4,339 research outputs found
Resource-driven Substructural Defeasible Logic
Linear Logic and Defeasible Logic have been adopted to formalise different
features relevant to agents: consumption of resources, and reasoning with
exceptions. We propose a framework to combine sub-structural features,
corresponding to the consumption of resources, with defeasibility aspects, and
we discuss the design choices for the framework
Ice melting and earthquake suppression in Greenland
It has been suggested that the Greenland ice sheet is the cause of earthquake
suppression in the region. With few exceptions, the observed seismicity
extends only along the continental margins of Greenland, which almost
coincide with the ice sheet margin. This pattern has been put forward as
further validation of the earthquake suppression hypothesis. In this review,
new evidence in terms of ice melting, post-glacial rebound and earthquake
occurrence is gathered and discussed to re-evaluate the connection between
ice mass unloading and earthquake suppression. In Greenland, the spatiotemporal
distribution of earthquakes indicates that seismicity is mainly con-
fined to regions where the thick layer of ice is absent and where significant ice
melting is presently occurring. A clear correlation between seismic activity
and ice melting in Greenland is not found. However, earthquake locations
and corresponding depth distributions suggest two distinct governing mechanisms:
post-glacial rebound promotes moderate-size crustal earthquakes at
Greenland’s regional scale, while current ice melting promotes shallow low
magnitude seismicity locall
Intermittent Sea Level Acceleration
Using instrumental observations from the Permanent Service for Mean Sea Level (PSMSL), we provide a new assessment of the global sea{level acceleration for the last 2 centuries (1820-2010). Our results, obtained
by a stack of tide gauge time series, con firm the existence of a global sea level acceleration (GSLA) and, coherently with independent assessments so far, they point to a value close to 0:01 mm/yr2. However, di fferently from
previous studies, we discuss how change points or abrupt inflections in individual sea level time series have contributed to the GSLA. Our analysis,
based on methods borrowed from econometrics, suggests the existence of
two distinct driving mechanisms for the GSLA, both involving a minority
of tide gauges globally. The first effectively implies a gradual increase in the rate of sea level rise at individual tide gauges, while the second is manifest
through a sequence of catastrophic variations of the sea level trend. These occurred intermittently since the end of the 19th century and became more frequent during the last four decades
Open Source Software and Technological Innovation: Competitive Issues
The new model of the products development and distribution represented by the open source movement has an important impact on the composition of the software market because it is a system alternative to the proprietary model. The antitrust analysis of the software market has to value, therefore, the strong effect of the open source development and distribution on the market contendibility and on the effectiveness to substitute the proprietary scheme
Sum of exit times in series of metastable states in probabilistic cellular automata
Reversible Probabilistic Cellular Automata are a special class
of automata whose stationary behavior is described by Gibbs--like
measures. For those models the dynamics can be trapped for a very
long time in states which are very different from the ones typical
of stationarity.
This phenomenon can be recasted in the framework of metastability
theory which is typical of Statistical Mechanics.
In this paper we consider a model presenting two not degenerate in
energy
metastable states which form a series, in the sense that,
when the dynamics is started at one of them, before reaching
stationarity, the system must necessarily visit the second one.
We discuss a rule for combining the exit times
from each of the metastable states
Anomalous secular sea-level acceleration in the Baltic Sea caused by isostatic adjustment
Observations from the global array of tide gauges show that global sealevel
has been rising at an average rate of 1.5-2 mm/yr during the last
~150 years [Douglas 1991, Spada and Galassi 2012]. Although a global
sea-level acceleration was initially ruled out [Douglas 1992], subsequent
studies [Douglas 1997, Church and White 2006, Jevrejeva et al.
2008, Church and White 2011] have coherently proposed values of ~1
mm/year/century [Olivieri and Spada 2013]. More complex non-linear
trends and abrupt sea-level variations have now also been recognized.
Globally, these could manifest a regime shift between the late Holocene
and the current rhythms of sea-level rise [Gehrels and Woodworth 2013],
while locally they result from ocean circulation anomalies, steric effects
and wind stress [Bromirski et al. 2011, Merrifield 2011]. Although isostatic
readjustment affects the local rates of secular sea-level change
[Milne and Mitrovica 1998, Peltier 2004], a possible impact on regional
acceleration has been so far discounted [Douglas 1992, Jevrejeva et al.
2008, Woodworth et al. 2009] since the process evolves on a millennium
time scale [Turcotte and Schubert 2002]. Here we report a previously unnoticed
anomaly in the long-term sea-level acceleration of the Baltic Sea
tide gauge records, and we explain it by the classical post-glacial rebound
theory and numerical modeling of glacial isostasy. Contrary to previous
assumptions, our findings demonstrate that isostatic compensation plays
a role in the regional secular sea-level acceleration
Empirical mode decomposition of long-term polar motion observation
We use the Empirical Mode Decomposition (EMD) method to study the decadal variations in polar motion and its long-term trend since year 1900. The existence of the so-called “Markowitz wobble”, a multidecadal fluctuation of the mean pole of rotation whose nature has long been debated since its discovery in 1960, is confirmed. In the EMD approach, the Markowitz wobble naturally arises as an empirical oscillatory term in polar motion, showing significant amplitude variations and a period of approximately 3 decades. The path of the time-averaged, non-cyclic component of polar motion matches the results of previous investigations based on classical spectral methods. However, our analysis also reveals previously unnoticed steep variations (change points) in the rate and the direction of secular polar motion
Metastability and small eigenvalues in Markov chains
In this letter we announce rigorous results that elucidate the relation
between metastable states and low-lying eigenvalues in Markov chains in a much
more general setting and with considerable greater precision as was so far
available. This includes a sharp uncertainty principle relating all low-lying
eigenvalues to mean times of metastable transitions, a relation between the
support of eigenfunctions and the attractor of a metastable state, and sharp
estimates on the convergence of probability distribution of the metastable
transition times to the exponential distribution.Comment: 5pp, AMSTe
ASSESSMENT OF KINETICS FOR BUTANOL PRODUCTION BY CLOSTRIDIUM ACETOBUTYLICUM
The economic scenario established at the beginning of the third millennium has revived the interest in Acetone-Butanol-Ethanol (ABE) fermentations. Recent developments in molecular techniques applied to solventogenic microrganisms in combination with recent advances in fermentation systems and downstream processing have contributed to improve ABE fermentation processes feasibility and competitiveness. The challenges raised over the last years as regards ABE production may be synthesized in: i) use of renewable resources as substrate; ii) selection of strains characterized by high ABE productivity; iii) development of new fermentation systems; iv) development of new downstream strategies for enhanced solvent recovery. The selection of unconventional substrates is favoured by the ability of clostridia strains to metabolize a wide range of carbohydrates like glucose, lactose, etc...., typically present in wastewater streams e.g. from food industries. Even though clostridia have been proven successful to produce ABE, information about kinetics of substrate conversion, cell growth and butanol production is still lacking. Studies available in literature most typically regard batch tests whose results do not apply easily to continuous processes. The strong interaction between the growth/acidogenesis phase and the solventogenesis phase should be taken into account.
The reactor systems investigated for the ABE fermentation belong to the batch and fed-batch typologies. Some attempts are reported in literature regarding continuous fermentation by means of clostridia strains confined in the reactor by immobilization or cell-recycling.
The present study reports the preliminary results of a research activity aiming at investigating the feasibility of the ABE production by Clostridium acetobutylicum ATCC824 in a continuous biofilm reactor adopting cheese whey as feedstock. The contribution regards the characterization of the kinetics related to the ABE production process by free C. acetobutylicum ATCC824 adopting as medium lactose solutions, in order to emulate the cheese whey. The conversion process is characterized in terms of cells, acids, solvents, pH, gas composition and total organic compounds as a function of time. Results are worked out to assess the kinetics of the cells growth and of the ABE production. The yields of the carbon source in cells, acids and solvents are also assessed. The investigation is carried out adopting both batch reactors and two continuous reactors. In particular, the continuous reactors are equipped to operate under controlled conversion regimes, acidogenesis or solventogenesis.
Tests carried out under batch conditions show that: i) cells growth follows the Monod kinetics for lactose concentration (CL) smaller than 100 g/L; ii) the butanol specific production rate increases linearly with CL; iii) the lactose conversion - measured at the end of the solventogenesis phase - decreases with CL; iv) the selectivity of butanol with respect to total solvents increases with CL and stabilizes at about 72%W for CL larger than 30 g/L. Preliminary tests carried out with the continuous reactor operated under solventogenesis regime show that steady state establishes with respect to cells and metabolites concentration at dilution rate of about 0.04 h-1
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