1,044 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
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)
Dungâvisiting beetle diversity is mainly affected by land use, while community specialization is driven by climate
Dung beetles are important actors in the selfâregulation 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 spaceâforâtime approach to test the effects of land use and climate on αâdiversity, local community specialization (H (2)âČ) on dung resources, and Îłâdiversity of dungâvisiting beetles. For this, we used pitfall traps baited with four different dung types at 115 study sites, distributed over a spatial extent of 300âkmâĂâ300âkm and 1000âm in elevation. Study sites were established in four local landâuse types: forests, grasslands, arable sites, and settlements, embedded in nearânatural, agricultural, or urban landscapes. Our results show that abundance and species density of dungâvisiting beetles were negatively affected by agricultural land use at both spatial scales, whereas Îłâdiversity at the local scale was negatively affected by settlements and on a landscape scale equally by agricultural and urban land use. Increasing precipitation diminished dungâvisiting beetle abundance, and higher temperatures reduced community specialization on dung types and Îłâdiversity. These results indicate that intensive land use and high temperatures may cause a loss in dungâvisiting beetle diversity and alter community networks. A decrease in dungâvisiting 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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