1,156 research outputs found
A Mathematical Model for the Origin of Name Brands and Generics
Firms in the U.S. spend over 200 billion dollars each year advertising their
products to consumers, around one percent of the country's gross domestic
product. It is of great interest to understand how that aggregate expenditure
affects prices, market efficiency, and overall welfare. Here, we present a
mathematical model for the dynamics of competition through advertising and find
a surprising prediction: when advertising is relatively cheap compared to the
maximum benefit advertising offers, rational firms split into two groups, one
with significantly less advertising (a "generic" group) and one with
significantly more advertising (a "name brand" group). Our model predicts that
this segmentation will also be reflected in price distributions; we use large
consumer data sets to test this prediction and find good qualitative agreement.Comment: 24 pages, 11 figure
Ideas previas acerca de la mecánica clásica de los ingresantes de Ingenierías y Física de la Universidad Nacional del Sur
Este trabajo de corte cuantitativo se propuso describir y comparar las ideas previas de los estudiantes del Curso de Nivelación en Física de la Universidad Nacional del Sur. Para ello se aplicó el Cuestionario sobre el Concepto de Fuerza (elaborado por David Hestenes en 1985) en su modalidad pretest y postest a una muestra significativa del estudiantado del curso a inicios de 2021, en el contexto de educación remota por aislamiento social y preventivo. A partir de los resultados del mismo se estudió el nivel de conceptualización newtoniano y su aprendizaje. También se analizó la frecuencia de aparición de ideas previas no-newtonianas y la correlación de los resultados del cuestionario con el rendimiento académico. Se encontró que los estudiantes respondieron en promedio un 27 % de respuestas correctas en la encuesta previa, y un 38 % en la encuesta posterior, obteniendo una ganancia de 16 %. Por otra parte, se concluyó que existe una correlación positiva débil (R=0,25) entre el rendimiento académico y el conocimiento conceptual final de la mecánica clásica
Holography and Defect Conformal Field Theories
We develop both the gravity and field theory sides of the Karch-Randall
conjecture that the near-horizon description of a certain D5-D3 brane
configuration in string theory, realized as AdS_5 x S^5 bisected by an AdS_4 x
S^2 "brane", is dual to N=4 Super Yang-Mills theory in R^4 coupled to an R^3
defect. We propose a complete Lagrangian for the field theory dual, a novel
"defect superconformal field theory" wherein a subset of the fields of N=4 SYM
interacts with a d=3 SU(N) fundamental hypermultiplet on the defect preserving
conformal invariance and 8 supercharges. The Kaluza-Klein reduction of wrapped
D5 modes on AdS_4 x S^2 leads to towers of short representations of OSp(4|4),
and we construct the map to a set of dual gauge-invariant defect operators O_3
possessing integer conformal dimensions. Gravity calculations of and
are given. Spacetime and N-dependence matches expectations from dCFT,
while the behavior as functions of lambda = g^2 N at strong and weak coupling
is generically different. We comment on a class of correlators for which a
non-renormalization theorem may still exist. Partial evidence for the
conformality of the quantum theory is given, including a complete argument for
the special case of a U(1) gauge group. Some weak coupling arguments which
illuminate the duality are presented.Comment: 47 pages, LaTeX, 2 figures, feynmf. v2: fixed minor errors, added
references. v3: fixed more typo
Master crossover behavior of parachor correlations for one-component fluids
The master asymptotic behavior of the usual parachor correlations, expressing
surface tension as a power law of the density difference
between coexisting liquid and vapor, is analyzed for a
series of pure compounds close to their liquid-vapor critical point, using only
four critical parameters , , and ,
for each fluid.
... The main consequences of these theoretical estimations are discussed in
the light of engineering applications and process simulations where parachor
correlations constitute one of the most practical method for estimating surface
tension from density and capillary rise measurements
Systematic Overestimation of Machine Learning Performance in Neuroimaging Studies of Depression
We currently observe a disconcerting phenomenon in machine learning studies
in psychiatry: While we would expect larger samples to yield better results due
to the availability of more data, larger machine learning studies consistently
show much weaker performance than the numerous small-scale studies. Here, we
systematically investigated this effect focusing on one of the most heavily
studied questions in the field, namely the classification of patients suffering
from Major Depressive Disorder (MDD) and healthy controls. Drawing upon a
balanced sample of MDD patients and healthy controls from our
recent international Predictive Analytics Competition (PAC), we first trained
and tested a classification model on the full dataset which yielded an accuracy
of 61%. Next, we mimicked the process by which researchers would draw samples
of various sizes ( to ) from the population and showed a strong
risk of overestimation. Specifically, for small sample sizes (), we
observe accuracies of up to 95%. For medium sample sizes () accuracies
up to 75% were found. Importantly, further investigation showed that
sufficiently large test sets effectively protect against performance
overestimation whereas larger datasets per se do not. While these results
question the validity of a substantial part of the current literature, we
outline the relatively low-cost remedy of larger test sets
Adrenocortical tumours in children and adolescents: The EXPeRT/PARTNER diagnostic and therapeutic recommendations
Adrenocortical tumours (ACTs) are rare during childhood. A complete surgical resection provides the best chance of cure, but the role and efficacy of the adjuvant therapy are still controversial. Various histologic criteria of malignancy for ACTs adopted in children do not facilitate comparative studies and are not completely shared. Therefore, a sharp demarcation between benign and malignant lesions has not been recognised, making it difficult to identify who potentially needs perioperative therapy. This manuscript presents the internationally harmonised recommendations for the diagnosis and treatment of ACTs in children and adolescents, established by the European Cooperative Study Group for Paediatric Rare Tumours (EXPeRT) group within the EU-funded project PARTNER (Paediatric Rare Tumours Network - European Registry)
Diversity and specialization responses to climate and land use differ between deadwood fungi and bacteria
Climate and land use are major determinants of biodiversity, and declines in species richness in cold and human exploited landscapes can be caused by lower rates of biotic interactions. Deadwood fungi and bacteria interact strongly with their hosts due to long-lasting evolutionary trajectories. However, how rates of biotic interactions (specialization) change with temperature and land-use intensity are unknown for both microbial groups. We hypothesize a decrease in species richness and specialization of communities with decreasing temperature and increasing land use intensity while controlling for precipitation. We used a full-factorial nested design to disentangle land use at habitat and landscape scale and temperature spanning an area of 300 × 300 km in Germany. We exposed four deadwood objects representing the main tree species in Central Europe (beech, oak, spruce, pine) in 175 study plots. Overall, we found that fungal and bacterial richness, community composition and specialization were weakly related to temperature and land use. Fungal richness was slightly higher in near-natural than in urban landscapes. Bacterial richness was positively associated with mean annual temperature, negatively associated with local temperature and highest in grassland habitats. Bacterial richness was positively related to the covariate mean annual precipitation. We found strong effects of host-tree identity on species richness and community composition. A generally high level of fungal host-tree specialization might explain the weak response to temperature and land use. Effects of host-tree identity and specialization were more pronounced in fungi. We suggest that host tree changes caused by land use and climate change will be more important for fungal communities, while changes in climate will affect bacterial communities more directly. Contrasting responses of the two taxonomic groups suggest a reorganization of deadwood microbial communities, which might have further consequences on diversity and decomposition in the Anthropocene
Dung‐visiting beetle diversity is mainly affected by land use, while community specialization is driven by climate
Dung beetles are important actors in the self‐regulation of ecosystems by driving nutrient cycling, bioturbation, and pest suppression. Urbanization and the sprawl of agricultural areas, however, destroy natural habitats and may threaten dung beetle diversity. In addition, climate change may cause shifts in geographical distribution and community composition. We used a space‐for‐time approach to test the effects of land use and climate on α‐diversity, local community specialization (H (2)′) on dung resources, and γ‐diversity of dung‐visiting beetles. For this, we used pitfall traps baited with four different dung types at 115 study sites, distributed over a spatial extent of 300 km × 300 km and 1000 m in elevation. Study sites were established in four local land‐use types: forests, grasslands, arable sites, and settlements, embedded in near‐natural, agricultural, or urban landscapes. Our results show that abundance and species density of dung‐visiting beetles were negatively affected by agricultural land use at both spatial scales, whereas γ‐diversity at the local scale was negatively affected by settlements and on a landscape scale equally by agricultural and urban land use. Increasing precipitation diminished dung‐visiting beetle abundance, and higher temperatures reduced community specialization on dung types and γ‐diversity. These results indicate that intensive land use and high temperatures may cause a loss in dung‐visiting beetle diversity and alter community networks. A decrease in dung‐visiting beetle diversity may disturb decomposition processes at both local and landscape scales and alter ecosystem functioning, which may lead to drastic ecological and economic damage
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