontologiques d'une approche atomiste dans ce domaine. L'ethnographe prĂ©suppose qu'il y a dans le monde, non seulement des personnes, mais des idĂ©es. Comment les idĂ©es sont-elles dans le monde ? Le "monisme ontologique" de Sperber consiste Ă refuser de multiplier les genres d'entitĂ©s. Les idĂ©es sont donc, selon lui, des entitĂ©s matĂ©rielles au mĂȘme titre que les personnes. Pourtant, le vĂ©ritable problĂšme ontologique qui se pose au sujet des idĂ©es n'est pas de dĂ©cider si elles sont des objets matĂ©riels ou plutĂŽt des objets immatĂ©riels, mais de savoir si leur mode d'ĂȘtre est celui des objets ou s'il est d'une autre catĂ©gorie que celle des objets
Philosophie des représentations collectives
Selon Peter Winch,, le mental et le social sont comme les deux cĂŽtĂ©s d'une mĂȘme piĂšce de monnaie. Autrement dit, â les relations sociales sont des relations internes â. En montrant que c'Ă©tait lĂ le point traitĂ© par Wittgenstein dans sa discussion sur les pratiques guidĂ©es par des rĂšgles, Winch a fait faire un pas dĂ©cisif Ă la philosophie sociale
L'idée d'un sens commun
Je traiterai du principe de charitĂ© sous l'angle d'une rĂ©flexion sur la philosophie du sens commun. J'expliquerai d'abord ce que signifie : poser la question d'un sens commun. J'en viendrai ensuite aux diverses raisons qui ont Ă©tĂ© donnĂ©es d'adopter un â principe de charitĂ© â Ă l'Ă©gard des propos qui nous sont tenus par un interlocuteur
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