Algebraically special solutions in AdS/CFT

We investigate the AdS/CFT interpretation of the class of algebraicallyspecial solutions of Einstein gravity with a negative cosmological constant.Such solutions describe a CFT living in a 2+1 dimensional time-dependentgeometry that, generically, has no isometries. The algebraically specialcondition implies that the expectation value of the CFT energy-momentum tensoris a local function of the boundary metric. When such a spacetime is slowlyvarying, the fluid/gravity approximation is valid and one can read off thevalues of certain higher order transport coefficients. To do this, we introducea formalism for studying conformal, relativistic fluids in 2+1 dimensions thatreduces everything to the manipulation of scalar quantities.

Observing the Sun with the Murchison Widefield Array

The Sun has remained a difficult source to image for radio telescopes,especially at the low radio frequencies. Its morphologically complex emissionfeatures span a large range of angular scales, emission mechanisms involved andbrightness temperatures. In addition, time and frequency synthesis, the keytool used by most radio interferometers to build up information about thesource being imaged is not effective for solar imaging, because many of thefeatures of interest are short lived and change dramatically over smallfractional bandwidths. Building on the advances in radio frequency technology, digital signalprocessing and computing, the kind of instruments needed to simultaneouslycapture the evolution of solar emission in time, frequency, morphology andpolarization over a large spectral span with the requisite imaging fidelity,and time and frequency resolution have only recently begun to appear. Of thisclass of instruments, the Murchison Widefield Array (MWA) is best suited forsolar observations. The MWA has now entered a routine observing phase and herewe present some early examples from MWA observations.

Counterexamples to $ C^{\infty} $ well posedness for some hyperbolic operators with triple characteristics

In this paper we prove that for a class of non-effectively hyperbolicoperators with smooth triple characteristics the Cauchy problem is well posedin the Gevrey 2 class, beyond the generic Gevrey class $ 3/2 $ (see e.g.\cite{Bro}). Moreover we show that this value is optimal.

Are news important to predict large losses?

In this paper we investigate the impact of news to predict extreme financialreturns using high frequency data. We consider several model specificationsdiffering for the dynamic property of the underlying stochastic process as wellas for the innovation process. Since news are essentially qualitative measures,they are firstly transformed into quantitative measures which are subsequentlyintroduced as exogenous regressors into the conditional volatility dynamics.Three basic sentiment indexes are constructed starting from three list of wordsdefined by historical market news response and by a discriminant analysis.Models are evaluated in terms of their predictive accuracy to forecastout-of-sample Value-at-Risk of the STOXX Europe 600 sectors at differentconfidence levels using several statistic tests and the Model Confidence Setprocedure of Hansen et al. (2011). Since the Hansen's procedure usuallydelivers a set of models having the same VaR predictive ability, we propose anew forecasting combination technique that dynamically weights the VaRpredictions obtained by the models belonging to the optimal final set. Ourresults confirms that the inclusion of exogenous information as well as theright specification of the returns' conditional distribution significantlydecrease the number of actual versus expected VaR violations towards one, asthis is especially true for higher confidence levels.

The Model Confidence Set package for R

This paper presents the R package MCS which implements the Model ConfidenceSet (MCS) procedure recently developed by Hansen et al. (2011). The Hansen'sprocedure consists on a sequence of tests which permits to construct a set of'superior' models, where the null hypothesis of Equal Predictive Ability (EPA)is not rejected at a certain confidence level. The EPA statistic tests iscalculated for an arbitrary loss function, meaning that we could test models onvarious aspects, for example punctual forecasts. The relevance of the packageis shown using an example which aims at illustrating in details the use of thefunctions provided by the package. The example compares the ability ofdifferent models belonging to the ARCH family to predict large financiallosses. We also discuss the implementation of the ARCH--type models and theirmaximum likelihood estimation using the popular R package rugarch developed byGhalanos (2014).

Extracting the Italian output gap: a Bayesian approach

During the last decades particular effort has been directed towardsunderstanding and predicting the relevant state of the business cycle with theobjective of decomposing permanent shocks from those having only a transitoryimpact on real output. This trend--cycle decomposition has a relevant impact onseveral economic and fiscal variables and constitutes by itself an importantindicator for policy purposes. This paper deals with trend--cycle decompositionfor the Italian economy having some interesting peculiarities which makes itattractive to analyse from both a statistic and an historical perspective. Wepropose an univariate model for the quarterly real GDP, subsequently extendedto include the price dynamics through a Phillips curve. This study considers aseries of the Italian quarterly real GDP recently released by OECD whichincludes both the 1960s and the recent global financial crisis of 2007--2008.Parameters estimate as well as the signal extraction are performed within theBayesian paradigm which effectively handles complex models where the parametersenter the log--likelihood function in a strongly nonlinear way. A new AdaptiveIndependent Metropolis--within--Gibbs sampler is then developed to efficientlysimulate the parameters of the unobserved cycle. Our results suggest thatinflation influences the Output Gap estimate, making the extracted Italian OGan important indicator of inflation pressures on the real side of the economy,as stated by the Phillips theory. Moreover, our estimate of the sequence ofpeaks and troughs of the Output Gap is in line with the OECD official dating ofthe Italian business cycle.

A general notion of activity for the Tutte polynomial

In the literature can be found several descriptions of the Tutte polynomialof graphs. Tutte defined it thanks to a notion of activity based on an orderingof the edges. Thereafter, Bernardi gave a non-equivalent notion of the activitywhere the graph is embedded in a surface. In this paper, we see that othernotions of activity can thus be imagined and they can all be embodied in a samenotion, the $\Delta$-activity. We develop a short theory which sheds light onthe connections between the different expressions of the Tutte polynomial.

Towards a q-analogue of the Harer-Zagier formula via rook placements

In 1986 Harer and Zagier computed a certain matrix integral to determine aninfluential closed-form formula for the number of (orientable) one-face maps onn vertices colored from N colors. Kerov (1997) provided a proof which computedthe same matrix integral differently, which gave an interpretation of thesenumbers as also counting the number of placements of non-attacking rooks onYoung diagrams. Bernardi (2010) provided a bijective proof of this formula byputting one-face maps in bijection with tree-rooted maps, which are orientablemaps with a designated spanning tree. In the first part of the paper, weexplore the connection between these rook placements and tree-rooted maps bydeveloping a bijection between these objects. Rook placements on Young diagramshave a q-analogue due to Garsia and Remmel (1986). In the second part of thepaper, we propose a statistic on rook placements that leads to a conjecturedidentity which is a q-analogue of part of the Harer-Zagier formula. Thisidentity is also expressed in terms of moments of orthogonal polynomials whichare rescaling of q-Hermite polynomials. We then use these moments to give arecurrence for the proposed q-analogue.

Cluster Radio Halos at the crossroads between astrophysics and cosmology in the SKA era

Giant Radio Halos (RH) are diffuse, Mpc-sized, synchrotron radio sourcesobserved in a fraction of merging galaxy clusters. The current scenario for theorigin of RHs assumes that turbulence generated during cluster mergersre-accelerates pre-existing fossil and/or secondary electrons in theintra-cluster-medium (ICM) to the energies necessary to produce the observedradio emission. Moreover, more relaxed clusters could host diffuse "off state"halos produced by secondary electrons. In this Chapter we use Monte Carlosimulations, that combine turbulent-acceleration physics and the generation ofsecondaries in the ICM, to calculate the occurrence of RHs in the Universe,their spectral properties and connection with properties of the hostingclusters. Predictions for SKA1 surveys are presented at low (100-300 MHz) andmid (1-2 GHz) frequencies assuming the expected sensitivities and spatialresolutions of SKA1. SKA1 will step into an unexplored territory allowing us tostudy the formation and evolution of RHs in a totally new range of clustermasses and redshift, allowing firm tests of the current theoretical hypothesis.In particular, the combination of SKA1-LOW and SUR will allow the discovery of~1000 ultrasteep- spectrum halos and to detect for the very first time "offstate" RHs. We expect that at least ~2500 giant RHs will be discovered bySKA1-LOW surveys up to z~0.6. Remarkably these surveys will be sensitive to RHsin a cluster mass range (down to ~10^14 solar masses) and redshifts (up to ~1)that are unexplored by current observations. SKA1 surveys will be highlycompetitive with present and future SZ-surveys in the detection ofhigh-redshift massive objects.

Ideal characteristics of the dental surgeon in the family health strategy