16,245 research outputs found
Deterministically driven random walks in a random environment on Z
We introduce the concept of a deterministic walk in a deterministic
environment on a countable state space (DWDE). For the deterministic walk in a
fixed environment we establish properties analogous to those found in Markov
chain theory, but for systems that do not in general have the Markov property.
In particular, we establish hypotheses ensuring that a DWDE on is either
recurrent or transient. An immediate consequence of this result is that a
symmetric DWDE on is recurrent. Moreover, in the transient case, we show
that the probability that the DWDE diverges to is either 0 or 1. In
certain cases we compute the direction of divergence in the transient case
The Effect of Fishermen's Quotas on Expected Catch Rates
Fishermen's quotas have the effect of truncating catches at the quota limit. Hence the expected catch is smaller than the quota. A simple search model is developed that provides an estimation of the factor by which expected catches are reduced.Environmental Economics and Policy, Research Methods/ Statistical Methods, Resource /Energy Economics and Policy, Risk and Uncertainty,
A Dual Digraph Approach for Leaderless Atomic Broadcast (Extended Version)
Many distributed systems work on a common shared state; in such systems,
distributed agreement is necessary for consistency. With an increasing number
of servers, these systems become more susceptible to single-server failures,
increasing the relevance of fault-tolerance. Atomic broadcast enables
fault-tolerant distributed agreement, yet it is costly to solve. Most practical
algorithms entail linear work per broadcast message. AllConcur -- a leaderless
approach -- reduces the work, by connecting the servers via a sparse resilient
overlay network; yet, this resiliency entails redundancy, limiting the
reduction of work. In this paper, we propose AllConcur+, an atomic broadcast
algorithm that lifts this limitation: During intervals with no failures, it
achieves minimal work by using a redundancy-free overlay network. When failures
do occur, it automatically recovers by switching to a resilient overlay
network. In our performance evaluation of non-failure scenarios, AllConcur+
achieves comparable throughput to AllGather -- a non-fault-tolerant distributed
agreement algorithm -- and outperforms AllConcur, LCR and Libpaxos both in
terms of throughput and latency. Furthermore, our evaluation of failure
scenarios shows that AllConcur+'s expected performance is robust with regard to
occasional failures. Thus, for realistic use cases, leveraging redundancy-free
distributed agreement during intervals with no failures improves performance
significantly.Comment: Overview: 24 pages, 6 sections, 3 appendices, 8 figures, 3 tables.
Modifications from previous version: extended the evaluation of AllConcur+
with a simulation of a multiple datacenters deploymen
A secular increase in continental crust nitrogen during the Precambrian
Recent work indicates the presence of substantial geologic nitrogen
reservoirs in the mantle and continental crust. Importantly, this geologic
nitrogen has exchanged between the atmosphere and the solid Earth over time.
Changes in atmospheric nitrogen (i.e. atmospheric mass) have direct effects on
climate and biological productivity. It is difficult to constrain, however, the
evolution of the major nitrogen reservoirs through time. Here we show a secular
increase in continental crust nitrogen through Earth history recorded in
glacial tills (2.9 Ga to modern), which act as a proxy for average upper
continental crust composition. Archean and earliest Palaeoproterozoic tills
contain 66 100 ppm nitrogen, whereas Neoproterozoic and Phanerozoic tills
contain 290 165 ppm nitrogen, whilst the isotopic composition has
remained constant at ~4\permil. Nitrogen has accumulated in the continental
crust through time, likely sequestered from the atmosphere via biological
fixation. Our findings support dynamic, non-steady state behaviour of nitrogen
through time, and are consistent with net transfer of atmospheric N to geologic
reservoirs over time.Comment: 14 pages, 2 figures, 2 tables, supplemental informatio
The Predictive Utility of Generalized Expected Utility Theories
Many alternative theories have been proposed to explain violations of expected utility
(EU) theory observed in experiments. Several recent studies test some of these alternative
theories against each other. Formal tests used to judge the theories usually count the
number of responses consistent with the theory, ignoring systematic variation in responses
that are inconsistent. We develop a maximum-likelihood estimation method which uses
all the information in the data, creates test statistics that can be aggregated across studies,
and enables one to judge the predictive utility-the fit and parsimony-of utility theories.
Analyses of 23 data sets, using several thousand choices, suggest a menu of theories which
sacrifice the least parsimony for the biggest improvement in fit. The menu is: mixed
fanning, prospect theory, EU, and expected value. Which theories are best is highly
sensitive to whether gambles in a pair have the same support (EU fits better) or not (EU
fits poorly). Our method may have application to other domains in which various theories
predict different subsets of choices (e.g., refinements of Nash equilibrium in noncooperative
games)
Small-body encounters using solar sail propulsion
Cometary Rendezvous and Flybys have large V requirements, which impose almost unattainable, and sometimes prohibitive, demands on the propellant budget of conventional, chemical propulsion. Ion Propulsion is a viable alternative, but as the number and difficulty of target objectives increases then the potential of this technology becomes rapidly less attractive. Solar sails exhibit an extremely high effective specific impulse over long mission durations. No propellant is required so that large changes in V could be realised without necessitating the introduction of complex gravity assists, which prolong mission duration and restrict launch opportunities. The endurance of the structures and materials are thus the only limiting factors dictating the number and range of bodies with which the solar-sail propelled vehicle can encounter throughout its lifetime. In this paper we have analysed a number of high-energy, small-body mission scenarios using a parameterised approach to sail control representation. The sail cone and clock angle histories were characterised by linear interpolation across a set of discrete nodes. The optimal control problem was thus transcribed to a Non-Linear Programming problem to select the optimal controls at the nodes that minimised the transfer time while enforcing the cartesian end-point boundary constraints (6 states for rendezvous, 3 for flypast). The Fortran77 optimisation package NPSOL 5.0 was used for this purpose with the variational equations of motion formulated in modified equinoctial orbital elements and integrated using a variable-order, adaptive step-size Adams-Moulton-Bashforth method. We present optimal rendezvous trajectories to Short-Period Comets such as 46P/Wirtanen in 484 days with a sail characteristic acceleration of 1.9 mms-2, and with 2P/Encke in 574 days with a characteristic acceleration of 1.0 mms-2. An analysis using high-performance sails has been conducted to permit fast flyby intercepts of newly discovered Long Period Comets (LPCs). Previous examples adopted were C/1995 O1/Hale- Bopp, C/1995 Y1/Hyakutake, C/1999 T1/McNaught-Hartley, C/1999 F1/Catalina, C/1999 N2/Lynn and C/1999 H1/Lee, to demonstrate the feasibility of a late launch to quickly intercept a new LPC using a solar sail. Since the time between discovery of a new LPC such as Hale-Bopp and perihelion passage was less then 2 years, this then leaves a very short time-span for orbit determination, preparation, planning and operational phases. Preliminary mission analysis shows that a Hale-Bopp perihelion flypast could have been achieved, with a sail characteristic acceleration of 5.0 mms-2, by launching just 209 days before comet perihelion passage. With a characteristic acceleration of 2.0 mms-2 Hale-Bopp could also have been intercepted at its descending node by launching 270 days before nodal descent. The sail could then have returned to rendezvous with the Earth 261 days later, giving a minimum total mission turn-around time of 531 days. An alternative, dual flyby scenario has been investigated, to continue on to C/1997 D1/Mueller, after which solar system escape was reached and arrival at Heliopause would occur in 12 years. Solar Electric Propulsion has been adopted as the primary propulsion system for the DAWN dual asteroid rendezvous mission scheduled for launch in 2006. The objective of this mission is to rendezvous with inner main-belt asteroids, Vesta and Ceres. We have also investigated solar sail adaptation to this mission, for the same launch date and 11 month orbiter stay-times. We have extended the mission objectives to two further asteroids, Lucina and Lutetia, with the aim of demonstrating a Mainbelt Asteroid Survey scenario
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