18,139 research outputs found
Positive Lyapunov Exponents for Quasiperiodic Szego cocycles
In this paper we first obtain a formula of averaged Lyapunov exponents for
ergodic Szego cocycles via the Herman-Avila-Bochi formula. Then using
acceleration, we construct a class of analytic quasi-periodic Szego cocycles
with uniformly positive Lyapunov exponents. Finally, a simple application of
the main theorem in [Y] allows us to estimate the Lebesgue measure of support
of the measure associated to certain class of C1 quasiperiodic 2- sided
Verblunsky coefficients. Using the same method, we also recover the [S-S]
results for Schrodinger cocycles with nonconstant real analytic potentials and
obtain some nonuniform hyperbolicity results for arbitrarily fixed Brjuno
frequency and for certain C1 potentials.Comment: 27 papge
A Direct Reputation Model for VO Formation
We show that reputation is a basic ingredient in the Virtual Organisation (VO) formation process. Agents can use their experiences gained in direct past interactions to model other’s reputation and deciding on either join a VO or determining who is the most suitable set of partners. Reputation values are computed using a reinforcement learning algorithm, so agents can learn and adapt their reputation models of their partners according to their recent behaviour. Our approach is especially powerful if the agent participates in a VO in which the members can change their behaviour to exploit their partners. The reputation model presented in this paper deals with the questions of deception and fraud that have been ignored in current models of VO formation
Columnar defects acting as passive internal field detectors
We have studied the angular dependence of the irreversible magnetization of
several YBaCuO and 2H-NbSe single crystals with columnar
defects tilted off the c-axis. At high magnetic fields, the irreversible
magnetization exhibits a well known maximum when the applied
field is parallel to the tracks. As the field is decreased below , the peak shifts away from the tracks' direction toward either the
c-axis or the ab-planes. We demonstrate that this shift results from the
misalignment between the external and internal field directions due to the
competition between anisotropy and geometry effects.Comment: 5 figure
Nitrogen circulation in a Mediterranean holm oak forest, La Castanya, Montseny, northeastern Spain
The importance of storm frequency as well as the groundwater and hyporheic inputs on nitrate (NO<sub>3</sub>-N) and ammonium (NH<sub>4</sub>-N) levels in stream water were studied in a small perennial Mediterranean catchment, Riera Major, in northeast Spain. NO<sub>3</sub>-N concentrations ranged from 0.15 to 1.9 mg l<sup>-1</sup>. Discharge explained 47% of the annual NO<sub>3</sub>-N concentration variance, but this percentage increased to 97% when single floods were analysed. The rate of change in nitrate concentration with respect to flow, ΔNO<sub>3</sub>-N/ΔQ, ranged widely from 0 to 20 μg NO<sub>3</sub>-N s l<sup>-2</sup>. The ΔNO<sub>3</sub>-N/ΔQ values fitted to a non linear model with respect to the storm flow magnitude (ΔQ) (r<sup>2</sup>=0.48, d.f.=22, P<0.01). High values of ΔNO<sub>3</sub>-N/ΔQ occurred at intermediate ΔQ values, whereas low ΔNO<sub>3</sub>-N/ΔQ values occurred during severe storms (ΔQ > 400 l s<sup>-1</sup>). N<sub>3</sub>-N concentrations exhibit anticlockwise hysteresis patterns with changing flow and the patterns observed for autumnal and winter storms indicated that groundwater was the main N<sub>3</sub>-N source for stream and hyporheic water. At baseflow, NO<sub>3</sub>-N concentration in groundwater was higher (t=4.75, d.f.=29, P>0.001) and co-varied with concentrations in the stream (r=0.91, d.f.=28, P<0.001). In contrast, NO<sub>3</sub>-N concentration in hyporheic water was identical to that in stream water. The role of the hyporheic zone as source or sink for ammonium was studied hyporheic was studied comparing its concentrations in stream and hyporheic zone before and after a major storm occurred in October 1994 that removed particulate organic matter stored in sediments. Results showed high ammonium concentrations (75±28 s.d. μg NH<sub>4</sub>-N l<sup>-1</sup>) before the storm flow in the hyporheic zone. After the storm, the ammonium concentration in the hyporheic dropped by 80% (13.6±8 μg N<sub>4</sub>-N l<sup>-1</sup>) and approached to the level found in stream water (11±8 μg NH<sub>4</sub>-N l<sup>-1</sup>) indicating that indisturbed hyporheic sediments act as a source for ammonium. After the storm, the ammonium concentrations in the stream, hyporheic and groundwater zones were very similar suggesting that stream ammonium concentrations are sustained mainly by input from groundwater. The present study provides evidence that storm flow magnitude is an important source of variability of nitrate concentration and fluxes in Mediterranean streams subjected to an irregular precipitation regime with prolonged dry periods.</p> <p style='line-height: 20px;'><b>Keywords: </b>nitrate, discharge regime, hyporheic zone, groundwater, Mediterranean, stream, Riera Major</p
- …