• Bulletin of the Korean Mathematical Society
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• v.57 no.1
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• pp.195-206
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• 2020

The fundamental idea of this article is to present an effective way to obtain the large groups in terms of the split extension obtained by a finite cyclic group and a free abelian group rank 2. The proof of the main result on largeness property of this specific split extension groups will be given by using the connection of large groups with the groups having deficiency one presentations.

• Management & Information Systems Review
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• v.26
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• pp.293-309
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• 2008

This study examined the market efficiency of korea stock market by comparing variance ratios(VR) of stock groups which is sorted by market capitalization. We compute variance ratios of KOSPI large capitalization, midium capitalization, and small capitalization for 546 trading days from 2006/01/02 to 2008/04/15. For our study, we also use high frequency data that is; intra-day 1 minute data. The characteristics of variance ratios of stock groups by market capitalization as follows: From 1 to 5 minute interval, variance ratios of three stock group increase far from zero(0). The longer time interval, the more variance ratios decrease, but only large capitalization converge on around zero. This means that the market of large capitalization is more efficient compare to other stock groups. The entire sample period can be divided two sub-period because the impact of sub prime crisis arised from U.S.A. influences Korea stock market. Before sub prime crisis, the VRs of mid cap and small cap do not converge on around zero except large cap although the time interval is longer. After sub prime crisis, the VRs of three stock groups decrease when time interval is longer, but only large cap converge on around zero. We conclude that large cap is more efficient than other stock groups in Korea Stock Market.

• Journal of the Korean Operations Research and Management Science Society
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• v.38 no.4
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• pp.65-78
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• 2013

Circular shareholding is established when at least three member firms in a business group hold stock in other member firms and form a series of ownership in a circular way. Although there have been many studies which investigated a negative effect of circular shareholding on firm's value, few studies have discussed how to resolve the problem given complicated ownership structures of large business groups. This paper is based on a mixed integer programming model, which was proposed in the author's previous research and can identify the ownership share divested in order to resolve circular shareholding. Since the optimization model becomes too complicated for large business groups and requires a sophisticated software to solve it, we propose a simple heuristic method that can find a good approximate solution to the model. Its applications to twelve Korean large business groups show that the heuristic method is not just computationally attractive but also provides near-optimal solutions in most cases.

• The Korean Journal of Applied Statistics
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• v.18 no.2
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• pp.395-410
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• 2005

For the analysis of large tables formed by many categorical variables, we suggest a method to group the variables into several disjoint groups in which the variables are completely associated within the groups. We use a simple function of Kullback-Leibler divergence as a similarity measure to find the groups. Since the groups are complete hierarchical sets, we can identify the association structure of the large tables by the marginal log-linear models. Examples are introduced to illustrate the suggested method.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.76.1-76.1
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• 2014

We investigate the solar flare and CME occurrence rate and probability depending on sunspot class and its area change. These CMEs are front-side, partial and full halo CMEs associated with X-ray flares. For this we use the Solar Region Summary(SRS) from NOAA, NGDC flare catalog, and SOHO/LASCO CME catalog for 16 years (from January 1996 to December 2011). We classify each sunspot class into two sub-groups: "Large" and "Small". In addition, for each class, we classify it into three sub-groups according to sunspot class area change: "Decrease", "Steady", and "Increase". In terms of sunspot class area, the solar flare and CME occurrence probabilities noticeably increase at compact and large sunspot groups (e.g., 'Fkc'). In terms of sunspot area change, solar flare and CME occurrence probabilities for the "Increase" sub-groups are noticeably higher than those for the other sub-groups. For example, in case of the (M+X)-class flares of 'Dkc' class, the flare occurrence probability of the "Increase" sub-group is three times higher than that of the "Steady" sub-group. In case of the 'Eai' class, the CME occurrence probability of the "Increase" sub-groups is five time higher than that of the "Steady" sub-group. Our results demonstrate statistically that magnetic flux and its emergence enhance solar flare and CME occurrence, especially for compact and large sunspot groups.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.349-353
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• 2015

We use multi-wavelength observations of galaxy groups to probe the formation models for galaxy formation in cosmological simulations, statistically. The observations include Chandra and XMM-Newton X-ray observations, optical photometry and radio observations at 1.4 GHz and 610 MHz. Using a large sample of galaxy groups observed by the XMM-Newton X-ray telescope as part of the XMM-Large Scale Survey, we carried out a statistical study of the redshift evolution of the luminosity gap for a well defined mass-selected group sample and show the relative success of some of the semi-analytic models in reproducing the observed properties of galaxy groups up to redshift z ~ 1.2. The observed trend argues in favour of a stronger evolution of the feedback from active galactic nuclei at z < 1 compared to the models. The slope of the relation between the magnitude of the brightest cluster galaxy and the value of the luminosity gap does not evolve with redshift and is well reproduced by the models. We find that the radio power of giant elliptic galaxies residing in galaxy groups with a large luminosity gap are lower compared to giant ellipticals of the same stellar masses but in typical galaxy groups.

• Journal of The Korean Astronomical Society
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• v.48 no.1
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• pp.75-82
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• 2015

We use a volume-limited sample of quasars in the Sloan Digital Sky Survey (SDSS) DR7 quasar catalog to identify quasar groups and address their statistical significance. This quasar sample has a uniform selection function on the sky and nearly a maximum possible contiguous volume that can be drawn from the DR7 catalog. Quasar groups are identified by using the Friend-of-Friend algorithm with a set of fixed comoving linking lengths. We find that the richness distribution of the richest 100 quasar groups or the size distribution of the largest 100 groups are statistically equivalent with those of randomly-distributed points with the same number density and sky coverage when groups are identified with the linking length of $70h^{-1}Mpc$. It is shown that the large-scale structures like the huge Large Quasar Group (U1.27) reported by Clowes et al. (2013) can be found with high probability even if quasars have no physical clustering, and does not challenge the initially homogeneous cosmological models. Our results are statistically more reliable than those of Nadathur (2013), where the test was made only for the largest quasar group. It is shown that the linking length should be smaller than $50h^{-1}Mpc$ in order for the quasar groups identified in the DR7 catalog not to be dominated by associations of quasars grouped by chance. We present 20 richest quasar groups identified with the linking length of $70h^{-1}Mpc$ for further analyses.

• Journal of the Korean Operations Research and Management Science Society
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• v.34 no.4
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• pp.73-89
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• 2009

Circular shareholdings among three companies are formed when company A owns stock in company B, company B owns stock in company C, and company C owns stock in company A. Since circular shareholdings among large family-controlled firms are used to give the controlling shareholder greater control or more opportunities to expropriate minority investors, the government has encouraged large business groups to gradually remove their circular shareholdings. In this paper, we propose a combinatorial optimization model that can answer the question, which equity investments among complicated investment relationships of one large business group should be removed to resolve its circular shareholdings. To the best knowledge of the authors, our research is the first one that has approached the circular shareholding problem in respect of management science. The proposed combinatorial optimization model are formulated into integer programming problem and applied to some Korean major business groups.

• Journal for History of Mathematics
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• v.15 no.2
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• pp.125-134
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• 2002

The purpose of this paper is to analyze the the effect on the academic improvement, applying various teaching methods throughout cooperative teaming in small groups to inspire slow-learners'interest on mathematics and impove capability to solve problems and achievement degree of studies. The subject of the study is as follows: 1. What is the difference of students'interest on mathematics between cooperative learning in small groups and large group learning\ulcorner 2. Is there any difference of slow-learners' achievement degree of studies between cooperative learning in small groups and large group learning?

• Advanced Composite Materials
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• v.16 no.3
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• pp.245-258
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• 2007

The single-modal Weibull model has been assessed on Tyranno ZMI Si-Zr-C-O fiber if a set of shape and scale parameters accurately reproduced the effect of the size of the diameter on strength. The tensile data of a single fiber have been divided into two expedient groups as 'small diameter' group and 'large diameter' group in deriving the parameters, which should be consistent if the Weibull model accurately reproduced the size effect. However, the derived Weibull parameters were inconsistent between the two groups. Thereby the authors have concluded that the parameters of the single-modal Weibull model are dependent on the fiber diameter, so that the model is inadequate to reproduce the strength size effect. On the other hand, Weibull parameters were found consistent between the two groups by excluding the data of 'large mirror zone' sample, which was defined as the sample around 10% mirror zone area of the fracture surface. What is more, the exclusion reduced the strength variance more drastically in the 'large diameter' group than in the 'small diameter' group, even though the 'large mirror zone' samples were found identical in the percentage between the two groups. The authors therefore conclude that diameter limitation to the 'small diameter' group level can lead to drastically less distributed strength values than the estimated strength through the Weibull scaling on the present Tyranno ZMI Si-Zr-C-O fiber.