393 research outputs found
When Gravity Fails: Local Search Topology
Local search algorithms for combinatorial search problems frequently
encounter a sequence of states in which it is impossible to improve the value
of the objective function; moves through these regions, called plateau moves,
dominate the time spent in local search. We analyze and characterize plateaus
for three different classes of randomly generated Boolean Satisfiability
problems. We identify several interesting features of plateaus that impact the
performance of local search algorithms. We show that local minima tend to be
small but occasionally may be very large. We also show that local minima can be
escaped without unsatisfying a large number of clauses, but that systematically
searching for an escape route may be computationally expensive if the local
minimum is large. We show that plateaus with exits, called benches, tend to be
much larger than minima, and that some benches have very few exit states which
local search can use to escape. We show that the solutions (i.e., global
minima) of randomly generated problem instances form clusters, which behave
similarly to local minima. We revisit several enhancements of local search
algorithms and explain their performance in light of our results. Finally we
discuss strategies for creating the next generation of local search algorithms.Comment: See http://www.jair.org/ for any accompanying file
An updated assessment of past and future warming over France based on a regional observational constraint
Building on CMIP6 climate simulations, updated global and regional observations, and recently introduced statistical methods, we provide an updated assessment of past and future warming over France. Following the IPCC AR6 and recent global-scale studies, we combine model results with observations to constrain climate change at the regional scale. Over mainland France, the forced warming in 2020 with respect to 1900â1930 is assessed to be 1.66 [1.41 to 1.90]ââC, i.e., in the upper range of the CMIP6 estimates, and is almost entirely human-induced. A refined view of the seasonality of this past warming is provided through updated daily climate normals. Projected warming in response to an intermediate emission scenario is assessed to be 3.8ââC (2.9 to 4.8ââC) in 2100 and rises up to 6.7 [5.2 to 8.2]ââC in a very high emission scenario, i.e., substantially higher than in previous ensembles of global and regional simulations. Winter warming and summer warming are expected to be about 15â% lower than and 30â% higher than the annual mean warming, respectively, for all scenarios and time periods. This work highlights the importance of combining various lines of evidence, including model and observed data, to deliver the most reliable climate information. This refined regional assessment can feed adaptation planning for a range of activities and provides additional rationale for urgent climate action. Code is made available to facilitate replication over other areas or political entities.</p
A Halo Model with Environment Dependence: Theoretical Considerations
We present a modification of the standard halo model with the goal of
providing an improved description of galaxy clustering. Recent surveys, like
the Sloan Digital Sky Survey (SDSS) and the Anglo-Australian Two-degree survey
(2dF), have shown that there seems to be a correlation between the clustering
of galaxies and their properties such as metallicity and star formation rate,
which are believed to be environment-dependent. This environmental dependence
is not included in the standard halo model where the host halo mass is the only
variable specifying galaxy properties. In our approach, the halo properties
i.e., the concentration, and the Halo Occupation Distribution --HOD--
prescription, will not only depend on the halo mass (like in the standard halo
model) but also on the halo environment. We examine how different environmental
dependence of halo concentration and HOD prescription affect the correlation
function. We see that at the level of dark matter, the concentration of haloes
affects moderately the dark matter correlation function only at small scales.
However the galaxy correlation function is extremely sensitive to the HOD
details, even when only the HOD of a small fraction of haloes is modified.Comment: 23 pages, 17 figures. Accepted for publication in MNRA
Physical properties underlying observed kinematics of satellite galaxies
We study the kinematics of satellites around isolated galaxies selected from
the Sloan Digital Sky Survey (SDSS) spectroscopic catalog. Using a model of the
phase-space density previously measured for the halos of LCDM dark matter
cosmological simulations, we determine the properties of the halo mass
distribution and the orbital anisotropy of the satellites as a function of the
colour-based morphological type and the stellar mass of the central host
galaxy. We place constraints on the halo mass and the concentration parameter
of dark matter and the satellite number density profiles. We obtain a
concentration-mass relation for galactic dark matter haloes that is consistent
with predictions of a standard LCDM cosmological model. At given halo or
stellar mass, red galaxies have more concentrated halos than their blue
counterparts. The fraction of dark matter within a few effective radii is
minimal for 11.25<log M_star<11.5. The number density profile of the satellites
appears to be shallower than of dark matter, with the scale radius typically 60
per cent larger than of dark matter. The orbital anisotropy around red hosts
exhibits a mild excess of radial motions, in agreement with the typical
anisotropy profiles found in cosmological simulations, whereas blue galaxies
are found to be consistent with an isotropic velocity distribution. Our new
constraints on the halo masses of galaxies are used to provide analytic
approximations of the halo-to-stellar mass relation for red and blue galaxies.Comment: 12 pages, 11 figures; accepted for publication in MNRAS (updated
references
Future global climate: scenario-based projections and near-term information
This chapter assesses simulations of future global climate change, spanning time horizons from the near term (2021â2040), mid-term (2041â2060), and long term (2081â2100) out to the year 2300. Changes are assessed relative to both the recent past (1995â2014) and the 1850â1900 approximation to the pre-industrial period
On the van der Waerden numbers w(2;3,t)
We present results and conjectures on the van der Waerden numbers w(2;3,t)
and on the new palindromic van der Waerden numbers pdw(2;3,t). We have computed
the new number w(2;3,19) = 349, and we provide lower bounds for 20 <= t <= 39,
where for t <= 30 we conjecture these lower bounds to be exact. The lower
bounds for 24 <= t <= 30 refute the conjecture that w(2;3,t) <= t^2, and we
present an improved conjecture. We also investigate regularities in the good
partitions (certificates) to better understand the lower bounds.
Motivated by such reglarities, we introduce *palindromic van der Waerden
numbers* pdw(k; t_0,...,t_{k-1}), defined as ordinary van der Waerden numbers
w(k; t_0,...,t_{k-1}), however only allowing palindromic solutions (good
partitions), defined as reading the same from both ends. Different from the
situation for ordinary van der Waerden numbers, these "numbers" need actually
to be pairs of numbers. We compute pdw(2;3,t) for 3 <= t <= 27, and we provide
lower bounds, which we conjecture to be exact, for t <= 35.
All computations are based on SAT solving, and we discuss the various
relations between SAT solving and Ramsey theory. Especially we introduce a
novel (open-source) SAT solver, the tawSolver, which performs best on the SAT
instances studied here, and which is actually the original DLL-solver, but with
an efficient implementation and a modern heuristic typical for look-ahead
solvers (applying the theory developed in the SAT handbook article of the
second author).Comment: Second version 25 pages, updates of numerical data, improved
formulations, and extended discussions on SAT. Third version 42 pages, with
SAT solver data (especially for new SAT solver) and improved representation.
Fourth version 47 pages, with updates and added explanation
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