639 research outputs found
Fragments of ML Decidable by Nested Data Class Memory Automata
The call-by-value language RML may be viewed as a canonical restriction of
Standard ML to ground-type references, augmented by a "bad variable" construct
in the sense of Reynolds. We consider the fragment of (finitary) RML terms of
order at most 1 with free variables of order at most 2, and identify two
subfragments of this for which we show observational equivalence to be
decidable. The first subfragment consists of those terms in which the
P-pointers in the game semantic representation are determined by the underlying
sequence of moves. The second subfragment consists of terms in which the
O-pointers of moves corresponding to free variables in the game semantic
representation are determined by the underlying moves. These results are shown
using a reduction to a form of automata over data words in which the data
values have a tree-structure, reflecting the tree-structure of the threads in
the game semantic plays. In addition we show that observational equivalence is
undecidable at every third- or higher-order type, every second-order type which
takes at least two first-order arguments, and every second-order type (of arity
greater than one) that has a first-order argument which is not the final
argument
Does the Sun Shrink with Increasing Magnetic Activity?
We have analyzed the full set of SOHO/MDI f- and p-mode oscillation
frequencies from 1996 to date in a search for evidence of solar radius
evolution during the rising phase of the current activity cycle. Like Antia et
al. (2000), we find that a significant fraction of the f-mode frequency changes
scale with frequency; and that if these are interpreted in terms of a radius
change, it implies a shrinking sun. Our inferred rate of shrinkage is about 1.5
km/y, which is somewhat smaller than found by Antia et al. We argue that this
rate does not refer to the surface, but rather to a layer extending roughly
from 4 to 8 Mm beneath the visible surface. The rate of shrinking may be
accounted for by an increasing radial component of the rms random magnetic
field at a rate that depends on its radial distribution. If it were uniform,
the required field would be ~7 kG. However, if it were inwardly increasing,
then a 1 kG field at 8 Mm would suffice.
To assess contribution to the solar radius change arising above 4Mm, we
analyzed the p-mode data. The evolution of the p-mode frequencies may be
explained by a magnetic^M field growing with activity. The implications of the
near-surface magnetic field changes depend on the anisotropy of the random
magnetic field. If the field change is predominantly radial, then we infer an
additional shrinking at a rate between 1.1-1.3 km/y at the photosphere. If on
the other hand the increase is isotropic, we find a competing expansion at a
rate of 2.3 km/y. In any case, variations in the sun's radius in the activity
cycle are at the level of 10^{-5} or less, hence have a negligible contribution
to the irradiance variations.Comment: 10 pages (ApJ preprint style), 4 figures; accepted for publication in
Ap
High frequency and high wavenumber solar oscillations
We determine the frequencies of solar oscillations covering a wide range of
degree (100< l <4000) and frequency (1.5 <\nu<10 mHz) using the ring diagram
technique applied to power spectra obtained from MDI (Michelson Doppler Imager)
data. The f-mode ridge extends up to degree of approximately 3000, where the
line width becomes very large, implying a damping time which is comparable to
the time period. The frequencies of high degree f-modes are significantly
different from those given by the simple dispersion relation \omega^2=gk. The
f-mode peaks in power spectra are distinctly asymmetric and use of asymmetric
profile increases the fitted frequency bringing them closer to the frequencies
computed for a solar model.Comment: Revised version. 1.2 mHz features identified as artifacts of data
analysis. Accepted for publication in Ap
Minor constituents of human milk (I) identification of cyclohexaneundecanoic acid and phytanic acid in human milk fat by a combination gas chromatograph-mass spectrometer
Phase mixing of standing Alfven waves with shear flows in solar spicules
Alfvenic waves are thought to play an important role in coronal heating and
solar wind acceleration. Here we investigate the dissipation of such waves due
to phase mixing at the presence of shear flow and field in the stratified
atmosphere of solar spicules. The initial flow is assumed to be directed along
spicule axis and to vary linearly in the x direction and the equilibrium
magnetic field is taken 2-dimensional and divergence-free. It is determined
that the shear flow and field can fasten the damping of standing Alfven waves.
In spite of propagating Alfven waves, standing Alfven waves in Solar spicules
dissipate in a few periods. As height increases, the perturbed velocity
amplitude does increase in contrast to the behavior of perturbed magnetic
field. Moreover, it should be emphasized that the stratification due to
gravity, shear flow and field are the facts that should be considered in MHD
models in spicules.Comment: Accepted for publication in Astrophysics & Space Scienc
Accurate Determination of the Solar Photospheric Radius
The Solar Diameter Monitor measured the duration of solar meridian transits
during the 6 years 1981 to 1987, spanning the declining half of solar cycle 21.
We have combined these photoelectric measurements with models of the solar
limb-darkening function, deriving a mean value for the solar near-equatorial
radius of 695.508 +- .026 Mm. Annual averages of the radius are identical
within the measurement error of +- .037 Mm.Comment: 6 pages, 2 figures Submitted to Astrophys. J. Lett., March 9 1998
Uses AASTeX Macro package aas2pp
Collapsible Pushdown Parity Games
International audienceThis paper studies a large class of two-player perfect-information turn-based parity games on infinite graphs, namely those generated by collapsible pushdown automata. The main motivation for studying these games comes from the connections from collapsible pushdown automata and higher-order recursion schemes, both models being equi-expressive for generating infinite trees. Our main result is to establish the decidability of such games and to provide an effective representation of the winning region as well as of a winning strategy. Thus, the results obtained here provide all necessary tools for an in-depth study of logical properties of trees generated by collapsible pushdown automata/recursion schemes
The Potential of Current- and Wind-Driven Transport for Environmental Management of the Baltic Sea
The ever increasing impact of the marine industry and transport on vulnerable sea areas puts the marine environment under exceptional pressure and calls for inspired methods for mitigating the impact of the related risks. We describe a method for preventive reduction of remote environmental risks caused by the shipping and maritime industry that are transported by surface currents and wind impact to the coasts. This method is based on characterizing systematically the damaging potential of the offshore areas in terms of potential transport to vulnerable regions of an oil spill or other pollution that has occurred in a particular area. The resulting maps of probabilities of pollution to be transported to the nearshore and the time it takes for the pollution to reach the nearshore are used to design environmentally optimized fairways for the Gulf of Finland, Baltic Proper, and south-western Baltic Se
Why compare marine ecosystems?
This paper is not subject to U.S. copyright. The definitive version was published in ICES Journal of Marine Science: Journal du Conseil 67 (2010): 1-9, doi:10.1093/icesjms/fsp221.Effective marine ecosystem-based management (EBM) requires understanding the key processes and relationships controlling the aspects of biodiversity, productivity, and resilience to perturbations. Unfortunately, the scales, complexity, and non-linear dynamics that characterize marine ecosystems often confound managing for these properties. Nevertheless, scientifically derived decision-support tools (DSTs) are needed to account for impacts resulting from a variety of simultaneous human activities. Three possible methodologies for revealing mechanisms necessary to develop DSTs for EBM are: (i) controlled experimentation, (ii) iterative programmes of observation and modelling ("learning by doing"), and (iii) comparative ecosystem analysis. We have seen that controlled experiments are limited in capturing the complexity necessary to develop models of marine ecosystem dynamics with sufficient realism at appropriate scales. Iterative programmes of observation, model building, and assessment are useful for specific ecosystem issues but rarely lead to generally transferable products. Comparative ecosystem analyses may be the most effective, building on the first two by inferring ecosystem processes based on comparisons and contrasts of ecosystem response to human-induced factors. We propose a hierarchical system of ecosystem comparisons to include within-ecosystem comparisons (utilizing temporal and spatial changes in relation to human activities), within-ecosystem-type comparisons (e.g. coral reefs, temperate continental shelves, upwelling areas), and cross-ecosystem-type comparisons (e.g. coral reefs vs. boreal, terrestrial vs. marine ecosystems). Such a hierarchical comparative approach should lead to better understanding of the processes controlling biodiversity, productivity, and the resilience of marine ecosystems. In turn, better understanding of these processes will lead to the development of increasingly general laws, hypotheses, functional forms, governing equations, and broad interpretations of ecosystem responses to human activities, ultimately improving DSTs in support of EBM
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