5,098 research outputs found
Elastic-Net Regularization in Learning Theory
Within the framework of statistical learning theory we analyze in detail the
so-called elastic-net regularization scheme proposed by Zou and Hastie for the
selection of groups of correlated variables. To investigate on the statistical
properties of this scheme and in particular on its consistency properties, we
set up a suitable mathematical framework. Our setting is random-design
regression where we allow the response variable to be vector-valued and we
consider prediction functions which are linear combination of elements ({\em
features}) in an infinite-dimensional dictionary. Under the assumption that the
regression function admits a sparse representation on the dictionary, we prove
that there exists a particular ``{\em elastic-net representation}'' of the
regression function such that, if the number of data increases, the elastic-net
estimator is consistent not only for prediction but also for variable/feature
selection. Our results include finite-sample bounds and an adaptive scheme to
select the regularization parameter. Moreover, using convex analysis tools, we
derive an iterative thresholding algorithm for computing the elastic-net
solution which is different from the optimization procedure originally proposed
by Zou and HastieComment: 32 pages, 3 figure
Sparse and stable Markowitz portfolios
We consider the problem of portfolio selection within the classical Markowitz
mean-variance framework, reformulated as a constrained least-squares regression
problem. We propose to add to the objective function a penalty proportional to
the sum of the absolute values of the portfolio weights. This penalty
regularizes (stabilizes) the optimization problem, encourages sparse portfolios
(i.e. portfolios with only few active positions), and allows to account for
transaction costs. Our approach recovers as special cases the
no-short-positions portfolios, but does allow for short positions in limited
number. We implement this methodology on two benchmark data sets constructed by
Fama and French. Using only a modest amount of training data, we construct
portfolios whose out-of-sample performance, as measured by Sharpe ratio, is
consistently and significantly better than that of the naive evenly-weighted
portfolio which constitutes, as shown in recent literature, a very tough
benchmark.Comment: Better emphasis of main result, new abstract, new examples and
figures. New appendix with full details of algorithm. 17 pages, 6 figure
A rapid procedure for the extraction of genomic DNA from intact Aspergillus spores
Genomic DNA of different species of Aspergillus was prepared from intact spores using the Nucleon MiY kit of Amersham. The method is rapid, does not involve mechanical disruption of the spores nor the use of phenol-chloroform extractions and yields DNA that is suitable for PCR amplification and Southern analysis. The method is also applicable to mycelium ground with glass beads
Migration aspirations and migration cultures:A case study of Ukrainian migration towards the European Union
An abundant body of research focused on macrolevel, mesolevel, and microlevel factors explaining why individuals move across international borders. In this paper, we aim to complement the existing literature by exploring how, within a single country, mesolevel factors differently impact migration aspirations, focusing on a case study of Ukraine. We particularly focus on how migration aspirations of individuals in two different regions can be explained by their international social networks with family members, on the one hand, and with friends, on the other. Furthermore, we explore whether regional migration characteristics play a role, as well as the interaction of such characteristics with individuals' frequency of contact with transnational networks. Our analyses are based on the EUMAGINE project and suggest that the interplay between regional migration characteristics and transnational social contact are key for explainingthe decline of migration systems over time
Magnetic monopole and string excitations in a two-dimensional spin ice
We study the magnetic excitations of a square lattice spin-ice recently
produced in an artificial form, as an array of nanoscale magnets. Our analysis,
based upon the dipolar interaction between the nanomagnetic islands, correctly
reproduces the ground-state observed experimentally. In addition, we find
magnetic monopole-like excitations effectively interacting by means of the
usual Coulombic plus a linear confining potential, the latter being related to
a string-like excitation binding the monopoles pairs, what indicates that the
fractionalization of magnetic dipoles may not be so easy in two dimensions.
These findings contrast this material with the three-dimensional analogue,
where such monopoles experience only the Coulombic interaction. We discuss,
however, two entropic effects that affect the monopole interactions: firstly,
the string configurational entropy may loose the string tension and then, free
magnetic monopoles should also be found in lower dimensional spin ices;
secondly, in contrast to the string configurational entropy, an entropically
driven Coulomb force, which increases with temperature, has the opposite effect
of confining the magnetic defects.Comment: 8 pages. Accepted by Journal of Applied Physics (2009
Destination choices of international students in the Netherlands:A mesoâlevel analysis of higher education institutions and cities
This study quantitatively investigates enrolments of international students using data that contains nearly every student in the Netherlands for the years 2016â2019. Using this data, we are able to perform a meso-level analysis where we could investigate the characteristics of higher education institutions (HEIs) and cities in international student mobility. This research contributes to the literature by studying variation between HEIs and by focusing on actual enrolments instead of relying on survey results. Such meso-level studies have thus far been very rare, especially on this scale. Although there are commonalities between types of degrees, we find mixed results for academic and city characteristics, and it appears that academic factors are more important for master students while for bachelor students city characteristics have stronger effects. The effect of having already existing stocks of international students appears to be important for all types of degrees. Aside from differences between bachelor and master students, our findings also suggest that HEIs might directly influence international student flows as this would explain some of our results. Although this study only focuses on the Netherlands, it opens up many avenues for future comparative research on the destination choices of international students and the role of HEIs
International student mobility and labour market outcomes: an investigation of the role of level of study, type of mobility, and international prestige hierarchies
Over the last decades, there has been increasing interest in the topic of international student mobility (ISM). However, there is surprisingly little analysis of the ways in which different characteristics and types of short-term ISM or the importance of host education systems and labour markets may affect early career outcomes of formerly mobile graduates. Therefore, in this study we explore, first, the relationship between participation in ISM at the Bachelor and Master level and graduatesâ wages and the duration of education-to-work transitions. Second, we investigate variations in ISMâs labour market outcomes according to the type of mobility: study, internships, or combinations of both. Third, we examine the relationship between labour market outcomes of formerly mobile students and the country of destinationâs position in higher education international prestige hierarchies and labour market competitiveness. We use the Dutch National Alumni Survey 2015, a representative survey of higher education graduates in the Netherlands, conducted 1.5 years after graduation. Before controlling for selection into ISM, the results suggest the existence of labour market returns to ISM and that the heterogeneity of ISM experiences matters, as labour market outcomes vary according to the level of study, the type of mobility and the positioning of the country of destination in international prestige hierarchies. However, after controlling for selection into ISM through propensity score matching, the differences in early career outcomes between formerly mobile and non-mobile graduates disappear, suggesting that they cannot be causally attributed to their ISM-experience. We explain these results with reference to the characteristics of the Dutch education system and labour market, where restricted possibilities for upward vertical mobility limit returns to ISM in the local labour market
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