31,552 research outputs found
Homotopy type of spaces of curves with constrained curvature on flat surfaces
Let be a complete flat surface, such as the Euclidean plane. We determine
the homeomorphism class of the space of all curves on which start and end
at given points in given directions and whose curvatures are constrained to lie
in a given open interval, in terms of all parameters involved. Any connected
component of such a space is either contractible or homotopy equivalent to an
-sphere, and every is realizable. Explicit homotopy equivalences
between the components and the corresponding spheres are constructed.Comment: 39 pages, 13 figures. Differs from previous version by many
improvements of the expositio
New results on the genetic cryptanalysis of TEA and reduced-round versions of XTEA
Congress on Evolutionary Computation. Portland, USA, 19-23 June 2004Recently, a simple way of creating very efficient distinguishers for cryptographic primitives such as block ciphers or hash functions, was presented by the authors. Here, this cryptanalysis attack is shown to be successful when applied over reduced round versions of the block cipher XTEA. Additionally, a variant of this genetic attack is introduced and its results over TEA shown to be the most powerful published to date
Top ranking economics journals impact variability and a ranking update to the year 2002
In this paper I address four questions concerning the quality of scientific economic papers. First, I validate the ex-ante procedure of computing the average impact of economic papers by comparing its results with the expost values. Second, I calibrate an estimator of papers normalised impact. Third, I compute the ranking variability of journals using a bootstrap procedure. Fourth, I test the statistical hypothesis that journalsâ ranking did not changed over the time interval between 1980 and 2000. I concluded that this hypothesis is rejected only for the âQuarterly Journal of Economicsâ and âEconometricaâ, which saw their citation impact improved.
Thermodynamics of nonsingular bouncing universes
Homogeneous and isotropic, nonsingular, bouncing world models are designed to
evade the initial singularity at the beginning of the cosmic expansion. Here,
we study the thermodynamics of the subset of these models governed by general
relativity. Considering the entropy of matter, radiation and that the entropy
of the apparent horizon is proportional to its area, we argue that these models
do not respect the generalised second law of thermodynamics, also away from the
bounce.Comment: 12 pages, 3 figures. Version published in the European Physical
Journal
Black holes with constant topological Euler density
A class of four dimensional spherically symmetric and static geometries with
constant topological Euler density is studied. These geometries are shown to
solve the coupled Einstein-Maxwell system when non-linear Born-Infeld-like
electrodynamics is employed.Comment: 7 pages, REVTeX 4, 1 figure, to appear in EP
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