217 research outputs found
The PACE 2022 Parameterized Algorithms and Computational Experiments Challenge: Directed Feedback Vertex Set
The Parameterized Algorithms and Computational Experiments challenge (PACE) 2022 was devoted to engineer algorithms solving the NP-hard Directed Feedback Vertex Set (DFVS) problem. The DFVS problem is to find a minimum subset in a given directed graph such that, when all vertices of and their adjacent edges are deleted from , the remainder is acyclic.
Overall, the challenge had 90 participants from 26 teams, 12 countries, and 3 continents that submitted their implementations to this year’s competition. In this report, we briefly describe the setup of the challenge, the selection of benchmark instances, as well as the ranking of the participating teams. We also briefly outline the approaches used in the submitted solvers
Active Brownian particles with velocity-alignment and active fluctuations
We consider a model of active Brownian particles with velocity-alignment in
two spatial dimensions with passive and active fluctuations. Hereby, active
fluctuations refers to purely non-equilibrium stochastic forces correlated with
the heading of an individual active particle. In the simplest case studied
here, they are assumed as independent stochastic forces parallel (speed noise)
and perpendicular (angular noise) to the velocity of the particle. On the other
hand, passive fluctuations are defined by a noise vector independent of the
direction of motion of a particle, and may account for example for thermal
fluctuations.
We derive a macroscopic description of the active Brownian particle gas with
velocity-alignment interaction. Hereby, we start from the individual based
description in terms of stochastic differential equations (Langevin equations)
and derive equations of motion for the coarse grained kinetic variables
(density, velocity and temperature) via a moment expansion of the corresponding
probability density function.
We focus here in particular on the different impact of active and passive
fluctuations on the onset of collective motion and show how active fluctuations
in the active Brownian dynamics can change the phase-transition behaviour of
the system. In particular, we show that active angular fluctuation lead to an
earlier breakdown of collective motion and to emergence of a new bistable
regime in the mean-field case.Comment: 5 figures, 22 pages, submitted to New Journal of Physic
Naïve orangutans (Pongo abeliiand Pongo pygmaeus) individually acquire nut‐cracking using hammer tools
Nut-cracking with hammer tools (henceforth: nut-cracking) has been argued to be one of the most complex tool-use behaviors observed in nonhuman animals. So far, only chimpanzees, capuchins, and macaques have been observed using tools to crack nuts in the wild (Boesch and Boesch, 1990; Gumert et al., 2009; Mannu and Ottoni, 2009). However, the learning mechanisms behind this behavior, and the extent of nut-cracking in other primate species are still unknown. The aim of this study was two-fold. First, we investigated whether another great ape species would develop nut-cracking when provided with all the tools and appropriate conditions to do so. Second, we examined the mechanisms behind the emergence of nut-cracking by testing a naïve sample. Orangutans (Pongo abelii and Pongo pygmaeus) have the second most extensive tool-use repertoire among the great apes (after chimpanzees) and show flexible problem-solving capacities. Orangutans have not been observed cracking nuts in the wild, however, perhaps because their arboreal habits provide limited opportunities for nut-cracking. Therefore, orangutans are a valid candidate species for the investigation of the development of this behavior. Four nut-cracking-naïve orangutans at Leipzig zoo (P. abelii; Mage = 16; age range = 10–19; 4F; at the time of testing) were provided with nuts and hammers but were not demonstrated the nut-cracking behavioral form. Additionally, we report data from a previously unpublished study by one of the authors (Martina Funk) with eight orangutans housed at Zürich zoo (six P. abelii and two P. pygmaeus; Mage = 14; age range = 2–30; 5F; at the time of testing) that followed a similar testing paradigm. Out of the twelve orangutans tested, at least four individuals, one from Leipzig (P. abelii) and three from Zürich (P. abelii and P. pygmaeus), spontaneously expressed nut-cracking using wooden hammers. These results demonstrate that nut-cracking can emerge in orangutans through individual learning and certain types of non-copying social learning
Chemical and physical small-scale structure in a pre-stellar core
We present a comparative study of several molecular lines and of the dust
contiunuum at 1.2mm in a pre-stellar core that is embedded in the Galactic
cirrus cloud MCLD123.5+24.9. Previous studies found that the core is
gravitationally stable and shows signs of inward motion. Using the Owens Valley
(OVRO) and Plateau de Bure (PdB) interferometers we obtained high-angular
resolution maps of the core in the carbon monosulfide CS 2-1 and the
cyanoacetylene HC3N 10-9 transitions. Together with CS 5-4, C34S 3-2, and
bolometer data obtained with the IRAM 30m telescope, we analyse the excitation
conditions and the structural properties of the cloud. On the one hand, the new
CS 2-1 observations reveal significant substructure on a scale of about 7",
i.e., the beam size, corresponding to about 1050 AU at an adopted distance of
150pc. On the other hand, the interferometric observations in the HC3N 10-9
transition shows just one single well resolved clump in the inner part of the
core. This core is well described by an intensity profile following from a
centrally peaked volume density distribution. We find no evidence for depletion
of CS onto dust grains. The inward motion seen in the CS 2-1 occurs one-sided
from the middle of the filamentary cloud towards the HC3N core.Comment: 8 pages, 8 figures, accepted by A&
Spin correlations in spin blockade
We investigate spin currents and spin-current correlations for double quantum
dots in the spin blockade regime. By analysing the time evolution of the
density matrix, we obtain the spin resolved currents and derive from a
generating function an expression for the fluctuations and correlations. Both
the charge current and the spin current turn out to be generally
super-Poissonian. Moreover, we study the influence of ac fields acting upon the
transported electrons. In particular, we focus on fields that cause spin
rotation or photon-assisted tunnelling.Comment: 14 pages, 9 figure
Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem [Discussion paper]
We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O3) concentrations, except in polar regions where the model now underestimates O3 concentrations. Halogen chemistry reduces the global tropospheric O3 burden by 18.6 %, with the O3 lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28 × 106 molecules cm−3 are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH4 lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH4 by Cl is small (∼ 2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼ 15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde
Semiclassical description of multiphoton processes
We analyze strong field atomic dynamics semiclassically, based on a full
time-dependent description with the Hermann-Kluk propagator. From the
properties of the exact classical trajectories, in particular the accumulation
of action in time, the prominent features of above threshold ionization (ATI)
and higher harmonic generation (HHG) are proven to be interference phenomena.
They are reproduced quantitatively in the semiclassical approximation.
Moreover, the behavior of the action of the classical trajectories supports the
so called strong field approximation which has been devised and postulated for
strong field dynamics.Comment: 10 pages, 11 figure
- …