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A core operator of evolutionary algorithms (EAs) is the mutation. Recently, much attention has been devoted to the study of mutation operators with dynamic and non-uniform mutation rates. Following up on this area of work, we propose a new mutation operator and analyze its performance on the (1 + 1) Evolutionary Algorithm (EA). Our analyses show that this mutation operator competes with pre-existing ones, when used by the (1 + 1) EA on classes of problems for which results on the other mutation operators are available. We show that the (1 + 1) EA using our mutation operator finds a (1/3)-approximation ratio on any non-negative submodular function in polynomial time. We also consider the problem of maximizing a symmetric submodular function under a single matroid constraint and show that the (1 + 1) EA using our operator finds a (1/3)-approximation within polynomial time. This performance matches that of combinatorial local search algorithms specifically designed to solve these problems and outperforms them with constant probability. Finally, we evaluate the performance of the (1 + 1) EA using our operator experimentally by considering two applications: (a) the maximum directed cut problem on real-world graphs of different origins, with up to 6.6 million vertices and 56 million edges and (b) the symmetric mutual information problem using a four month period air pollution data set. In comparison with uniform mutation and a recently proposed dynamic scheme, our operator comes out on top on these instances.
Invertebrate herbivores are ubiquitous in most terrestrial ecosystems, and theory predicts that their impact on plant community biomass should depend on diversity and productivity of the associated plant communities. To elucidate general patterns in the relationship between invertebrate herbivory, plant diversity, and productivity, we carried out a long-term herbivore exclusion experiment at multiple grassland sites in a mountainous landscape of central Germany. Over a period of five years, we used above-and belowground insecticides as well as a molluscicide to manipulate invertebrate herbivory at 14 grassland sites, covering a wide range of plant species diversity (13-38 species/m(2)) and aboveground plant productivity (272-1125 g.m(-2).yr(-1)), where plant species richness and productivity of the sites were not significantly correlated. Herbivore exclusion had significant effects on the plant communities: it decreased plant species richness and evenness, and it altered plant community composition. In particular, exclusion of belowground herbivores promoted grasses at the expense of herbs. In contrast to our expectation, herbivore effects on plant community biomass were not influenced by productivity. However, effect size of invertebrate herbivores was negatively correlated with plant diversity of the grasslands: the effect of herbivory on biomass tended to be negative at sites of high diversity and positive at sites of low diversity. In general, the effects of aboveground herbivores were relatively small as compared to belowground herbivores, which were important drivers of plant community composition. Our study is the first to show that variation in the effects of invertebrate herbivory on plant communities across a landscape is significantly influenced by plant species richness.
Molecules often fragment after photoionization in the gas phase. Usually, this process can only be investigated spectroscopically as long as there exists electron correlation between the photofragments. Important parameters, like their kinetic energy after separation, cannot be investigated. We are reporting on a femtosecond time-resolved Auger electron spectroscopy study concerning the photofragmentation dynamics of thymine. We observe the appearance of clearly distinguishable signatures from thymine′s neutral photofragment isocyanic acid. Furthermore, we observe a time-dependent shift of its spectrum, which we can attribute to the influence of the charged fragment on the Auger electron. This allows us to map our time-dependent dataset onto the fragmentation coordinate. The time dependence of the shift supports efficient transformation of the excess energy gained from photoionization into kinetic energy of the fragments. Our method is broadly applicable to the investigation of photofragmentation processes.
Molecules often fragment after photoionization in the gas phase. Usually, this process can only be investigated spectroscopically as long as there exists electron correlation between the photofragments. Important parameters, like their kinetic energy after separation, cannot be investigated. We are reporting on a femtosecond time-resolved Auger electron spectroscopy study concerning the photofragmentation dynamics of thymine. We observe the appearance of clearly distinguishable signatures from thymine′s neutral photofragment isocyanic acid. Furthermore, we observe a time-dependent shift of its spectrum, which we can attribute to the influence of the charged fragment on the Auger electron. This allows us to map our time-dependent dataset onto the fragmentation coordinate. The time dependence of the shift supports efficient transformation of the excess energy gained from photoionization into kinetic energy of the fragments. Our method is broadly applicable to the investigation of photofragmentation processes.
Restoration of semi-natural grassland communities
involves a combination of (1) sward disturbance to
create a temporal window for establishment, and (2)
target species introduction, the latter usually by seed
sowing. With great regularity, particular species
establish only poorly. More reliable establishment
could improve outcome of restoration projects and
increase cost-effectiveness. We investigated the
abiotic germination niche of ten poorly establishing
calcareous grassland species by simultaneously
exploring the effects of moisture and light availability
and temperature fluctuation on percentage germina-
tion and speed of germination. We also investigated
the effects of three different pre-treatments used to
enhance seed germination – cold-stratification, osmo-
tic priming and priming in combination with gibberellic
acid (GA 3 ) – and how these affected abiotic
germination niches. Species varied markedly in width
of abiotic germination niche, ranging from Carex flacca
with very strict abiotic requirements, to several species
reliably germinating across the whole range of abiotic
conditions. Our results suggest pronounced differ-
ences between species in gap requirements for
establishment. Germination was improved in most
species by at least one pre-treatment. Evidence for
positive effects of adding GA 3 to seed priming
solutions was limited. In several species, pre-treated
seeds germinated under a wider range of abiotic
conditions than untreated seeds. Improved knowledge
of species-specific germination niches and the effects
of seed pre-treatments may help to improve species
establishment by sowing, and to identify species for
which sowing at a later stage of restoration or
introduction as small plants may represent a more
viable strategy.
Transparent conductive materials based on indium oxide remain yet irreplaceable in various optoelectronic applications. Amorphous oxides appear especially attractive for technology as they are isotropic, demonstrate relatively high electron mobility and can be processed at low temperatures. Among them is indium zinc oxide (IZO) with a large zinc content that is crucial for keeping the amorphous state but redundant for the doping. In this work we investigated water-free and water containing IZO films obtained by radio frequency sputtering. The correlation between temperature driven changes of the chemical state, the optical and electrical properties as well as the progression of crystallization was in focus. Such characterization methods as: scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, temperature dependent Hall-effect measurements and others were applied. Temperature dependent electrical properties of amorphous IZO and IZO:H2O films were found to evolve similarly. Based on our experience in In2O3:H2O (In2O3:H or IOH) we proposed an explanation for the changes observed. Water admixture was found to decrease crystallization temperature of IZO significantly from similar to 550 degrees C to similar to 280 degrees C. Herewith, the presence and concentration of water and/or hydroxyls was found to determine Zn distribution in the film. In particular, Zn enrichment was detected at the film's surface respective to the high water and/or hydroxyl amount. Raman spectra revealed a two-dimensional crystallization of w-ZnO which precedes regardless water presence an extensive In2O3 crystallization. An abrupt loss of electron mobility as a result of crystallization was attributed to the formation of ZnO interlayer on grain boundaries.
HESS observations of the binary system PSR B1259-63/LS 2883 around the 2010/2011 periastron passage
(2013)
Aims. We present very high energy (VHE; E > 100 GeV) data from the gamma-ray binary system PSR B1259-63/LS 2883 taken around its periastron passage on 15th of December 2010 with the High Energy Stereoscopic System (H. E. S. S.) of Cherenkov Telescopes. We aim to search for a possible TeV counterpart of the GeV flare detected by the Fermi LAT. In addition, we aim to study the current periastron passage in the context of previous observations taken at similar orbital phases, testing the repetitive behaviour of the source.
Methods. Observations at VHEs were conducted with H.E.S.S. from 9th to 16th of January 2011. The total dataset amounts to similar to 6 h of observing time. The data taken around the 2004 periastron passage were also re-analysed with the current analysis techniques in order to extend the energy spectrum above 3 TeV to fully compare observation results from 2004 and 2011.
Results. The source is detected in the 2011 data at a significance level of 11.5 sigma revealing an averaged integral flux above 1 TeV of (1.01 +/- 0.18(stat) +/- 0.20(sys)) x 10(-12) cm(-2) s(-1). The differential energy spectrum follows a power-law shape with a spectral index Gamma = 2.92 +/- 0.30(stat) +/- 0.20(sys) and a flux normalisation at 1 TeV of N-0 = (1.95 +/- 0.32(stat) +/- 0.39(sys)) x 10(-12) TeV-1 cm(-2) s(-1). The measured light curve does not show any evidence for variability of the source on the daily scale. The re-analysis of the 2004 data yields results compatible with the published ones. The differential energy spectrum measured up to similar to 10 TeV is consistent with a power law with a spectral index Gamma = 2.81 +/- 0.10(stat) +/- 0.20(sys) and a flux normalisation at 1 TeV of N-0 = (1.29 +/- 0.08(stat) +/- 0.26(sys)) x 10(-12) TeV-1 cm(-2) s(-1).
Conclusions. The measured integral flux and the spectral shape of the 2011 data are compatible with the results obtained around previous periastron passages. The absence of variability in the H.E.S.S. data indicates that the GeV flare observed by Fermi LAT in the time period covered also by H.E.S.S. observations originates in a different physical scenario than the TeV emission. Moreover, the comparison of the new results to the results from the 2004 observations made at a similar orbital phase provides a stronger evidence of the repetitive behaviour of the source.
The quasar PKS 1510-089 (z = 0.361) was observed with the H.E.S.S. array of imaging atmospheric Cherenkov telescopes during high states in the optical and GeV bands, to search for very high energy (VHE, defined as E >= 0.1 TeV) emission. VHE gamma-rays were detected with a statistical significance of 9.2 standard deviations in 15.8 h of H. E. S. S. data taken during March and April 2009. A VHE integral flux of I(0.15 TeV < E < 1.0TeV) = (1.0 +/- 0.2(stat) +/- 0.2(sys)) x 10(-11) cm(-2) s(-1) is measured. The best-fit power law to the VHE data has a photon index of G = 5.4 +/- 0.7(stat) +/- 0.3(sys). The GeV and optical light curves show pronounced variability during the period of H.E.S.S. observations. However, there is insufficient evidence to claim statistically significant variability in the VHE data. Because of its relatively high redshift, the VHE flux from PKS 1510-089 should suffer considerable attenuation in the intergalactic space due to the extragalactic background light (EBL). Hence, the measured gamma-ray spectrum is used to derive upper limits on the opacity due to EBL, which are found to be comparable with the previously derived limits from relatively-nearby BL Lac objects. Unlike typical VHE-detected blazars where the broadband spectrum is dominated by nonthermal radiation at all wavelengths, the quasar PKS 1510-089 has a bright thermal component in the optical to UV frequency band. Among all VHE detected blazars, PKS 1510-089 has the most luminous broad line region. The detection of VHE emission from this quasar indicates a low level of gamma - gamma absorption on the internal optical to UV photon field.