Efficient Marginal Likelihood Computation for Gaussian Process Regression

In a Bayesian learning setting, the posterior distribution of a predictive model arises from a trade-off between its prior distribution and the conditional likelihood of observed data. Such distribution functions usually rely on additional hyperparameters which need to be tuned in order to achieve optimum predictive performance; this operation can be efficiently performed in an Empirical Bayes fashion by maximizing the posterior marginal likelihood of the observed data. Since the score function of this optimization problem is in general characterized by the presence of local optima, it is necessary to resort to global optimization strategies, which require a large number of function evaluations. Given that the evaluation is usually computationally intensive and badly scaled with respect to the dataset size, the maximum number of observations that can be treated simultaneously is quite limited. In this paper, we consider the case of hyperparameter tuning in Gaussian process regression. A straightforward implementation of the posterior log-likelihood for this model requires O(N^3) operations for every iteration of the optimization procedure, where N is the number of examples in the input dataset. We derive a novel set of identities that allow, after an initial overhead of O(N^3), the evaluation of the score function, as well as the Jacobian and Hessian matrices, in O(N) operations. We prove how the proposed identities, that follow from the eigendecomposition of the kernel matrix, yield a reduction of several orders of magnitude in the computation time for the hyperparameter optimization problem. Notably, the proposed solution provides computational advantages even with respect to state of the art approximations that rely on sparse kernel matrices.Comment: 20 pages, 3 figure

I monasteri delle clarisse a Cagliari e Oristano (secoli XIV-XVI). Fondazione, ruolo sociale, patrimonio artistico

ITALIANO: L’articolo si focalizza sui monasteri di clarisse fondati tra XIV e XVI secolo nelle città di Cagliari e di Oristano: la prima capitale del regno di Sardegna catalano-aragonese poi spagnolo, la seconda capitale del regno o “giudicato” di Arborea, inglobato nel regno di Sardegna a partire dal 1420. Queste due città furono le prime, nell’isola, ad avere un monastero di clarisse. Di questi monasteri l’articolo ripercorre le origini, le principali vicende e il ruolo sociale. Si sofferma, inoltre, sul superstite patrimonio artistico medievale conservato nella chiesa clariana di Oristano e sulle architetture di epoca moderna dei monasteri cagliaritani. / ENGLISH: The article focuses on the monasteries of Poor Clares founded between the 14th and 16th centuries in the cities of Cagliari and Oristano: the first one was the capital of the catalan-aragonese Kingdom of Sardinia, the second one was the capital of the kingdom or “giudicato” of Arborea, incorporated into the Kingdom of Sardinia in 1420. These two cities were the first, in the island, to have a monastery of Poor Clares. The article traces the origins of these nunneries, the main events and their social role. It focuses also on the surviving medieval heritage in the Clarian church of Oristano and on the modern age architecture of the nunneries in Cagliari

Ricostruire il paesaggio storico e la memoria dei luoghi. Le opere difensive nell’agro meridionale di Cagliari attraverso una relazione descrittiva del 1707

On 1st August, 1707, the Patrimonial Council of the Kingdom of Sardinia approved the contents of the report drawn up by Don Manuel Bellejo, Prince of Tapia, regarding the interventions to be carried out in the defensive works ion land to the south of Cagliari. The so-called War of Succession to the Throne of Spain, fought by land, sea and on several continents, put a strain on the military sector of the various states. The port cities assumed strategic importance, as evidenced by the investments made in strengthening the strongholds of the Kingdom. The danger of invasion of Sardinia was  evident in the report of Don Manuel, who identified a weak point of Cagliari in the territory between the promontory of Sant’Elia and the Villanova district. With an ideal bird’s eye-view , Bellejo accompanies the reader through the anti-invasion garrisons, ringing places, natural features, and  paths, illustrating the defensive works inspected or necessary. The wealth of details allows to relocate the ancient garrisons on historical and current cartography and to reconstruct the conformation of the territory. For this purpose, the maps and military projects produced between the 16th and 18th centuries are fundamental aids. The comparison between ancient maps and today’s aerial photogrammetry provides solid tools for the study of the document, which opens an interesting window on the Cagliari of the time: a short, but intense historical phase that narrowly precedes the great transformations of the Savoy era.Il 1 agosto 1707, il Consiglio Patrimoniale del Regno di Sardegna approva i contenuti della relazione redatta da don Manuel Bellejo, principe di Tapia, riguardante gli interventi da realizzarsi nelle opere difensive a meridione di Cagliari. La cosiddetta Guerra di Successione al Trono di Spagna, combattuta per terra, per mare e su più continenti, mette a dura prova il comparto militare dei vari stati; le città portuali assumono importanza strategica, come testimoniano gli investimenti destinati anche alle piazzeforti del regno sardo.Il pericolo di invasione della Sardegna traspare con chiarezza dalla relazione di don Manuel, il quale individua nel territorio compreso tra il promontorio di Sant’Elia e il quartiere Villanova un punto nevralgico di Cagliari. Con un ideale ‘volo d’uccello’, Bellejo accompagna il lettore attraverso i presidi antisbarco, inanellando luoghi, emergenze naturali, percorsi ed illustrando le opere difensive ispezionate o da realizzare. La dovizia di dettagli consente di ricollocare gli antichi presidi sulla cartografia storica e attuale e di ricostruire la conformazione del territorio. A tal fine, sono di fondamentale ausilio le mappe e i progetti militari prodotti tra Sei e Settecento. Il confronto tra le cartografie antiche e le aerofotogrammetrie attuali fornisce solidi strumenti per lo studio del documento, il quale apre una interessante finestra sulla Cagliari dell’epoca: una breve, ma intensa fase storica che precede, di poco, le grandi trasformazioni di epoca sabauda

Numerical investigation on the use of Dimethyl Ether (DME) as an alternative fuel for compression-ignition engines

Dimethyl Ether (DME) is an oxygenated fuel that could favour the transition of the heavy-duty transportation sector to carbon neutrality thanks to its similarities in terms of thermophysical properties with diesel fuel, which will facilitate the retrofitting of existing architectures, and the possibility to achieve good trade-offs between NOx emissions, soot formation and overall combustion efficiency. The possibility of producing it from a multitude of carbon-neutral sources and the low hydrogen-to-carbon ratio would allow for an overall lower CO2 output, making an attractive option in limiting the global warming impact of the heavy-duty transportation sector. In the present work, a numerical analysis of the combustion process of DME is carried out. First, the numerical setup is validated against experimental data available for a constant volume vessel with an initial density of 14.8 kg/m3, discussing the capabilities of a chemistry-based combustion model using tabulated kinetics of homogeneous reactors: the Tabulated Well Mixed (TWM) model. Ignition delay times (IDT) are compared for a wide range of temperatures, from 750 K to 1100 K, and oxygen concentrations, from 15% to 21%. The same setup is then applied in the simulation of a heavy-duty internal combustion engine (ICE). A first validation was done to assess the performance of the numerical methodology in a traditional Mixing Controlled Compression Ignition (MCCI) scenario. Then, two other points were simulated: an MCCI condition with 35% of EGR and a Late-Premixed Charge Compression Ignition (L-PCCI) one, with 35% of EGR and an SOIe of 4 CAD aTDC. Local temperature distributions were compared, analyzing the effect of these technologies in NOx emission mitigation and their impact on gross indicated efficiency (& eta;g), showing the advantages that using DME can have on a real-world application

Sardinia aperture array demonstrator: measurement campaigns of radio frequency interferences

Measurement campaigns of radio frequency interferences (RFIs) represent a fundamental aspect to optimize data collected by radio astronomical observations. In order to monitor the state of unwanted man-made signals, every radio telescope needs to have a radio frequency map in the frequencies range of its receivers. The Sardinia Aperture Array Demonstrator (SAD) is an Italian aperture array demonstrator composed of prototypical Vivaldi antennas designed to work at frequencies below 500 MHz. The antennas are located at the Sardinia Radio Telescope (SRT) site and they are arranged with a versatile approach that is able to provide different array configurations. In this paper, we present the results of measurement campaigns conducted with the SAD antennas at the SRT observing site with the aim to monitor the evolution of RFI scenario from 2016 to date. The signal acquisition chain and the software tool used for RFI detection are, also, presented

Preliminary tests to design an ad hoc signal acquisition chain for the Sardinia Aperture Array Demonstrator

The Sardinia Aperture Array Demonstrator (SAD) is an Italian facility, which is composed of 128 prototypical Vivaldi antennas, specifically designed to operate across the 50-500 MHz frequency range. As well known, one of the major burden at low frequency are the radio frequency interferences, thus after several accurate measurement campaigns we realized that a specific signal conditioning is needed in order to feed the digital beamformer with the proper signal level. In this paper, we report the results of the preliminary tests that we carried out in order to design an ad hoc receiving chain for the SAD array

Application of simulation modeling for wildfire exposure and transmission assessment in Sardinia, Italy

Abstract The development of comprehensive fire management and risk assessment strategies is of prominent concern in Southern Europe, due to the expanding scale of wildfire risk. In this work, we applied simulation modeling to analyze fine-scale (100-m resolution) wildfire exposure and risk transmission in the 24,000 km2 island of Sardinia (Italy). Sardinia contains a variety of ecological, cultural, anthropic and touristic resources that each summer are threatened by wildfires, and represents well the Mediterranean Basin environments and conditions. Wildfire simulations based on the minimum travel time algorithm were used to characterize wildfire exposure and risk transmission in terms of annual burn probability, flame length, structures exposed and type and amount of transmission. We focused on the historical conditions associated with large (>50 ha) and very large (>200 ha) wildfires that occurred in Sardinia in the period 1998–2016, and combined outputs from wildfire simulation modeling with land uses, building footprint locations, weather, and historical ignition data. The outputs were summarized for weather zones, main wind scenarios and land uses. Our study characterized spatial variations in wildfire spread, exposure and risk transmission among and within weather zones and the main winds associated with large events. This work provides a novel quantitative approach to inform wildfire risk management and planning in Mediterranean areas. The proposed methodology can serve as reference for wildfire risk assessment and can be replicated elsewhere. Findings can be used to better understand the spatial dynamics and patterns of wildfire risk and evaluate expected wildfire behavior or transmission potential in Sardinia and neighboring regions

Draft Methodology for FAIR-by-Design Learning Materials

This document describes a methodology for FAIR-by-design production of learning materials based on the backward instructional process that is extended with additional activities focusing on the implementation of the FAIR guiding principles. A general discussion on important aspect of implementation such as granularity, scope, metadata schema, interoperability and publication in relevant repositories is provided together with a step by step six stage workflow and checklists that help implement the FAIR-by-design process. The outlined methodology will be used as a blueprint for a train-the-trainer course aiming to present the practical FAIR-by-design instructional design

Adaptation of an IRAM W-Band SIS Receiver to the INAF Sardinia Radio Telescope: A Feasibility Study and Preliminary Tests

Radio telescopes are used by astronomers to observe the naturally occurring radio waves generated by planets, interstellar molecular clouds, galaxies, and other cosmic objects. These telescopes are equipped with radio receivers that cover a portion of the radio frequency (RF) and millimetre-wave spectra. The Sardinia Radio Telescope (SRT) is an Italian instrument designed to operate between 300 MHz and 116 GHz. Currently, the SRT maximum observational frequency is 26.5 GHz. A feasibility study and preliminary tests were performed with the goal of equipping the SRT with a W-band (84–116 GHz) mono-feed radio receiver, whose results are presented in this paper. In particular, we describe the adaptation to the SRT of an 84–116 GHz cryogenic receiver developed by the Institute de Radio Astronomie Millimétrique (IRAM) for the Plateau de Bure Interferometer (PdBI) antennas. The receiver was upgraded by INAF with a new electronic control system for the remote control from the SRT control room, with a new local oscillator (LO), and with a new refrigeration system. Our feasibility study includes the design of new receiver optics. The single side band (SSB) receiver noise temperature measured in the laboratory, Trec ≈ 66 K at 86 GHz, is considered sufficiently low to carry out the characterisation of the SRT active surface and metrology system in the 3 mm band