•This paper proposes an improved grey wolf optimization (IGWO) for optimizing KELM.•A new hierarchical mechanism was established in the proposed IGWO.•Effectiveness of IGWO strategy is validated on ...functions and three practical applications.•Experimental results reveal the improved performance of the proposed algorithm.
Since its introduction, kernel extreme learning machine (KELM) has been widely used in a number of areas. The parameters in the model have an important influence on the performance of KELM. Therefore, model parameters must be properly adjusted before they can be put into practical use. This study proposes a new parameter learning strategy based on an improved grey wolf optimization (IGWO) strategy, in which a new hierarchical mechanism was established to improve the stochastic behavior, and exploration capability of grey wolves. In the proposed mechanism, random local search around the optimal grey wolf was introduced in Beta grey wolves, and random global search was introduced in Omega grey wolves. The effectiveness of IGWO strategy is first validated on 10 commonly used benchmark functions. Results have shown that the proposed IGWO can find good balance between exploration and exploitation. In addition, when IGWO is applied to solve the parameter adjustment problem of KELM model, it also provides better performance than other seven meta-heuristic algorithms in three practical applications, including students’ second major selection, thyroid cancer diagnosis and financial stress prediction. Therefore, the method proposed in this paper can serve as a good candidate tool for tuning the parameters of KELM, thus enabling the KELM model to achieve more promising results in practical applications.
ABSTRACT
We present a suite of the first 3D GRMHD collapsar simulations, which extend from the self-consistent jet launching by an accreting Kerr black hole (BH) to the breakout from the star. We ...identify three types of outflows, depending on the angular momentum, l, of the collapsing material and the magnetic field, B, on the BH horizon: (i) subrelativistic outflow (low l and high B), (ii) stationary accretion shock instability (SASI; high l and low B), (iii) relativistic jets (high l and high B). In the absence of jets, free-fall of the stellar envelope provides a good estimate for the BH accretion rate. Jets can substantially suppress the accretion rate, and their duration can be limited by the magnetization profile in the star. We find that progenitors with large (steep) inner density power-law indices (≳ 2), face extreme challenges as gamma-ray burst (GRB) progenitors due to excessive luminosity, global time evolution in the light curve throughout the burst and short breakout times, inconsistent with observations. Our results suggest that the wide variety of observed explosion appearances (supernova/supernova + GRB/low-luminosity GRBs) and the characteristics of the emitting relativistic outflows (luminosity and duration) can be naturally explained by the differences in the progenitor structure. Our simulations reveal several important jet features: (i) strong magnetic dissipation inside the star, resulting in weakly magnetized jets by breakout that may have significant photospheric emission and (ii) spontaneous emergence of tilted accretion disc-jet flows, even in the absence of any tilt in the progenitor.
Based on genotype-phenotype correlation analysis of 80 Wolf-Hirschhorn syndrome (WHS) patients, as well as on review of relevant literature, we add further insights to the following aspects of WHS: ...(1) clinical delineation and phenotypic categories; (2) characterization of the basic genomic defect, mechanisms of origin and familiarity; (3) identification of prognostic factors for mental retardation; (4) chromosome mapping of the distinctive clinical signs, in an effort to identify pathogenic genes. Clinically, we consider that minimal diagnostic criteria for WHS, defining a "core" phenotype, are typical facial appearance, mental retardation, growth delay and seizures (or EEG anomalies). Three different categories of the WHS phenotype were defined, generally correlating with the extent of the 4p deletion. The first one comprises a small deletion not exceeding 3.5 Mb, that is usually associated with a mild phenotype, lacking major malformations. This category is likely under-diagnosed. The second and by far the more frequent category is identified by large deletions, averaging between 5 and 18 Mb, and causes the widely recognizable WHS phenotype. The third clinical category results from a very large deletion exceeding 22-25 Mb causing a severe phenotype, that can hardly be defined as typical WHS. Genetically, de novo chromosome abnormalities in WHS include pure deletions but also complex rearrangements, mainly unbalanced translocations. With the exception of t(4p;8p), WHS-associated chromosome abnormalities are neither mediated by segmental duplications, nor associated with a parental inversion polymorphism on 4p16.3. Factors involved in prediction of prognosis include the extent of the deletion, the occurrence of complex chromosome anomalies, and the severity of seizures. We found that the core phenotype maps within the terminal 1.9 Mb region of chromosome 4p. Therefore, WHSCR-2 should be considered the critical region for this condition. We also confirmed that the pathogenesis of WHS is multigenic. Specific and independent chromosome regions were characterized for growth delay and seizures, as well as for the additional clinical signs that characterize this condition. With the exception of parental balanced translocations, familial recurrence is uncommon.
Context. Various binary black hole formation channels have been proposed since the first gravitational event GW150914 was discovered by the Advanced Laser Interferometer Gravitational-Wave ...Observatory (AdLIGO). The immediate progenitor of the binary black hole is a close binary system composed of a black hole and a helium star, which can be the outcome of the classical isolated binary evolution through the common envelope, or alternatively of the massive close evolution through chemically homogeneous channel. Aims. We study the spin angular momentum evolution of the helium star in order to constrain the spin of the second-born black hole. This work focuses on the common envelope formation channel, however, some of our conclusions are also relevant for the chemically homogeneous evolution channel. Methods. We perform detailed stellar structure and binary evolution calculations that take into account, mass-loss, internal differential rotation, and tidal interactions between the helium star and the black hole companion, where we also calculate the strength of the tidal interactions from first principles based on the structure of the helium stars. We systematically explore the parameter space of initial binary properties, including initial black hole and helium star masses, initial rotation of the helium star as well as metallicity. Results. We argue that the natal spin of the first-born black hole through the common envelope scenario is negligible (≲0.1), and therefore the second-born black hole’s spin dominates the measured effective spin, χeff, from gravitational wave events of double black hole mergers. We find that tides can be only important when orbital periods are shorter than 2 days. Upon core collapse, the helium star produces a black hole (the second-born black hole in the system) with a spin that can span the entire range from zero to maximally spinning. We show that the bimodal distribution of the spin of the second-born black hole obtained in recent papers is mainly due to oversimplifying assumptions. We find an anti-correlation between the merging timescale of the two black holes, Tmerger, and the effective spin χeff. Finally, we provide new prescriptions for the tidal coefficient E2 for both H-rich and the He-rich stars. Conclusions. To understand the spin of the second-born black hole, careful treatment of both tides and stellar winds is needed. We predict that, with future improvements to AdLIGO’s sensitivity, the sample of merging binary black hole systems will show an overdensity of sources with positive but small χeff originating from lower-mass black hole mergers born at low redshift.
To overcome the limitation of single search strategy of grey wolf optimizer (GWO) in solving various function optimization problems, we propose a multi-strategy ensemble GWO (MEGWO) in this paper. ...The proposed MEGWO incorporates three different search strategies to update the solutions. Firstly, the enhanced global-best lead strategy can improve the local search ability of GWO by fully exploiting the search space around the current best solution. Secondly, the adaptable cooperative strategy embeds one-dimensional update operation into the framework of GWO to provide a higher population diversity and promote the global search ability. Thirdly, the disperse foraging strategy forces a part of search agents to explore a promising area based on a self-adjusting parameter, which contributes to the balance between the exploitation and exploration. We conducted numerical experiments based on various functions form CEC2014. The obtained results are compared with other three modified GWO and seven state-of-the-art algorithms. Furthermore, feature selection is employed to investigate the effectiveness of MEGWO on real-world applications. The experimental results show that the proposed algorithm which integrate multiple improved search strategies, outperforms other variants of GWO and other algorithms in terms of accuracy and convergence speed. It is validated that MEGWO is an efficient and reliable algorithm not only for optimization of functions with different characteristics but also for real-world optimization problems.
•A multi-strategy ensemble GWO is proposed to boost the precision and efficiency of the original GWO.•A parameter self-adjusting strategy is utilized to balance the exploitation and exploration of the proposed MEGWO.•Wilcoxons signed-rank test and performance profile are used to investigate the significance of the MEGWO.•Feature selection is employed to evaluate the effectiveness of MEGWO on real-world applications.
Aims. We aim to determine the secular evolution of the orbital period of the short-period binary system WR 127 (WN3b+O9.5V, P ≈ 9.555 d ). Methods. We performed new low-resolution spectroscopic ...observations of WR 127 with the 2.5 m CMO SAI telescope to construct the radial velocity curves of the components. Our results suggest component masses of M WR sin 3 ( i ) = 11.8 ± 1.4 M ⊙ and M O sin 3 ( i ) = 17.2 ± 1.4 M ⊙ . By comparing these values with archival radial velocity curves we were able to create an ( O − C ) plot with an accuracy sufficient to search for the orbital period change in WR 127. Results. We report the reliable detection of a secular increase in the orbital period of WR 127 at a rate of Ṗ = 0.83 ± 0.14 s yr −1 , which corresponds to a dynamical mass-loss rate from the Wolf-Rayet (WR) star of Ė WR = (2.6 ± 0.5) × 10 −5 M ⊙ yr −1 . Conclusions. The mass-loss rate from WR stars in three Wolf-Rayet+OB binaries (WR 127, CX Cep, and V444 Cyg) as inferred from spectroscopic and photometric measurements suggests a preliminary empirical correlation between a WR star’s mass and its dynamical mass-loss rate of Ṁ WR ∼ M WR 1.8 . This relation is important for the understanding of the evolution of massive close binaries that include WR stars as such an evolution is a precursor of gravitational-wave binary merging events with neutron stars and black holes.
Wind power has demonstrated high-efficiency utilization in electricity system, accordingly, accurate and stable forecasting of wind speed is of vital significance in power grid security management ...and market economics. However, most former studies only consider either the accuracy or stability, with difficulty achieving the two targets simultaneously, which is insufficient for an effective forecasting method. This paper proposes a novel hybrid forecasting system that includes an effective data decomposition technique, a multi-objective optimization algorithm, a forecasting algorithm, and a set of comprehensive evaluation methods. In this system, the complete ensemble empirical mode decomposition (CEEMD) divides the original wind speed sequence into a set of intrinsic mode functions and then extreme learning machine (ELM) optimized by the multi-objective grey wolf optimization (MOGWO) is applied to achieve excellent forecasting performance. To validate the forecasting performance of the developed forecasting system, wind speed data at 10-min interval collected from Shandong Peninsula, China is considered as case study and comprehensive evaluations are introduced. The results demonstrate that the proposed hybrid system transcends the other compared single and traditional models and simultaneously realizes high accuracy and strong stability. Thus, the proposed CEEMD-MOGWO-ELM system can be effectively and satisfactorily used for smart-grid operation and management.
•A hybrid system with multi-objective optimization is proposed to forecast wind speed.•The developed hybrid system can achieve accurate and stable forecasting concurrently.•Comprehensive valuation methods are used to effectively measure the proposed system.•The proposed hybrid model outperforms the benchmark models considered in this study.
•Gray wolf optimizer (GWO) is employed in solving the optimal reactive power dispatch (ORPD) problems.•Three case studies have been utilized to show the effectiveness of GWO.•GWO able to find minimum ...loss and voltage deviation solution than those determined by other techniques.
This paper presents the use of a new meta-heuristic technique namely gray wolf optimizer (GWO) which is inspired from gray wolves’ leadership and hunting behaviors to solve optimal reactive power dispatch (ORPD) problem. ORPD problem is a well-known nonlinear optimization problem in power system. GWO is utilized to find the best combination of control variables such as generator voltages, tap changing transformers’ ratios as well as the amount of reactive compensation devices so that the loss and voltage deviation minimizations can be achieved. In this paper, two case studies of IEEE 30-bus system and IEEE 118-bus system are used to show the effectiveness of GWO technique compared to other techniques available in literature. The results of this research show that GWO is able to achieve less power loss and voltage deviation than those determined by other techniques.
In this study, a multi-strategy driven reinforced hierarchical operator for a grey wolf optimizer (RHGWO) is proposed to solve the feature selection (FS) problem, whereby tedious data are converted ...into information and often modeled as a combinatorial optimization problem. First, a multi-strategy mechanism is proposed to provide the GWO algorithm with exploration capabilities, including memory-based diversity and Lévy flight-based extension search. Next, a hierarchical segmentation technique is proposed to allocate exploration and exploitation, thereby providing exploration capability for superior wolves to search diverse regions and exploitation capability for inferior wolves to converge to the promising area. Subsequently, a chaotic elite learning strategy is designed for leaders to prevent misdirection. Finally, a more rational nonlinear parameter transformation is designed. Multiple experiments validate the adaptability and versatility of the proposed RHGWO algorithm.
Context.
Classical Wolf-Rayet (WR) stars are massive, hydrogen-depleted, post main-sequence stars that exhibit emission-line dominated spectra. For a given metallicity
Z
, stars exceeding a certain ...initial mass
M
single
WR
(Z) can reach the WR phase through intrinsic mass-loss or eruptions (single-star channel). In principle, stars of lower masses can reach the WR phase via stripping through binary interactions (binary channel). Because winds become weaker at low
Z
, it is commonly assumed that the binary channel dominates the formation of WR stars in environments with low metallicity such as the Small and Large Magellanic Clouds (SMC, LMC). However, the reported WR binary fractions of 30−40% in the SMC (
Z
= 0.002) and LMC (
Z
= 0.006) are comparable to that of the Galaxy (
Z
= 0.014), and no evidence for the dominance of the binary channel at low
Z
could be identified observationally. Here, we explain this apparent contradiction by considering the minimum initial mass
M
spec
WR
(Z) needed for the stripped product to appear as a WR star.
Aims.
By constraining
M
spec
WR
(Z) and
M
single
WR
(Z), we estimate the importance of binaries in forming WR stars as a function of
Z
.
Methods.
We calibrated
M
spec
WR
using the lowest-luminosity WR stars in the Magellanic Clouds and the Galaxy. A range of
M
single
WR
values were explored using various evolution codes. We estimated the additional contribution of the binary channel by considering the interval
M
spec
WR
(Z),
M
single
WR
(Z), which characterizes the initial-mass range in which the binary channel can form additional WR stars.
Results.
The WR-phenomenon ceases below luminosities of log
L
≈ 4.9, 5.25, and 5.6
L
⊙
in the Galaxy, the LMC, and the SMC, respectively, which translates to minimum He-star masses of 7.5, 11, 17
M
⊙
and minimum initial masses of
M
spec
WR
= 18, 23, 37
M
⊙
. Stripped stars with lower initial masses in the respective galaxies would tend not to appear as WR stars. The minimum mass necessary for self-stripping,
M
single
WR
(Z), is strongly model-dependent, but it lies in the range 20−30, 30−60, and ≳40
M
⊙
for the Galaxy, LMC, and SMC, respectively. We find that that the additional contribution of the binary channel is a non-trivial and model-dependent function of
Z
that cannot be conclusively claimed to be monotonically increasing with decreasing
Z
.
Conclusions.
The WR spectral appearance arises from the presence of strong winds. Therefore, both
M
spec
WR
and
M
single
WR
increase with decreasing metallicity. Considering this, we show that one should not a-priori expect that binary interactions become increasingly important in forming WR stars at low
Z
, or that the WR binary fraction grows with decreasing
Z
.
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