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Auteur : Grafiati
Publié le 11 mars 2023

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1

McNeish, Daniel, et Laura M. Stapleton. « Modeling Clustered Data with Very Few Clusters ». Multivariate Behavioral Research 51, no 4 (7 juin 2016) : 495–518. http://dx.doi.org/10.1080/00273171.2016.1167008.

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2

Niccodemi, Gianmaria, et Tom Wansbeek. « A New Estimator for Standard Errors with Few Unbalanced Clusters ». Econometrics 10, no 1 (21 janvier 2022) : 6. http://dx.doi.org/10.3390/econometrics10010006.

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In linear regression analysis, the estimator of the variance of the estimator of the regression coefficients should take into account the clustered nature of the data, if present, since using the standard textbook formula will in that case lead to a severe downward bias in the standard errors. This idea of a cluster-robust variance estimator (CRVE) generalizes to clusters the classical heteroskedasticity-robust estimator. Its justification is asymptotic in the number of clusters. Although an improvement, a considerable bias could remain when the number of clusters is low, the more so when regressors are correlated within cluster. In order to address these issues, two improved methods were proposed; one method, which we call CR2VE, was based on biased reduced linearization, while the other, CR3VE, can be seen as a jackknife estimator. The latter is unbiased under very strict conditions, in particular equal cluster size. To relax this condition, we introduce in this paper CR3VE-λ, a generalization of CR3VE where the cluster size is allowed to vary freely between clusters. We illustrate the performance of CR3VE-λ through simulations and we show that, especially when cluster sizes vary widely, it can outperform the other commonly used estimators.
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Ibragimov, Rustam, et Ulrich K. Müller. « Inference with Few Heterogeneous Clusters ». Review of Economics and Statistics 98, no 1 (mars 2016) : 83–96. http://dx.doi.org/10.1162/rest_a_00545.

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Frontera, Jennifer A., Lorna E. Thorpe, Naomi M. Simon, Adam de Havenon, Shadi Yaghi, Sakinah B. Sabadia, Dixon Yang et al. « Post-acute sequelae of COVID-19 symptom phenotypes and therapeutic strategies : A prospective, observational study ». PLOS ONE 17, no 9 (29 septembre 2022) : e0275274. http://dx.doi.org/10.1371/journal.pone.0275274.

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Background Post-acute sequelae of COVID-19 (PASC) includes a heterogeneous group of patients with variable symptomatology, who may respond to different therapeutic interventions. Identifying phenotypes of PASC and therapeutic strategies for different subgroups would be a major step forward in management. Methods In a prospective cohort study of patients hospitalized with COVID-19, 12-month symptoms and quantitative outcome metrics were collected. Unsupervised hierarchical cluster analyses were performed to identify patients with: (1) similar symptoms lasting ≥4 weeks after acute SARS-CoV-2 infection, and (2) similar therapeutic interventions. Logistic regression analyses were used to evaluate the association of these symptom and therapy clusters with quantitative 12-month outcome metrics (modified Rankin Scale, Barthel Index, NIH NeuroQoL). Results Among 242 patients, 122 (50%) reported ≥1 PASC symptom (median 3, IQR 1–5) lasting a median of 12-months (range 1–15) post-COVID diagnosis. Cluster analysis generated three symptom groups: Cluster1 had few symptoms (most commonly headache); Cluster2 had many symptoms including high levels of anxiety and depression; and Cluster3 primarily included shortness of breath, headache and cognitive symptoms. Cluster1 received few therapeutic interventions (OR 2.6, 95% CI 1.1–5.9), Cluster2 received several interventions, including antidepressants, anti-anxiety medications and psychological therapy (OR 15.7, 95% CI 4.1–59.7) and Cluster3 primarily received physical and occupational therapy (OR 3.1, 95%CI 1.3–7.1). The most severely affected patients (Symptom Cluster 2) had higher rates of disability (worse modified Rankin scores), worse NeuroQoL measures of anxiety, depression, fatigue and sleep disorder, and a higher number of stressors (all P<0.05). 100% of those who received a treatment strategy that included psychiatric therapies reported symptom improvement, compared to 97% who received primarily physical/occupational therapy, and 83% who received few interventions (P = 0.042). Conclusions We identified three clinically relevant PASC symptom-based phenotypes, which received different therapeutic interventions with varying response rates. These data may be helpful in tailoring individual treatment programs.
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Webb, Jeremy J., et Alison Sills. « The initial properties of young star clusters in M83 ». Monthly Notices of the Royal Astronomical Society 501, no 2 (12 décembre 2020) : 1933–39. http://dx.doi.org/10.1093/mnras/staa3832.

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ABSTRACT The initial sizes and masses of massive star clusters provide information about the cluster formation process and also determine how cluster populations are modified and destroyed, which have implications for using clusters as tracers of galaxy assembly. Young massive cluster populations are often assumed to be unchanged since cluster formation; therefore, their distributions of masses and radii are used as the initial values. However, the first few hundred million years of cluster evolution does change both cluster mass and cluster radius, through both internal and external processes. In this paper, we use a large suite of N-body cluster simulations in an appropriate tidal field to determine the best initial mass and initial size distributions of young clusters in the nearby galaxy M83. We find that the initial masses follow a power-law distribution with a slope of −2.7 ± 0.4 , and the half-mass radii follow a lognormal distribution with a mean of 2.57 ± 0.04 pc and a dispersion of 1.59 ± 0.01 pc. The corresponding initial projected half-light radius function has a mean of 2.7 ± 0.3 pc and a dispersion of 1.7 ± 0.2 pc. The evolution of the initial mass and size distribution functions is consistent with mass-loss and expansion due to stellar evolution, independent of the external tidal field and the cluster’s initial density profile. Observed cluster sizes and masses should not be used as the initial values, even when clusters are only a few hundred million years old.
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Huang, Francis L. « Using Cluster Bootstrapping to Analyze Nested Data With a Few Clusters ». Educational and Psychological Measurement 78, no 2 (24 novembre 2016) : 297–318. http://dx.doi.org/10.1177/0013164416678980.

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Cluster randomized trials involving participants nested within intact treatment and control groups are commonly performed in various educational, psychological, and biomedical studies. However, recruiting and retaining intact groups present various practical, financial, and logistical challenges to evaluators and often, cluster randomized trials are performed with a low number of clusters (~20 groups). Although multilevel models are often used to analyze nested data, researchers may be concerned of potentially biased results due to having only a few groups under study. Cluster bootstrapping has been suggested as an alternative procedure when analyzing clustered data though it has seen very little use in educational and psychological studies. Using a Monte Carlo simulation that varied the number of clusters, average cluster size, and intraclass correlations, we compared standard errors using cluster bootstrapping with those derived using ordinary least squares regression and multilevel models. Results indicate that cluster bootstrapping, though more computationally demanding, can be used as an alternative procedure for the analysis of clustered data when treatment effects at the group level are of primary interest. Supplementary material showing how to perform cluster bootstrapped regressions using R is also provided.
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Kezerashvili, R. Ya. « Few-body neutron and kaonic clusters ». Physical Sciences and Technology 3, no 1 (2016) : 63–72. http://dx.doi.org/10.26577/phst-2016-1-95.

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8

Reid, Neil, et Bruce W. Smith. « Assessing the Success of an Industrial Cluster ». International Journal of Applied Geospatial Research 3, no 3 (juillet 2012) : 21–36. http://dx.doi.org/10.4018/jagr.2012070102.

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Industrial clusters have received considerable attention as a regional development strategy. While their efficacy has been debated by academics, clusters have become popular among practitioners. Despite clusters’ acceptance, there have been few attempts to measure their success or their impact on constituent firms. This paper outlines and discusses the metrics developed to evaluate the success of the northwest Ohio greenhouse cluster. The cluster was launched in 2004 to help the industry become more competitive though collaborative problem solving. In identifying success metrics, the authors were cognizant of the fact that they had to reflect the cluster’s objectives and goals. Thus metrics that measured the impact of branding and marketing efforts, reducing energy costs, and increasing collaboration among cluster stakeholders were developed. The work reported in this paper is only the beginning phases of a longer-term, on-going effort to track the progress and success of the northwest Ohio greenhouse cluster.
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9

MacKinnon, James G., et Matthew D. Webb. « The wild bootstrap for few (treated) clusters ». Econometrics Journal 21, no 2 (6 mai 2018) : 114–35. http://dx.doi.org/10.1111/ectj.12107.

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10

Peng, Dong Liang, K. Sumiyama, H. Yamada, Takehiko Hihara et T. Uchida. « Preparation of Magnetically Soft, Highly-Densified Fe Cluster-Assembled Films by Impact Cluster Deposition ». Advanced Materials Research 11-12 (février 2006) : 595–98. http://dx.doi.org/10.4028/www.scientific.net/amr.11-12.595.

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Highly-densified Fe cluster-assembled films were obtained at room temperature by an energetic cluster deposition. Fe clusters were produced using a plasma-gas-condensation (PGC)-type cluster deposition apparatus with a high cluster productivity. Ionized clusters in a cluster beam were electrically accelerated and directly deposited onto a substrate together with neutral clusters from the same cluster source. By increasing the impact energy of the ionized clusters up to about 1 eV/atom, the obtained cluster-assemblies have packing fractions higher than 0.8 without any serious size change, and result in a soft magnetic behavior up to a frequency range of few hundred MHz.
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11

Armstrong, Jeremy R., Aksel S. Jensen, Artem G. Volosniev et Nikolaj T. Zinner. « Clusters in Separated Tubes of Tilted Dipoles ». Mathematics 8, no 4 (1 avril 2020) : 484. http://dx.doi.org/10.3390/math8040484.

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A few-body cluster is a building block of a many-body system in a gas phase provided the temperature at most is of the order of the binding energy of this cluster. Here we illustrate this statement by considering a system of tubes filled with dipolar distinguishable particles. We calculate the partition function, which determines the probability to find a few-body cluster at a given temperature. The input for our calculations—the energies of few-body clusters—is estimated using the harmonic approximation. We first describe and demonstrate the validity of our numerical procedure. Then we discuss the results featuring melting of the zero-temperature many-body state into a gas of free particles and few-body clusters. For temperature higher than its binding energy threshold, the dimers overwhelmingly dominate the ensemble, where the remaining probability is in free particles. At very high temperatures free (harmonic oscillator trap-bound) particle dominance is eventually reached. This structure evolution appears both for one and two particles in each layer providing crucial information about the behavior of ultracold dipolar gases. The investigation addresses the transition region between few- and many-body physics as a function of temperature using a system of ten dipoles in five tubes.
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12

Banerjee, Sambaran, Holger Baumgardt et Pavel Kroupa. « Stellar-mass black holes in star clusters : implications for gravitational-wave radiation ». Proceedings of the International Astronomical Union 5, S266 (août 2009) : 213–18. http://dx.doi.org/10.1017/s1743921309991074.

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AbstractWe study the dynamics of stellar-mass black holes (BHs) in star clusters, with particular attention to the formation of BH–BH binaries, which are interesting as sources of gravitational waves (GWs). We examine the properties of these BH–BH binaries through direct N-body simulations of Plummer clusters of N ≤ 105 low-mass stars with an initial population of stellar-mass BHs, using the nbody6 code. We find that the stellar-mass BHs segregate rapidly into the cluster core and form a dense subcluster of BHs in which BH–BH binaries form through three-body encounters. While most BH binaries are ejected from the cluster by recoils due to superelastic encounters with the single BHs, we find that for clusters with N ≳ 5 × 104, typically a few of them harden sufficiently so that they can merge via GW emission within the cluster. Also, for each of such clusters there are a few escaping BH binaries that merge within a Hubble time, with most merger times being within a few Gyr. These results imply that the intermediate-age massive clusters constitute the most important class of star cluster candidates that can produce dynamical BH–BH mergers at the present epoch. The BH–BH merger rates obtained from our computations imply a significant detection rate (~30 yr−1) for the proposed Advanced LIGO GW detector.
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13

Larsen, Søren S. « Young and intermediate-age massive star clusters ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 368, no 1913 (28 février 2010) : 867–87. http://dx.doi.org/10.1098/rsta.2009.0255.

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An overview of our current understanding of the formation and evolution of star clusters is given, with the main emphasis on high-mass clusters. Clusters form deeply embedded within dense clouds of molecular gas. Left-over gas is cleared within a few million years and, depending on the efficiency of star formation, the clusters may disperse almost immediately or remain gravitationally bound. Current evidence suggests that a small percentage of star formation occurs in clusters that remain bound, although it is not yet clear whether this fraction is truly universal. Internal two-body relaxation and external shocks will lead to further, gradual dissolution on time scales of up to a few hundred million years for low-mass open clusters in the Milky Way, while the most massive clusters (>10 5 M ⊙ ) have lifetimes comparable to or exceeding the age of the Universe. The low-mass end of the initial cluster mass function is well approximated by a power-law distribution, , but there is mounting evidence that quiescent spiral discs form relatively few clusters with masses M >2×10 5 M ⊙ . In starburst galaxies and old globular cluster systems, this limit appears to be higher, at least several ×10 6 M ⊙ . The difference is likely related to the higher gas densities and pressures in starburst galaxies, which allow denser, more massive giant molecular clouds to form. Low-mass clusters may thus trace star formation quite universally, while the more long-lived, massive clusters appear to form preferentially in the context of violent star formation.
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Assmann, P., M. Fellhauer et M. I. Wilkinson. « Star clusters as building blocks for dSph galaxy formation ». Proceedings of the International Astronomical Union 5, S266 (août 2009) : 353–56. http://dx.doi.org/10.1017/s174392130999127x.

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AbstractWe study numerically the formation of dSph galaxies. Intense starbursts, e.g., in gas-rich environments, typically produce a few to a few hundred young star clusters within a region of just a few hundred pc. The dynamical evolution of these star clusters may explain the formation of the luminous component of dwarf spheroidal (dSph) galaxies. Here, we perform a numerical experiment to show that the evolution of star cluster complexes in dark-matter haloes can explain the formation of the luminous components of dSph galaxies.
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15

TAKAOKA, G. H., J. MATSUO, C. E. ASCHERON et I. YAMADA. « FUNDAMENTAL ASPECTS OF THE IONIZED CLUSTER-BEAM DEPOSITION PROCESS ». Surface Review and Letters 03, no 01 (février 1996) : 1013–16. http://dx.doi.org/10.1142/s0218625x96001819.

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The electron diffraction measurement of Sb beams ejected by adiabatic expansion from a nozzle source showed that a considerable amount of clusters with a highly disordered atomic arrangement was produced. The cluster size was found to be larger than a few tens of atoms per cluster, though the exact size could not be determined. It was reported that after impact of clusters ejected from the nozzle source, cluster islands were formed on a substrate surface, and that the film formation started by direct deposition of clusters as the rapid nucleation of cluster islands. These results supported the notion that large atomic clusters existed in the beam.
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16

Taljaard, Monica, Steven Teerenstra, Noah M. Ivers et Dean A. Fergusson. « Substantial risks associated with few clusters in cluster randomized and stepped wedge designs ». Clinical Trials : Journal of the Society for Clinical Trials 13, no 4 (3 mars 2016) : 459–63. http://dx.doi.org/10.1177/1740774516634316.

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17

Kiremire, Enos Masheija Rwantale. « Generating Cluster Formulas Using The Primary Clusters And The K(n) Parameters ». International Journal of Chemistry 10, no 2 (27 avril 2018) : 85. http://dx.doi.org/10.5539/ijc.v10n2p85.

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The formulas of transition metal clusters can be regarded as multiples of their respective PRIMARY CLUSTERS and K(n) parameters. A primary cluster of a transition metal skeletal element can be defined as that cluster of a mono-skeletal element which obeys the 18 electron rule. Such clusters, among others include, Cr(CO)6, Fe(CO)5, Ni(CO)4, and Zn(CO)3 and the respective K values of the skeletal elements are 6, 5, 4 and 3. The selected K(n) series are given and the derived hypothetical golden clusters are given as examples for illustrations. Selected known golden clusters are also found to be multiples of K(n) parameters and the 18 valence fragment cluster, AuL3.5. The graphical representations of a few selected examples of golden clusters are given.
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Ma, Xiangcheng, Michael Y. Grudić, Eliot Quataert, Philip F. Hopkins, Claude-André Faucher-Giguère, Michael Boylan-Kolchin, Andrew Wetzel, Ji-hoon Kim, Norman Murray et Dušan Kereš. « Self-consistent proto-globular cluster formation in cosmological simulations of high-redshift galaxies ». Monthly Notices of the Royal Astronomical Society 493, no 3 (21 février 2020) : 4315–32. http://dx.doi.org/10.1093/mnras/staa527.

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ABSTRACT We report the formation of bound star clusters in a sample of high-resolution cosmological zoom-in simulations of z ≥ 5 galaxies from the Feedback In Realistic Environments project. We find that bound clusters preferentially form in high-pressure clouds with gas surface densities over $10^4\, \mathrm{ M}_{\odot }\, {\rm pc}^{-2}$, where the cloud-scale star formation efficiency is near unity and young stars born in these regions are gravitationally bound at birth. These high-pressure clouds are compressed by feedback-driven winds and/or collisions of smaller clouds/gas streams in highly gas-rich, turbulent environments. The newly formed clusters follow a power-law mass function of dN/dM ∼ M−2. The cluster formation efficiency is similar across galaxies with stellar masses of ∼107–$10^{10}\, \mathrm{ M}_{\odot }$ at z ≥ 5. The age spread of cluster stars is typically a few Myr and increases with cluster mass. The metallicity dispersion of cluster members is ∼0.08 dex in $\rm [Z/H]$ and does not depend on cluster mass significantly. Our findings support the scenario that present-day old globular clusters (GCs) were formed during relatively normal star formation in high-redshift galaxies. Simulations with a stricter/looser star formation model form a factor of a few more/fewer bound clusters per stellar mass formed, while the shape of the mass function is unchanged. Simulations with a lower local star formation efficiency form more stars in bound clusters. The simulated clusters are larger than observed GCs due to finite resolution. Our simulations are among the first cosmological simulations that form bound clusters self-consistently in a wide range of high-redshift galaxies.
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Grandjean, Didier, Eduardo Coutiño-Gonzalez, Ngo Tuan Cuong, Eduard Fron, Wouter Baekelant, Saleh Aghakhani, Philomena Schlexer et al. « Origin of the bright photoluminescence of few-atom silver clusters confined in LTA zeolites ». Science 361, no 6403 (16 août 2018) : 686–90. http://dx.doi.org/10.1126/science.aaq1308.

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Silver (Ag) clusters confined in matrices possess remarkable luminescence properties, but little is known about their structural and electronic properties. We characterized the bright green luminescence of Ag clusters confined in partially exchanged Ag–Linde Type A (LTA) zeolites by means of a combination of x-ray excited optical luminescence-extended x-ray absorption fine structure, time-dependent–density functional theory calculations, and time-resolved spectroscopy. A mixture of tetrahedral Ag4(H2O)x2+(x= 2 andx= 4) clusters occupies the center of a fraction of the sodalite cages. Their optical properties originate from a confined two-electron superatom quantum system with hybridized Ag and water O orbitals delocalized over the cluster. Upon excitation, one electron of the s-type highest occupied molecular orbital is promoted to the p-type lowest unoccupied molecular orbitals and relaxes through enhanced intersystem crossing into long-lived triplet states.
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Mirmelstein, M., M. Shimon et Y. Rephaeli. « Detection likelihood of cluster-induced CMB polarization ». Astronomy & ; Astrophysics 644 (30 novembre 2020) : A36. http://dx.doi.org/10.1051/0004-6361/201834657.

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Nearby galaxy clusters can potentially induce sub-microkelvin polarization signals in the cosmic microwave background (CMB) at characteristic scales of a few arcminutes. We explore four such polarization signals induced in a rich nearby fiducial cluster and calculate the likelihood of their detection by a telescope project with capabilities such as those of the Simons Observatory (SO). In our feasibility analysis, we include instrumental noise, primordial CMB anisotropy, statistical thermal Sunyaev-Zeldovich (SZ) cluster signal, and point source confusion, assuming a few percent of the nominal telescope observation time of an SO-like project. Our analysis indicates that the thermal SZ intensity can be sensitively mapped in rich nearby clusters and that the kinematic SZ intensity can be measured with high statistical significance toward a fast moving nearby cluster. The detection of polarized SZ signals will be quite challenging but could still be feasible toward several very rich nearby clusters with very high SZ intensity. The polarized SZ signal from a sample of ∼20 clusters can be statistically detected at S/N ∼ 3, if observed for several months.
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Joshi, Ravi, Benjamin Waldschmidt, Jörg Engstler, Rolf Schäfer et Jörg J. Schneider. « Generation and agglomeration behaviour of size-selected sub-nm iron clusters as catalysts for the growth of carbon nanotubes ». Beilstein Journal of Nanotechnology 2 (1 novembre 2011) : 734–39. http://dx.doi.org/10.3762/bjnano.2.80.

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Mass-selected, ligand-free Fe N clusters with N = 10–30 atoms (cluster diameter: 0.6–0.9 nm) were implanted into [Al@SiO x ] surfaces at a low surface coverage corresponding to a few thousandths up to a few hundredths of a monolayer in order to avoid initial cluster agglomeration. These studies are aimed towards gaining an insight into the lower limit of the size regime of carbon nanotube (CNT) growth by employing size-selected sub-nm iron clusters as catalyst or precatalyst precursors for CNT growth. Agglomeration of sub-nm iron clusters to iron nanoparticles with a median size range between three and six nanometres and the CNT formation hence can be observed at CVD growth temperatures of 750 °C. Below 600 °C, no CNT growth is observed.
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Michaelian, K. « Evolving few-ion clusters of Na and Cl ». American Journal of Physics 66, no 3 (mars 1998) : 231–40. http://dx.doi.org/10.1119/1.18851.

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Colless, Donald H. « Taxa, individuals, clusters and a few other things ». Biology & ; Philosophy 21, no 3 (juin 2006) : 353–67. http://dx.doi.org/10.1007/s10539-005-8240-2.

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Belyaev, V. B., W. Sandhas et I. I. Shlyk. « Meson-nuclear clusters in the few-body approaches ». EPJ Web of Conferences 3 (2010) : 03033. http://dx.doi.org/10.1051/epjconf/20100303033.

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Gordillo, M. C. « Paramagnetism in few-fermion clusters with repulsive interactions ». Journal of Physics B : Atomic, Molecular and Optical Physics 51, no 15 (12 juillet 2018) : 155301. http://dx.doi.org/10.1088/1361-6455/aad036.

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Balcon, Emilie. « France to focus on a few biotech clusters ». Nature Biotechnology 22, no 9 (27 août 2004) : 1063–64. http://dx.doi.org/10.1038/nbt0904-1063.

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Zinnecker, Hans. « Embedded and open star clusters : basic questions ». Proceedings of the International Astronomical Union 5, S266 (août 2009) : 17–23. http://dx.doi.org/10.1017/s1743921309990810.

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AbstractThis paper provides an introduction to IAU Symposium 266 on star clusters as basic building blocks in space and time. We define clusters as bound systems and discriminate them from general stellar clusterings or groups and unbound associations. We give a few examples of young, embedded, compact clusters which may evolve into looser, open clusters after dynamical relaxation due to mass loss and secular relaxation processes. We ask how and where star clusters form (in normal and interacting galaxies) and provide statistics of open clusters in terms of cluster masses and ages in the solar neighborhood (where observational data are most complete). Finally, we list a number of basic questions for current and future star cluster research and discuss the prospects for cluster studies with the next generation infrared and submillimeter telescopes (Herschel; JWST, E–ELT; ALMA, NOEMA).
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Gao, Wanying, Kehang Li et Chunling Wei. « Satellite Cluster Formation Reconfiguration Based on the Bifurcating Potential Field ». Aerospace 9, no 3 (4 mars 2022) : 137. http://dx.doi.org/10.3390/aerospace9030137.

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The satellite cluster formation reconfiguration has received considerable attention in recent years. However, the traditional centralized control methods are challenging to apply to satellite clusters because of the enormous fuel consumption, and few studies have addressed the mathematical characterization of satellite clusters. This research aims to propose a mathematical characterization method for satellite clusters and investigate the formation reconfiguration control of satellite clusters. This study provided the five-element characterization method to represent the cluster characteristics and internal correlation characteristics of orbiting satellite clusters. In addition, a control method based on bifurcating potential fields was proposed to realize satellite cluster formation’s dynamic migration and rapid reconfiguration. A cluster with 50 satellites was simulated to verify the feasibility and effectiveness of the proposed formation control algorithm. The results show that various formation topologies were achieved by simply changing the bifurcation parameter and configuration adjustment parameters. The five descriptive elements of the satellite cluster can intuitively and effectively reflect the running state of the satellite cluster.
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Negueruela, Ignacio. « Clusters rich in red supergiants ». Proceedings of the International Astronomical Union 11, A29B (août 2015) : 461–63. http://dx.doi.org/10.1017/s1743921316005858.

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AbstractIn the past few years, several clusters containing large numbers of red supergiants have been discovered. These clusters are amongst the most massive young clusters known in the Milky Way, with stellar masses reaching a few 104M⊙. They have provided us, for the first time, with large homogeneous samples of red supergiants of a given age. These large populations make them, despite heavy extinction along their sightlines, powerful laboratories to understand the evolutionary status of red supergiants. While some of the clusters, such as the eponymous RSGC1, are so obscured that their members are only observable in the near-IR, some of them are easily accessible, allowing for an excellent characterisation of cluster and stellar properties. The information gleaned so far from these clusters gives strong support to the idea that late-M type supergiants represent a separate class, characterised by very heavy mass loss. It also shows that the spectral-type distribution of red supergiants in the Milky Way is very strongly peaked towards M1, while suggesting a correlation between spectral type and evolutionary stage.
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Fellhauer, Michael. « Building Dwarf Galaxies out of Merged Young Star Clusters ». Symposium - International Astronomical Union 207 (2002) : 730–32. http://dx.doi.org/10.1017/s0074180900224704.

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Young star clusters in interacting galaxies are often found in groups or clusters of star clusters containing up to 100 single clusters. In our project we study the future fate of these clusters of star clusters. We find that the star clusters merge on time scales of a few dynamical crossing times of the super-cluster. The resulting merger object has similarities with observed dwarf ellipticals (dE). Furthermore, if destructive processes like tidal heating, dynamical friction or interaction with disc or bulge of the parent galaxy are taken into account our merger objects may evolve into objects resembling dwarf spheroidal galaxies (dSph), without the need of a high dark matter content.
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31

Hattori, M. « A Metal Enriched Dark Cluster of Galaxies at Z = 1 ». Symposium - International Astronomical Union 187 (2002) : 129–38. http://dx.doi.org/10.1017/s0074180900113841.

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Looking for and studying very distant galaxy clusters, clusters at z > 1, are one of the prime subjects of the modern observational cosmology. If the metallicity of the hot intra-cluster medium in very distant galaxy clusters is measured for example, it provides fruitful informations for us to understand the formation and evolution of galaxies. However, difficulty of the study is that there is few confirmed very distant galaxy clusters yet. We first have to search for very distant clusters but it requires very deep observations. A random selection of sky is not practical. We have to select the sky. In this article, it is demonstrated that missing lens problem has close connection with very distant cluster of galaxies and dark lens searches could open a new window for studying very distant cluster of galaxies.
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Jones, Michael G., Ananthan Karunakaran, Paul Bennet, David J. Sand, Kristine Spekkens, Burçin Mutlu-Pakdil, Denija Crnojević, Steven Janowiecki, Lukas Leisman et Catherine E. Fielder. « Gas-rich, Field Ultra-diffuse Galaxies Host Few Gobular Clusters ». Astrophysical Journal Letters 942, no 1 (28 décembre 2022) : L5. http://dx.doi.org/10.3847/2041-8213/acaaab.

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Abstract We present Hubble Space Telescope imaging of 14 gas-rich, low-surface-brightness galaxies in the field at distances of 25–36 Mpc, with mean effective radii and g-band central surface brightnesses of 1.9 kpc and 24.2 mag arcsec−2. Nine meet the standard criteria to be considered ultra-diffuse galaxies (UDGs). An inspection of point-like sources brighter than the turnover magnitude of the globular cluster luminosity function and within twice the half-light radii of each galaxy reveals that, unlike those in denser environments, gas-rich, field UDGs host very few old globular clusters (GCs). Most of the targets (nine) have zero candidate GCs, with the remainder having one or two candidates each. These findings are broadly consistent with expectations for normal dwarf galaxies of similar stellar mass. This rules out gas-rich, field UDGs as potential progenitors of the GC-rich UDGs that are typically found in galaxy clusters. However, some in galaxy groups may be directly accreted from the field. In line with other recent results, this strongly suggests that there must be at least two distinct formation pathways for UDGs, and that this subpopulation is simply an extreme low surface brightness extension of the underlying dwarf galaxy population. The root cause of their diffuse stellar distributions remains unclear, but the formation mechanism appears to only impact the distribution of stars (and potentially dark matter), without strongly impacting the distribution of neutral gas, the overall stellar mass, or the number of GCs.
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33

Kröger, H., et R. Perne. « Few-body binding-energy correlations ». Canadian Journal of Physics 63, no 3 (1 mars 1985) : 366–70. http://dx.doi.org/10.1139/p85-057.

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Few-nucleon binding-energy correlations are discussed. The first-order correction in the propagator potential range ratio is estimated. The class of potentials is generalized from the previously considered rank-one separable potential. Similar results are predicted for few-atomic molecule clusters.
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Lin, Hai, Ruoshan Kong et Jiali Liu. « Genetic Algorithm Based Clustering for Large-Scale Sensor Networks ». Cybernetics and Information Technologies 15, no 6 (1 décembre 2015) : 168–77. http://dx.doi.org/10.1515/cait-2015-0077.

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Abstract Despite the success of various clustering algorithms for Wireless Sensor Networks (WSNs), there are few works that consider the interference between clusters. Obviously, interference-free clustering makes the communication more efficient and achieves energy saving. In this paper we propose a new clustering method for large-scale sensor networks. With this method the network is partitioned into clusters. Intra-cluster communication in a cluster has no interference by its neighbor clusters. Moreover, the proposed clustering is based on a Genetic Algorithm (GA), which can achieve optimal performance in terms of the number of isolated nodes. This is demonstrated by the simulation analysis.
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35

Kroupa, Pavel. « Star-cluster formation and evolution ». Proceedings of the International Astronomical Union 2, S237 (août 2006) : 230–37. http://dx.doi.org/10.1017/s1743921307001524.

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AbstractStar clusters are observed to form in a highly compact state and with low star-formation efficiencies, and only 10 per cent of all clusters appear to survive to middle- and old-dynamical age. If the residual gas is expelled on a dynamical time the clusters disrupt. Massive clusters may then feed a hot kinematical stellar component into their host-galaxy's field population thereby thickening galactic disks, a process that theories of galaxy formation and evolution need to accommodate. If the gas-evacuation time-scale depends on cluster mass, then a power-law embedded-cluster mass function may transform within a few dozen Myr to a mass function with a turnover near 105M, thereby possibly explaining this universal empirical feature. Discordant empirical evidence on the mass function of star clusters leads to the insight that the physical processes shaping early cluster evolution remain an issue of cutting-edge research.
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Ma, Jun, Xizi Cao, Xiaopeng Xing, Xuefeng Wang et Joel H. Parks. « Adsorption of O2 on anionic silver clusters : spins and electron binding energies dominate in the range up to nano sizes ». Physical Chemistry Chemical Physics 18, no 2 (2016) : 743–48. http://dx.doi.org/10.1039/c5cp06116d.

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37

Baumgardt, H., et P. Kroupa. « The Influence of Gas Expulsion on the Evolution of Star Clusters ». Proceedings of the International Astronomical Union 3, S246 (septembre 2007) : 36–40. http://dx.doi.org/10.1017/s1743921308015238.

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AbstractWe present new results on the dynamical evolution and dissolution of star clusters due to residual gas expulsion and the effect this has on the mass function and other properties of star cluster systems. To this end, we have carried out a large set of N-body simulations, varying the star formation efficiency, gas expulsion time scale and strength of the external tidal field, obtaining a three-dimensional grid of models which can be used to predict the evolution of individual star clusters or whole star cluster systems by interpolating between our runs. When applied to the Milky Way globular cluster system, we find that gas expulsion is the main dissolution mechanism for star clusters, destroying about 80% of all clusters within a few 10s of Myers. Together with later dynamical evolution, it seems possible to turn an initial power-law mass function into a log-normal one with properties similar to what has been observed for the Milky Way globular clusters.
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38

Djorgovski, S., et H. Penner. « A Search for Post-Collapse Cores ». Symposium - International Astronomical Union 113 (1985) : 73–76. http://dx.doi.org/10.1017/s0074180900147242.

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We report the preliminary results of a surface photometry survey of globular cluster cores. Two new cores with post-collapse morphology have been found, and two possible candidates. The estimated fraction of clusters with this core morphology in the Galaxy is only a few percent. Most clusters do not show a morphology similar to the predictions of the central black hole models.
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Walker, Alistair R. « RR Lyrae stars in the Magellanic Clouds ». Symposium - International Astronomical Union 148 (1991) : 307–12. http://dx.doi.org/10.1017/s0074180900200636.

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RR Lyrae variables are a component of the oldest stellar populations. In the Magellanic Clouds they occur in a few clusters and in the general field. Recent observations have concentrated on obtaining accurate photometry for the cluster variables and the evolved stars of their parent clusters; these data have been used to determine abundances, distances and ages. Results from this work are reviewed.
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40

Cox, Georgina R., Jo Robinson, Michelle Williamson, Anne Lockley, Yee Tak Derek Cheung et Jane Pirkis. « Suicide Clusters in Young People ». Crisis 33, no 4 (1 juillet 2012) : 208–14. http://dx.doi.org/10.1027/0227-5910/a000144.

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Background: Suicide clusters have commonly been documented in adolescents and young people. Aims: The current review conducts a literature search in order to identify and evaluate postvention strategies that have been employed in response to suicide clusters in young people. Methods: Online databases, gray literature, and Google were searched for relevant articles relating to postvention interventions following a suicide cluster in young people. Results: Few studies have formally documented response strategies to a suicide cluster in young people, and at present only one has been longitudinally evaluated. However, a number of strategies show promise, including: developing a community response plan; educational/psychological debriefings; providing both individual and group counseling to affected peers; screening high risk individuals; responsible media reporting of suicide clusters; and promotion of health recovery within the community to prevent further suicides. Conclusions: There is a gap in formal evidence-based guidelines detailing appropriate postvention response strategies to suicide clusters in young people. The low-frequency nature of suicide clusters means that long-term systematic evaluation of response strategies is problematic. However, some broader suicide prevention strategies could help to inform future suicide cluster postvention responses.
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Bibeau, Jeffrey P., Shawn Gray et Enrique M. De La Cruz. « Clusters of a Few Bound Cofilins Sever Actin Filaments ». Journal of Molecular Biology 433, no 7 (avril 2021) : 166833. http://dx.doi.org/10.1016/j.jmb.2021.166833.

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42

Schultz, Danielle, et Nancy Lin. « Few Atom Silver Clusters Display Broad Spectrum Antimicrobial Activity ». Biophysical Journal 112, no 3 (février 2017) : 593a. http://dx.doi.org/10.1016/j.bpj.2016.11.3190.

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43

Liu, Hui Heng, et Wei Chen. « A Novel Weighted-Cooperative Spectrum Sensing Scheme Using Clustering ». Applied Mechanics and Materials 195-196 (août 2012) : 277–82. http://dx.doi.org/10.4028/www.scientific.net/amm.195-196.277.

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This paper proposes a novel weighted-cooperative spectrum sensing scheme using clustering for cognitive radio system. We firstly classify the secondary users into a few clusters according to several existent methods, and then use cluster-head to collect the observation results come from different secondary users in the same cluster and make a cluster-decision. Considering the different distances between the clusters and the fusion center, different weightings are used to weight the cluster-decisions before combining. The simulation results show that our proposed method improve the probability of detection and reduce the probability of error.
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44

Nayak, Prasantha K., et Annapurni Subramaniam. « Methods to Identify Star Clusters in The Large Magellanic Cloud (LMC) ». Mapana - Journal of Sciences 17, no 1 (1 janvier 2018) : 57–73. http://dx.doi.org/10.12723/mjs.44.6.

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The Large Magellanic Cloud (LMC) hosts a few thousand star clusters which are ideal tools to study stellar evolution, star formation history, cluster formation and dissolution processes in the galaxy. Although many surveys (like IRSF, OGLE II etc) of the LMC have been carried out, a large fraction of clusters, mainly poor ones are yet to be identified. Also, the parameters of already identified clusters are not well studied. In this context, we have tried to explore that among the available and upcoming surveys which survey (optical/NIR) can be used to efficiently detect and study the clusters in the LMC. We have found that the available OGLE-III optical data is ideal for this purpose, but only for young clusters, whereas Deeper optical data from DECAM survey, OGLE-IV and skymapper are ideal to study poor and old clusters. We have also found that one can combine the ongoing VISTA data with upcoming optical data (OGLE IV) and estimate the cluster parameters more accurately.
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45

Blaisten-Barojas, Estela. « MOLECULAR DYNAMICS STUDY OF CLUSTER GROWTH AND POLYMER DEGRADATION ». International Journal of Modern Physics B 06, no 23n24 (décembre 1992) : 3643–55. http://dx.doi.org/10.1142/s0217979292001705.

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This work is an overview on two molecular dynamics studies of processes at temperatures characteristic of a flame — growth of silicon clusters from binary cluster collisions and thermal degradation of polymers. In the first study, silicon clusters grow as a consequence of cluster-cluster collisions by forming transient agglomerates that coalesce in a few picoseconds. The collision energy accommodates within the cluster favoring the formation of globular larger clusters regardless of the collision energy and of the impact parameter. On the average the probability for the clusters to stick upon collision is almost 1, showing clearly that the process is irreversible. The second study concerns simple polymeric chains undergoing fragmentation when they burn. These fragments are products of thermal degradation. The consorted sequence of depolymerization reactions arises after fragmentation. As a result, a sample of degrading fragments is formed where the polymer chains have dramatically coiled. These fragments self trap themselves in coiled conformations due to the cooling effect produced by the depolymerization reaction.
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46

Felinger, Attila, et Jänos Liszi. « Simulation of Fractal-Like Crystal Growth ». Zeitschrift für Naturforschung A 46, no 1-2 (1 février 1991) : 203–5. http://dx.doi.org/10.1515/zna-1991-1-231.

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AbstractNon-equilibrium crystallization was simulated on a two dimensional square lattice. Several clusters were grown simultaneously by using the model of diffusion limited aggregation. The growing process was reversible, i.e. dissolution of particles from the boundary of any cluster was made possible. The rate of growth and dissolution was determined by a stochastic method. The simulation resulted in an aggregate pattern having a few large and several small clusters. The fractal dimensions of the large clusters were found in the range of D = 1.62-1.72.
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47

Drukier, G. A. « On the Retention of Globular Cluster Neutron Stars ». Symposium - International Astronomical Union 174 (1996) : 347–48. http://dx.doi.org/10.1017/s0074180900001753.

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Although globular clusters are known to contain a population of neutron stars, the recent finding by Lyne and Lorimer (1994) that field neutron stars are formed with a mean “kick” velocity of 450 km s−1 implies that globular clusters would retain very few neutron stars. The number of neutron stars is important when discussing cluster mortality and the formation rate of millisecond pulsars.
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48

Bravo-Alfaro, H., J. M. Islas et C. Caretta. « The Substructure in the Cluster Abell85 ». Proceedings of the International Astronomical Union 2, S235 (août 2006) : 192. http://dx.doi.org/10.1017/s1743921306005989.

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We revisit the complex structure of the A85-A87 cluster, based on a volume complete VLA-HI imaging survey, as well as optical photometry and spectroscopy. HI imaging studies of a few nearby clusters have shown that most of the HI rich galaxies around clusters are located within infalling groups. The case of A 85 is a very peculiar one, with all the 11 HI-detected galaxies projected east of A 85, and all but one with much lower radial velocities (~ 15,000 km s−1 than the cluster's systemic velocity (16,500 km s−1) In order to quantify the degree of substructure in A 85 we estimate the parameter δ defined by Dressler and Schectman, and plot it in Fig. 1a: many large circles in the same area indicate a great possibility of substructuring (Bravo-Alfaro et al. 2007 in prep).
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Kononova, Irina, Pavel Kononov et Vyacheslav Moshnikov. « Step-by-Step Modeling and Experimental Study on the Sol–Gel Porous Structure of Percolation Nanoclusters ». Coatings 13, no 2 (16 février 2023) : 449. http://dx.doi.org/10.3390/coatings13020449.

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Non-conventional crystallization techniques have been developed in recent years. Non-conventional crystallization techniques use primary structural elements (for example, clusters) rather than atoms and molecules. Modern nanomaterial science is going through great changes as an entirely new approach of non-conventional growth mechanisms is emerging due to cluster coupling, catalyzing interest in cluster physics. The formation of fractal and percolation clusters has increased. We carried out step-by-step modeling and an experimental study of the formation of fractal and percolation clusters based on tin dioxide and silicon dioxide and formed by sol–gel technology. In this paper, the growth of fractal aggregates (clusters) from sol particles SnO2 and SiO2 based on the modified models of diffusion-limited and cluster–cluster aggregation is discussed. A percolation model using simulated fractal clusters of SnO2 and SiO2 particles is proposed. Experimental data on the sol–gel percolation structure of porous nanocomposites are presented. The modeling of SnO2 and SiO2 particles, which also consist of clusters (the next step in the hierarchy), is shown. We propose a generalized hierarchical three-dimensional percolation cluster model that allows calculating the surface area, knowing the experimental sizes of macropores and taking into account the micro- and mesopores (sizes less than a few nanometers).
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Roberts, Morton S. « Interstellar Matter in Globular Clusters ». Symposium - International Astronomical Union 126 (1988) : 411–22. http://dx.doi.org/10.1017/s0074180900042637.

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“Is there any point to which you would wish to draw my attention?”“To the curious incident of the dog in the night-time.”“The dog did nothing in the night-time.”“That was the curious incident,” remarked Sherlock Holmes.Memoirs of Sherlock HolmesThe source for intracluster matter is seen in various mass loss processes ongoing within clusters and is supported by the theoretical need for mass loss to explain the morphology of cluster colormagnitude diagrams. A variety of techniques ranging from X-ray to radio wavelengths have been employed to search for such matter but with few exceptions has not been found. The amount of material expected to collect between cleansing passages through the galactic plane has variously been estimated at between ∼ 102 and ∼ 103 M⊙. In contrast, observed upper limits for many clusters are well below these values, often > 1 M⊙. The few detections are at levels of ≲10−2 M⊙.
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