1,787 research outputs found
Cosmic Sculpture: A new way to visualise the Cosmic Microwave Background
3D printing presents an attractive alternative to visual representation of
physical datasets such as astronomical images that can be used for research,
outreach or teaching purposes, and is especially relevant to people with a
visual disability. We here report the use of 3D printing technology to produce
a representation of the all-sky Cosmic Microwave Background (CMB) intensity
anisotropy maps produced by the Planck mission. The success of this work in
representing key features of the CMB is discussed as is the potential of this
approach for representing other astrophysical data sets. 3D printing such
datasets represents a highly complementary approach to the usual 2D projections
used in teaching and outreach work, and can also form the basis of
undergraduate projects. The CAD files used to produce the models discussed in
this paper are made available.Comment: Accepted for publication in the European Journal of Physic
Chaotic Explosions
We investigate chaotic dynamical systems for which the intensity of
trajectories might grow unlimited in time. We show that (i) the intensity grows
exponentially in time and is distributed spatially according to a fractal
measure with an information dimension smaller than that of the phase space,(ii)
such exploding cases can be described by an operator formalism similar to the
one applied to chaotic systems with absorption (decaying intensities), but
(iii) the invariant quantities characterizing explosion and absorption are
typically not directly related to each other, e.g., the decay rate and fractal
dimensions of absorbing maps typically differ from the ones computed in the
corresponding inverse (exploding) maps. We illustrate our general results
through numerical simulation in the cardioid billiard mimicking a lasing
optical cavity, and through analytical calculations in the baker map.Comment: 7 pages, 5 figure
Success determinants of small and medium enterprises: The case of Portugal
The success determinants of Portuguese small and medium enterprises (SMEs) are examined
using a sample of 207 Portuguese firms from several industries listed in the Dun & Bradstreet
database that are at least five years old and with ten or more employees. The empirical study is
based on the Lussier (1995) success versus failure prediction model. Our results do not fully
support the international research on the determinants of SMEs’ success. The determinants of
Portuguese SMEs’ success are: capital, record-keeping and financial control, planning,
professional advice, age of owner, and marketing skills. Using logistic regression, the model
adequately fits the data, and accurately predicts 43.4 percent of the failed businesses and 87.0
percent of the successful businesses for an overall accuracy rate of 73.2 percent.info:eu-repo/semantics/acceptedVersio
Recreational, Cultural and Aesthetic Services from Estuarine and Coastal Ecosystems
The role of economic analysis in guiding the sustainable development of estuarine and coastal ecosystems is investigated based on a comprehensive review of the literature on the valuation of the recreation, cultural and aesthetic services. The implications of the findings for the sustainable management of coral reefs, Marine Protected Areas, and Small Island Developing States are discussed. Finally, the potential of meta-analytical benefit transfer and scaling up of values at various aggregation levels is demonstrated in the context of coastal tourism and recreation in Europe. The results of the study support the conclusion that the non-material values provided by coastal and estuarine ecosystems in terms of recreational, cultural and aesthetic services represent a substantial component of human well-being.Aesthetic Values, Coastal Recreation, Coral Reefs, Cultural Values, Ecosystem Services Valuation, Ecosystem Services, Estuarine Ecosystems, Marine Protected Areas, Non-market Valuation, Non-use Values, Passive Values, Recreational Fishing, Small Island Developing States, Spiritual and Religious Values.
Detecting customer defections: an application of continuous duration models
The considerable increase of business competition in the Portuguese fixed telecommunications industry for the last decades has given rise to a phenomenon of customer defection, which has serious consequences for the business financial performance and, therefore, for the economy. As such, researchers have recognised the importance of an in-depth study of customer defection in different industries and geographic locations. This study aims to understand and predict customer lifetime in a contractual setting in order to improve the practice of customer portfolio management. A duration model is developed to understand and predict the residential customer defection in the fixed telecommunications industry in Portugal. The models are developed by using large-scale data from an internal database of a Portuguese company which presents bundled offers of ADSL, fixed line telephone, pay-TV and home-video. The model is estimated with a large number of covariates, which includes customer’s basic information, demographics, churn flag, customer historical information about usage, billing, subscription, credit, and other. The results of this study are very useful to the computation of the customer lifetime value
Estimating a customer churn model in the ADSL industry in Portugal: The use of a Semi-Markov model
Customer churn has been stated as one of the main reasons of profitability losses in the
telecommunications industry. As such, it seems critical to have an a priori knowledge about the risk
of a given customer to churn at any moment, in order to take preventive measures to avoid the
defection of potentially profitable customers. This study intends to develop a duration model of the
residential customer churn in this industry in Portugal. We found empirical evidence that the
variables that influence customer churn are the total number of overdue bills since ever, average
monthly spending, average value of additional internet traffic, payment method, equipment renting,
and the subscription of a flat plan. We also found that the probability of a customer to churn is
neither constant over time nor across customers.info:eu-repo/semantics/publishedVersio
On the use of discounted cash flow method on the customer valuation
The Discounted Cash Flow Method has been widely argued as the best method to asset valuation. This article is about the valuation of customers. The use of historic customer profitability and the Discounted Cash Flow Method to customer valuation are discussed. Moreover, the components of customer lifetime value is presented and described
Performance assessment of portuguese secondary schools: the society and educational authorities perspectives
This paper describes a performance assessment of Portuguese secondary
schools using data envelopment analysis (DEA). The assessment adopts two
perspectives: a society perspective where schools are viewed as promoting
students achievement given student characteristics in terms of academic
abilities and socio-economic background; and an educational authorities
perspective where schools are viewed as transforming a set of resources
(including student academic abilities, socio-economic background, and also
school resources, such as teachers) into students' achievement. Two types of
DEA analysis are performed for each perspective: one using an output oriented
model allowing factor weights to vary freely from school to school and another
using a model that restricts factor weights to be equal for all schools. The first
model is well suited for identifying inefficient schools, whereas the latter is
best suited for improving discrimination between efficient schools when
pursuing the identification of benchmarks. Our sample comprised a small
number of schools and therefore the results obtained are mainly illustrative of
the potential of the method. Future research will explore the use of this method
for the assessment of schools at a national leve
Fast algorithm for topologically disordered lattices with constant coordination number
We present a stochastic algorithm for constructing a topologically disordered
(i.e., non-regular) spatial lattice with nodes of constant coordination number,
the CC lattice. The construction procedure dramatically improves on an earlier
proposal [Phys. Rev. E. 97, 022144 (2018)] with respect to both computational
complexity and finite-size scaling properties - making the CC lattice an
alternative to proximity graphs which, especially in higher dimensions, is
significantly faster to build. Among other applications, physical systems such
as certain amorphous materials with low concentration of coordination defects
are an important example of disordered, constant-coordination lattices in
nature. As a concrete application, we characterize the criticality of the 3D
Ising model on the CC lattice. We find that its phase transition belongs to the
clean Ising universality class, establishing that the disorder present in the
CC lattice is a non-relevant perturbation in the sense of renormalization group
theory.Comment: 9 pages, 8 figure
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