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   https://doi.org/10.1109/TVCG.2020.3002166  

   Das, Subhajit; Saket, Bahador; Kwon, Bum Chul; Endert, Alex (January 2020, IEEE Transactions on Visualization and Computer Graphics)

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2. Group actions on cluster algebras and cluster categories

   https://doi.org/10.1016/j.aim.2019.01.009  

   Paquette, Charles; Schiffler, Ralf (March 2019, Advances in Mathematics)
3. Operator Relationship between Conventional Coupled Cluster and Unitary Coupled Cluster

   https://doi.org/10.3390/sym14030494  

   Freericks, James K. (March 2022, Symmetry)

   The chemistry community has long sought the exact relationship between the conventional and the unitary coupled cluster ansatz for a single-reference system, especially given the interest in performing quantum chemistry on quantum computers. In this work, we show how one can use the operator manipulations given by the exponential disentangling identity and the Hadamard lemma to relate the factorized form of the unitary coupled-cluster approximation to a factorized form of the conventional coupled cluster approximation (the factorized form is required, because some amplitudes are operator-valued and do not commute with other terms). By employing the Trotter product formula, one can then relate the factorized form to the standard form of the unitary coupled cluster ansatz. The operator dependence of the factorized form of the coupled cluster approximation can also be removed at the expense of requiring even more higher-rank operators, finally yielding the conventional coupled cluster. The algebraic manipulations of this approach are daunting to carry out by hand, but can be automated on a computer for small enough systems. 

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4. Within-cluster resampling for multilevel models under informative cluster size

   https://doi.org/10.1093/biomet/asz035  

   Lee, D.; Kim, J. K.; Skinner, C. J. (July 2019, Biometrika)

   Summary A within-cluster resampling method is proposed for fitting a multilevel model in the presence of informative cluster size. Our method is based on the idea of removing the information in the cluster sizes by drawing bootstrap samples which contain a fixed number of observations from each cluster. We then estimate the parameters by maximizing an average, over the bootstrap samples, of a suitable composite loglikelihood. The consistency of the proposed estimator is shown and does not require that the correct model for cluster size is specified. We give an estimator of the covariance matrix of the proposed estimator, and a test for the noninformativeness of the cluster sizes. A simulation study shows, as in Neuhaus & McCulloch (2011), that the standard maximum likelihood estimator exhibits little bias for some regression coefficients. However, for those parameters which exhibit nonnegligible bias, the proposed method is successful in correcting for this bias. 

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5. The Next Generation Virgo Cluster Survey. XXXIII. Stellar Population Gradients in the Virgo Cluster Core Globular Cluster System

   https://doi.org/10.3847/1538-4357/ac63cf  

   Ko, Youkyung; Peng, Eric W.; Côté, Patrick; Ferrarese, Laura; Liu, Chengze; Longobardi, Alessia; Lançon, Ariane; Muñoz, Roberto P.; Puzia, Thomas H.; Alamo-Martínez, Karla A.; et al (June 2022, The Astrophysical Journal)

   Abstract We present a study of the stellar populations of globular clusters (GCs) in the Virgo Cluster core with a homogeneous spectroscopic catalog of 692 GCs within a major-axis distance R maj = 840 kpc from M87. We investigate radial and azimuthal variations in the mean age, total metallicity, [Fe/H], and α -element abundance of blue (metal-poor) and red (metal-rich) GCs using their co-added spectra. We find that the blue GCs have a steep radial gradient in [Z/H] within R maj = 165 kpc, with roughly equal contributions from [Fe/H] and [ α /Fe], and flat gradients beyond. By contrast, the red GCs show a much shallower gradient in [Z/H], which is entirely driven by [Fe/H]. We use GC-tagged Illustris simulations to demonstrate an accretion scenario where more massive satellites (with more metal- and α -rich GCs) sink further into the central galaxy than less massive ones, and where the gradient flattening occurs because of the low GC occupation fraction of low-mass dwarfs disrupted at larger distances. The dense environment around M87 may also cause the steep [ α /Fe] gradient of the blue GCs, mirroring what is seen in the dwarf galaxy population. The progenitors of red GCs have a narrower mass range than those of blue GCs, which makes their gradients shallower. We also explore spatial inhomogeneity in GC abundances, finding that the red GCs to the northwest of M87 are slightly more metal-rich. Future observations of GC stellar population gradients will be useful diagnostics of halo merger histories. 

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