22,066 research outputs found
Coevolutionary Dynamics in a Minimal Substrate
One of the central difficulties of coevolutionary methods arises from 'intransitive superiority' - in a two-player game, for example, the fact that A beats B, and B beats C, does not exclude the possibility that C beats A. Such cyclic superiority in a coevolutionary substrate is hypothesized to cause cycles in the dynamics of the population such that it 'chases its own tail' - traveling through some part of strategy space more than once despite apparent improvement with each step. It is often difficult to know whether an application domain contains such difficulties and to verify this hypothesis in the failure of a given coevolutionary set-up. In this paper we wish to elucidate some of the issues and concepts in an abstract domain where the dynamics of coevolution can be studied simply and directly. We define three simple 'number games' that illustrate intransitive superiority and resultant oscillatory dynamics, as well as some other relevant concepts. These include the distinction between a player's perceived performance and performance with respect to an external metric, and the significance of strategies with a multi-dimensional nature. These features alone can also cause oscillatory behavior and coevolutionary failure
On the Plutinos and Twotinos of the Kuiper Belt
We illuminate dynamical properties of Kuiper Belt Objects (KBOs) in the 3:2
(``Plutino'') and 2:1 (``Twotino'') Neptunian resonances within the model of
resonant capture and migration. We analyze a series of numerical integrations,
each involving the 4 migratory giant planets and 400 test particles distributed
throughout trans-Neptunian space, to measure efficiencies of capture as
functions of migration speed. Snapshots of the spatial distribution of resonant
KBOs reveal that Twotinos cluster +/- 75 degrees away from Neptune's longitude,
while Plutinos cluster +/- 90 degrees away. Longitudinal clustering persists
even for surveys that are not volume-limited in their ability to detect
resonant KBOs. Remarkably, between -90 degrees and -60 degrees of Neptune's
longitude, we find the sky density of Twotinos to nearly equal that of
Plutinos, despite the greater average distance of Twotinos. We couple our
findings to observations to crudely estimate that the intrinsic Twotino
population is within a factor of 3 of the Plutino population. Most strikingly,
the migration model predicts that more Twotinos may lie at longitudes behind
that of Neptune than ahead of it. The magnitude of the asymmetry amplifies
dramatically with faster rates of migration and can be as large as 300%. A
differential measurement of the sky density of 2:1 resonant objects behind of
and in front of Neptune's longitude would powerfully constrain the migration
history of that planet.Comment: AJ, in press, to appear in December 2002 issue. For version with
higher resolution figures, see
http://astron.berkeley.edu/~echiang/ppp/ppp.htm
Mutualism, Parasitism, and Evolutionary Adaptation
Our investigations concern the role of symbiosis as an enabling mechanism in evolutionary adaptation. Previous work has illustrated how the formation of mutualist groups can guide genetic variation so as to enable the evolution of ultimately independent organisms that would otherwise be unobtainable. The new experiments reported here show that this effect applies not just in genetically related organisms but may also occur from symbiosis between distinct species. In addition, a new detail is revealed: when the symbiotic group members are drawn from two separate species only one of these species achieves eventual independence and the other remains parasitic. It is nonetheless the case that this second species, formerly mutualistic, was critical in enabling the independence of the first. We offer a biological example that is suggestive of the effect and discuss the implications for evolving complex organisms, natural and artificial
Modeling Building Block Interdependency
The Building-Block Hypothesis appeals to the notion of problem decomposition and the assembly of solutions from sub-solutions. Accordingly, there have been many varieties of GA test problems with a structure based on building-blocks. Many of these problems use deceptive fitness functions to model interdependency between the bits within a block. However, very few have any model of interdependency between building-blocks; those that do are not consistent in the type of interaction used intra-block and inter-block. This paper discusses the inadequacies of the various test problems in the literature and clarifies the concept of building-block interdependency. We formulate a principled model of hierarchical interdependency that can be applied through many levels in a consistent manner and introduce Hierarchical If-and-only-if (H-IFF) as a canonical example. We present some empirical results of GAs on H-IFF showing that if population diversity is maintained and linkage is tight then the GA is able to identify and manipulate building-blocks over many levels of assembly, as the Building-Block Hypothesis suggests
Migration as Adaptation? Exploring The Scope for Co-ordinating Environmental and Migration Policies in the European Union
Parity meter for charge qubits: an efficient quantum entangler
We propose a realization of a charge parity meter based on two double quantum
dots alongside a quantum point contact. Such a device is a specific example of
the general class of mesoscopic quadratic quantum measurement detectors
previously investigated by Mao et al. [Phys. Rev. Lett. 93, 056803 (2004)]. Our
setup accomplishes entangled state preparation by a current measurement alone,
and allows the qubits to be effectively decoupled by pinching off the parity
meter. Two applications of the parity meter are discussed: the measurement of
Bell's inequality in charge qubits and the realization of a controlled NOT
gate.Comment: 8 pages, 4 figures; v2: discussion of measurement time, references
adde
Dynamic Poisson Factorization
Models for recommender systems use latent factors to explain the preferences
and behaviors of users with respect to a set of items (e.g., movies, books,
academic papers). Typically, the latent factors are assumed to be static and,
given these factors, the observed preferences and behaviors of users are
assumed to be generated without order. These assumptions limit the explorative
and predictive capabilities of such models, since users' interests and item
popularity may evolve over time. To address this, we propose dPF, a dynamic
matrix factorization model based on the recent Poisson factorization model for
recommendations. dPF models the time evolving latent factors with a Kalman
filter and the actions with Poisson distributions. We derive a scalable
variational inference algorithm to infer the latent factors. Finally, we
demonstrate dPF on 10 years of user click data from arXiv.org, one of the
largest repository of scientific papers and a formidable source of information
about the behavior of scientists. Empirically we show performance improvement
over both static and, more recently proposed, dynamic recommendation models. We
also provide a thorough exploration of the inferred posteriors over the latent
variables.Comment: RecSys 201
Calibration of atmospheric hydrogen measurements
Interest in atmospheric hydrogen (H-2) has been growing in recent years with the prospect of H-2 being a potential alternative to fossil fuels as an energy carrier. This has intensified research for a quantitative understanding of the atmospheric hydrogen cycle and its total budget, including the expansion of the global atmospheric measurement network. However, inconsistencies in published observational data constitute a major limitation in exploring such data sets. The discrepancies can be mainly attributed to difficulties in the calibration of the measurements. In this study various factors that may interfere with accurate quantification of atmospheric H-2 were investigated including drifts of standard gases in high pressure cylinders. As an experimental basis a procedure to generate precise mixtures of H-2 within the atmospheric concentration range was established. Application of this method has enabled a thorough linearity characterization of the commonly used GC-HgO reduction detector. We discovered that the detector response was sensitive to the composition of the matrix gas. Addressing these systematic errors, a new calibration scale has been generated defined by thirteen standards with dry air mole fractions ranging from 139-1226 nmol mol(-1). This new scale has been accepted as the official World Meteorological Organisation's (WMO) Global Atmospheric Watch (GAW) H-2 mole fraction scale
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