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Université de Fribourg

Ecology of active and passive players and their impact on information selection

Bianconi, Ginestra ; Laureti, Paolo ; Yu, Yi-Kou ; Zhang, Yi-Cheng

In: Physica A: Statistical Mechanics and its Applications, 2004, vol. 332, p. 519-532

Is visitors’ attendance a fair indicator of a web site's quality? Internet sub-domains are usually characterized by power-law distributions of visits, thus suggesting a rich-get-richer process. If this is the case, the number of visits is not a relevant measure of quality. If, on the other hand, there are active players, i.e., visitors who can tell the value of the information available, better...

Université de Fribourg

Selection by pairwise comparisons with limited resources

Laureti, Paolo ; Mathiesen, Joachim ; Zhang, Yi-Cheng

In: Physica A: Statistical Mechanics and its Applications, 2004, vol. 338, p. 596-604

We analyze different methods of sorting and selecting a set of objects by their intrinsic value, via pairwise comparisons whose outcome is uncertain. After discussing the limits of repeated Round Robins, two new methods are presented: The ran-fil requires no previous knowledge on the set under consideration, yet displaying good performances even in the least favorable case. The min-ent method...

Université de Fribourg

Aggregating partial, local evaluations to achieve global ranking

Laureti, Paolo ; Moret, Lionel ; Zhang, Yi-Cheng

In: Physica A: Statistical Mechanics and its Applications, 2005, vol. 345, 3-4, p. 705-712

We analyze some voting models mimicking online evaluation systems intended to reduce the information overload. The minimum number of operations needed for a system to be effective is analytically estimated. When herding effects are present, linear preferential attachment marks a transition between trustful and biased reputations.

Université de Fribourg

Supply and demand law under limited information

Zhang, Yi-Cheng

We present a model for the supply-demand law with quality and limited information capability. We postulate that imperfect information permeates in almost all economic transactions to varying degrees. Through a simple model we outline a research agenda that re-examines many standard issues in economics. Our analysis shows that whereas imperfect information can be improved, it leads to new...

Université de Fribourg

Equilibrium cluster formation in concentrated protein solutions and colloids

Stradner, Anna ; Sedgwick, Helen ; Cardinaux, Frédéric ; Poon, Wilson C. K. ; Egelhaaf, Stefan U. ; Schurtenberger, Peter

In: Nature, 2004, vol. 432, p. 492-495

Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting...

Université de Fribourg

Weak coupling instabilities of two-dimensional lattice electrons

Binz, Benedikt ; Baeriswyl, Dionys ; Douçot, B.

In: Physica C: Superconductivity, 2004, vol. 408-410, p. 250-251

We present a consistent and systematic study of a two-dimensional extended Hubbard model in the weak coupling limit. Quite generally, the electron gas is unstable towards a superconducting state even in the absence of phonons, since high-energy spin fluctuations create an effective attraction between the quasi-particles. However in the special case of a half-filled nearest-neighbor tight-binding...

Université de Fribourg

Adaptivity and ‘Per learning’

Wakeling, Joseph Rushton

One of the key points addressed by Per Bak in his models of brain function was that biological neural systems must be able not just to learn, but also to adapt—to quickly change their behaviour in response to a changing environment. I discuss this in the context of various simple learning rules and adaptive problems, centred around the Chialvo-Bak ‘minibrain’ model (Neurosci. 90 (1999)...

Université de Fribourg

Laser-based precision magnetometry in fundamental and applied research

Weis, Antoine ; Wynands, Robert

In: Optics and Lasers in Engineering, 2005, vol. 43, p. 387-401

In this paper we review research done by the recently founded Fribourg group for Atomic Physics (FRAP) using laser-assisted spin coherence spectroscopy of atomic samples in applied and fundamental physics. Spin coherence in optically pumped ensembles of paramagnetic atoms may be extremely long-lived when the samples are well shielded from external perturbations, like static or oscillating...

Université de Fribourg

Muon capture by ³He nuclei followed by proton and deuteron production

Bystritsky, V. M. ; Boreiko, V. F. ; Filipowicz, M. ; Gerasimov, V. V. ; Huot, Olivier ; Knowles, Paul E. ; Mulhauser, Françoise ; Pavlov, V. N. ; Schaller, Lukas A. ; Schneuwly, Hubert ; Sandukovsky, V. G. ; Stolupin, V. A. ; Volnykh, V. P. ; Woźniak, J.

In: Physical Review A, 2004, vol. 69, p. 012712-12727

The paper describes an experiment aimed at studying muon capture by He nuclei in pure ³He and D₂+³He mixtures at various densities. Energy distributions of protons and deuterons produced via µ₋+³He-->p + n + n + νµ and µ₋+³He-->d + n + νµ are measured for the energy intervals 10–49 MeV and 13–31 MeV, respectively. Muon capture rates lambdacapp(ΔEp) and...

Université de Fribourg

CD81-induced behavioural changes during chronic cocaine administration: in vivo gene delivery with regulatable lentivirus

Bahi, Amine ; Boyer, Frederic ; Kafri, Tal ; Dreyer, Jean-Luc

In: European Journal of Neuroscience, 2004, vol. 19, p. 1621-1633

CD81, a tetraspanin transmembrane protein involved in cell adhesion, is up-regulated in the mesolimbic dopaminergic pathway 24 h following acute administration of high doses of cocaine [Brenz-Verca et al., (2001) Mol. Cell. Neurosci., 17, 303-316]. Further evidence consecutive with this observation and based on microarray analysis are presented here. In addition, a regulatable lentivirus was...