3,898 research outputs found
Linear solutions for cryptographic nonlinear sequence generators
This letter shows that linear Cellular Automata based on rules 90/150
generate all the solutions of linear difference equations with binary constant
coefficients. Some of these solutions are pseudo-random noise sequences with
application in cryptography: the sequences generated by the class of shrinking
generators. Consequently, this contribution show that shrinking generators do
not provide enough guarantees to be used for encryption purposes. Furthermore,
the linearization is achieved through a simple algorithm about which a full
description is provided
The influence of riparian-hyporheic zone on the hydrological responses in an intermittent stream
Stream and riparian groundwater hydrology has been studied in a small intermittent stream draining a forested catchment for a system representative of a Mediterranean climate. The relationship between precipitation and stream runoff and the interactions between stream water and the surrounding riparian groundwater have been analysed under a wide spectrum of meteorological conditions. The hypothesis that the hydrological condition of the near-stream groundwater compartment can regulate the runoff generation during precipitation events was tested. Stream runoff is characterised by a summer dry period, and precipitation input explained only 25% of runoff variability over the study period (r<sup>2</sup> =0.25, d.f.=51, p<0.001). The variability of precipitation v. stream runoff is explained partly by the hydrogeological properties of the riparian near-stream zone. This zone is characterised by high hydrological conductivity values and abrupt changes in groundwater level in summer. The summer dry period begins with a rapid decrease in near-stream groundwater level, and ends just after the first autumnal rain when the original groundwater level recovers suddenly. Within this period, storms do not cause major stream runoff since water infiltrates rapidly into the riparian compartment until it is refilled during the subsequent winter and spring; then the precipitation explains the 80% of the stream runoff variability (r<sup>2</sup>=0.80, d.f.=34, p<0.001). These results suggest that the hydrological interaction between the riparian groundwater compartment and the stream channel is important in elucidating the hydrological responses during drought periods in small Mediterranean streams.</p> <p style='line-height: 20px;'><b>Keywords:</b> riparian zone, groundwater hydrology, runoff, intermittent stream, Mediterranean climate</p
Global Linear Complexity Analysis of Filter Keystream Generators
An efficient algorithm for computing lower bounds on the global linear
complexity of nonlinearly filtered PN-sequences is presented. The technique
here developed is based exclusively on the realization of bit wise logic
operations, which makes it appropriate for both software simulation and
hardware implementation. The present algorithm can be applied to any arbitrary
nonlinear function with a unique term of maximum order. Thus, the extent of its
application for different types of filter generators is quite broad.
Furthermore, emphasis is on the large lower bounds obtained that confirm the
exponential growth of the global linear complexity for the class of nonlinearly
filtered sequences
Molecular evolution of aphids and their primary ( Buchnera sp.) and secondary endosymbionts: implications for the role of symbiosis in insect evolution.
Aphids maintain an obligate, endosymbiotic association with Buchnera sp., a bacterium closely related to Escherichia coli. Bacteria are housed in specialized cells of organ-like structures called bacteriomes in the hemocoel of the aphid and are maternally transmitted. Phylogenetic studies have shown that the association had a single origin, dated about 200-250 million years ago, and that host and endosymbiont lineages have evolved in parallel since then. However, the pattern of deepest branching within the aphid family remains unsolved, which thereby hampers tin appraisal of, for example, the role played by horizontal gene transfer in the early evolution of Buchnera. The main role of Buchnera in this association is the biosynthesis and provisioning of essential amino acids to its aphid host. Physiological and metabolic studies have recently substantiated such nutritional role. In addition, genetic studies of Buchnera from several aphids have shown additional modifications, such as strong genome reduction, high A+T content compared to free-living bacteria, differential evolutionary rates, a relative increase in the number of non-synonymous substitutions, and gene amplification mediated by plasmids. Symbiosis is an active process in insect evolution cis revealed by the intermediate values of the previous characteristics showed by secondary symbionts compared to free-living bacteria and Buchnera
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Modelling the inorganic nitrogen behaviour in a small Mediterranean forested catchment, Fuirosos (Catalonia)
The aim of this work was to couple a nitrogen (N) sub-model to already existent hydrological lumped (LU4-N) and semi-distributed (LU4-R-N and SD4-R-N) conceptual models, to improve our understanding of the factors and processes controlling nitrogen cycling and losses in Mediterranean catchments. The N model adopted provides a simplified conceptualization of the soil nitrogen cycle considering mineralization, nitrification, immobilization, denitrification, plant uptake, and ammonium adsorption/desorption. It also includes nitrification and denitrification in the shallow perched aquifer. We included a soil moisture threshold for all the considered soil biological processes. The results suggested that all the nitrogen processes were highly influenced by the rain episodes and that soil microbial processes occurred in pulses stimulated by soil moisture increasing after rain. Our simulation highlighted the riparian zone as a possible source of nitrate, especially after the summer drought period, but it can also act as an important sink of nitrate due to denitrification, in particular during the wettest period of the year. The riparian zone was a key element to simulate the catchment nitrate behaviour. The lumped LU4-N model (which does not include the riparian zone) could not be validated, while both the semi-distributed LU4-R-N and SD4-R-N model (which include the riparian zone) gave satisfactory results for the calibration process and acceptable results for the temporal validation process
Nitrogen concentrations in a small Mediterranean stream: 1. Nitrate 2. Ammonium
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
Modeling contact formation between atomic-sized gold tips via molecular dynamics
The formation and rupture of atomic-sized contacts is modelled by means of
molecular dynamics simulations. Such nano-contacts are realized in scanning
tunnelling microscope and mechanically controlled break junction experiments.
These instruments routinely measure the conductance across the nano-sized
electrodes as they are brought into contact and separated, permitting
conductance traces to be recorded that are plots of conductance versus the
distance between the electrodes. One interesting feature of the conductance
traces is that for some metals and geometric configurations a jump in the value
of the conductance is observed right before contact between the electrodes, a
phenomenon known as jump-to-contact. This paper considers, from a computational
point of view, the dynamics of contact between two gold nano-electrodes.
Repeated indentation of the two surfaces on each other is performed in two
crystallographic orientations of face-centred cubic gold, namely (001) and
(111). Ultimately, the intention is to identify the structures at the atomic
level at the moment of first contact between the surfaces, since the value of
the conductance is related to the minimum cross-section in the contact region.
Conductance values obtained in this way are determined using first principles
electronic transport calculations, with atomic configurations taken from the
molecular dynamics simulations serving as input structures.Comment: 6 pages, 4 figures, conference submissio
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