Risk Minimization through Portfolio Replication

We use a replica approach to deal with portfolio optimization problems. A given risk measure is minimized using empirical estimates of asset values correlations. We study the phase transition which happens when the time series is too short with respect to the size of the portfolio. We also study the noise sensitivity of portfolio allocation when this transition is approached. We consider explicitely the cases where the absolute deviation and the conditional value-at-risk are chosen as a risk measure. We show how the replica method can study a wide range of risk measures, and deal with various types of time series correlations, including realistic ones with volatility clustering.Comment: 12 pages, APFA5 conferenc

Thermal correlators of anyons in two dimensions

The anyon fields have trivial $\alpha$-commutator for $\alpha$ not integer. For integer $\alpha$ the commutators become temperature-dependent operator valued distributions. The $n$-point functions do not factorize as for quasifree states.Comment: 14 pages, LaTeX (misprints corrected, a reference added

El protagonismo de Hispania en la consolidación de la Primacía Petrina. A propósito de dos libros recientes

This article deals with the critical analysis of two recent books of a very different nature and format, starting at the common perspective of the universal primacy of the bishop of Rome. The work by A. Ferreiro sets out to demonstrate that in Roman and Visigothic Hispania the papal primacy was accepted by the Hispanic episcopate from the 3rd to the 7th centuries without great opposition, although there were moments of friction. The book by J. Vilella, a prosopographical study of the figure of Osius of Cordoba, highlights the leading role of the Hispanic bishop in the Council of Sardica in 343, considered to be the first step towards the rupture of the Eastern and Western Churches.En el presente artículo se aborda el análisis crítico de dos libros recientes, de naturaleza y formato muy diferentes, desde la perspectiva común del Primado universal del obispo de Roma. La obra de A. Ferreiro se propone demostrar que en la Hispania romana y visigoda el primado pontificio fue aceptado desde los siglos III al VII sin gran oposición por el episcopado hispano, aunque no faltaran momentos de fricción. En el libro de J. Vilella, un estudio prosopográfico de la figura de Osio de Córdoba, se resalta el protagonismo del obispo hispano en el concilio de Sárdica del 343 considerado como el primer paso hacia la ruptura de las Iglesias de Oriente y Occidente

Mathematical structure of the temporal gauge

The mathematical structure of the temporal gauge of QED is critically examined in both the alternative formulations characterized by either positivity or regularity of the Weyl algebra. The conflict between time translation invariance and Gauss law constraint is shown to lead to peculiar features. In the positive case only the correlations of exponentials of fields exist (non regularity), the space translations are not strongly continuous, so that their generators do not exist, a theta vacuum degeneracy occurs, associated to a spontaneous symmetry breaking. In the indefinite case the spectral condition only holds in terms of positivity of the energy, gauge invariant theta-vacua exist on the observables, with no extension to time translation invariant states on the field algebra, the vacuum is faithful on the longitudinal algebra and a KMS structure emerges. Functional integral representations are derived in both cases, with the alternative between ergodic measures on real random fields or complex Gaussian random fields.Comment: Late

Misinformation on Misinformation: Conceptual and Methodological Challenges

Copyright © 2021 The Author(s). Alarmist narratives about online misinformation continue to gain traction despite evidence that its prevalence and impact are overstated. Drawing on research questioning the use of big data in social science and reception studies, we identify six misconceptions about misinformation and examine the conceptual and methodological challenges they raise. The first three misconceptions concern the prevalence and circulation of misinformation. First, the internet is not rife with misinformation or news, but with memes and entertaining content. Second, scientists focus on social media because it is methodologically convenient, but misinformation is not just a social media problem. Third, falsehoods do not spread faster than the truth; how we define (mis)information influences our results and their practical implications. The second three misconceptions concern the impact and the reception of misinformation. First, people do not believe everything they see on the internet: sheer volume of engagement should not be conflated with belief. Second, the influence of misinformation on people’s behavior is overblown since it often ‘preaches to the choir’. Third, people are more likely to be uninformed than misinformed; surveys overestimate misperceptions and say little about the causal influence of misinformation. To appropriately understand and fight misinformation, future research needs to address these challenges

El Concilio de Nicea y los inicios de una nueva relación entre el poder imperial y la jerarquía eclesiástica

Nicaea (325) and Chalcedon (451) are the two most significant councils in the History of the ancient Church for their theological definitions of Trinitarian dogma. However, Nicaea is the worst known of all the ecumenical councils because the official Acts have not been preserved, if they existed at all, and the most relevant historical source, the Vita Constantini by Eusebius of Caesarea, is not very objective due to the ideology of the author and his involvement in the council itself. All this has conditioned our knowledge, both of the development of the assembly and of the role played by the bishops. This should be interpreted, as this article intends to do, in the light of the new relations that were established between Constantine and the Christian episcopate before the synod (after the emperor’s entry into Rome in 311) and the leading role assigned to the bishops in the administration of the Empire.Nicea (325) y Calcedonia (451), son los dos concilios más importantes en la historia de la Iglesia por sus definiciones teológicas del dogma trinitario. Sin embargo, es el peor conocido de todos los ecuménicos porque no se han conservado, si es que existieron, las Actas oficiales, y la fuente histórica más importante, la Vita Constantini de Eusebio de Cesarea es muy poco objetiva por la ideología del autor y su protagonismo en el propio concilio. Todo ello ha condicionado el conocimiento, tanto del desarrollo de la asamblea como del papel desempeñado por los obispos. Este debe ser interpretado, tal como se pretende en el presente artículo, en función de las nuevas relaciones que se establecieron entre Constantino y el episcopado cristiano antes del propio sínodo desde la entrada del emperador en Roma en el 311 y el protagonismo que se adjudicó a los obispos en la administración del Imperio

On the Form Factors of Relevant Operators and their Cluster Property

We compute the Form Factors of the relevant scaling operators in a class of integrable models without internal symmetries by exploiting their cluster properties. Their identification is established by computing the corresponding anomalous dimensions by means of Delfino--Simonetti--Cardy sum--rule and further confirmed by comparing some universal ratios of the nearby non--integrable quantum field theories with their independent numerical determination.Comment: Latex file, 35 pages with 5 Postscript figure

Social facilitation on the development of foraging behaviors in a population of autonomous robots

Abstract. In this paper we propose an adaptive algorithm based on a combination of selective reproduction, individual learning, and social learning. Social learning consists of a simple facilitation process that regulates the strength of individual learning on the basis of the number of individuals located nearby. By testing this model in an experimental scenario, in which a population of 10 mobile robots has to develop a simple foraging behavior, we demonstrate how the model proposed produces effective results. By comparing the results obtained in different experimental conditions we also show how the method proposed outperforms other alternative algorithms based on genetic evolution or individual learning. Finally, we briefly discuss how the model proposed can help us to understand the role of social learning in biological organisms

Research note: Fighting misinformation or fighting for information?

Copyright © 2022 The Author(s). A wealth of interventions have been devised to reduce belief in fake news or the tendency to share such news. By contrast, interventions aimed at increasing trust in reliable news sources have received less attention. In this article we show that, given the very limited prevalence of misinformation (including fake news), interventions aimed at reducing acceptance or spread of such news are bound to have very small effects on the overall quality of the information environment, especially compared to interventions aimed at increasing trust in reliable news sources. To make this argument, we simulate the effect that such interventions have on a global information score, which increases when people accept reliable information and decreases when people accept misinformation.Agence nationale de la recherche, grants ANR-17-EURE-0017, FrontCog, ANR-10-IDEX-0001-02 PSL, and ANR-21-CE28-0016-01