87,260 research outputs found
Higher structures in deformation theory
No full textIn questo lavoro vengono affrontati due problemi principali. Il primo è lo studio della formalità per algebre di Lie differenziali graduate (DGLA) ed algebre L-infinito, e il secondo è lo studio del prodotto di Baker-Campbell-Hausdorff. Ringuardo al primo problema vengono mostrati due risultati principali: il primo mette in relazione due ostruzioni alla formalità presenti in letteratura (la classe di Eulero e i prodotti tripli di Lie-Massey), e il secondo è un'estensione del criterio di formalità ottenuto attraverso la classe di Eulero, adattato alla nozione di formalità superiore. Nella seconda parte del lavoro, attraverso un approccio algebrico e combinatorio, otteniamo un algoritmo efficiente per calcolare i coefficienti del prodotto di Baker-Campbell-Hausdorff nella base di Lyndon dell'algebra di Lie libera su due generatori.In this thesis we work on two main topics. The first one is the notion of formality for differential graded Lie algebras (DGLAs) and L-infinity algebras, and the second one is the study of the Baker-Campbell-Hausdorff product. Concerning the first problem we obtained two major results: the first one establishes a relation between two obstructions to formality which are present in literature (the Euler class and triple Lie-Massey products), and the second one is an exrtension of the formality criterion obtained from the Euler class, adapted to the notion of formality of higher degree. In the second part of this work, with and algebraic and combinatoric approach, we developed an efficient algorithm to compute the coefficients of the Baker-Campbell-Hausdorff product in the Lyndon basis of the free Lie algebra on two generators
Interacting path systems for credit portfolios risk analysis
No full textThis Note introduces an algorithm (referred to as interacting path systems algorithm, IPaS) based on the first Author multilevel splitting technique and suited to the analysis of multiple defaults in credit portfolios. A full development of this Note incorporating technical details and a survey of the use of Interacting Particle Systems in the field of credit risk, \it Interacting path systems for credit risk, is submitted for publication in \it Recent Advancements in the Theory and Practice of Credit Derivatives, Eds T. Bielecki, D. Brigo, F. Patras, Bloomberg Press (2011). The reader is referred to this article for further details
Interacting path systems for credit portfolios risk analysis
No full textThis Note introduces an algorithm (referred to as interacting path systems algorithm, IPaS) based on the first Author multilevel splitting technique and suited to the analysis of multiple defaults in credit portfolios. A full development of this Note incorporating technical details and a survey of the use of Interacting Particle Systems in the field of credit risk, \it Interacting path systems for credit risk, is submitted for publication in \it Recent Advancements in the Theory and Practice of Credit Derivatives, Eds T. Bielecki, D. Brigo, F. Patras, Bloomberg Press (2011). The reader is referred to this article for further details
Maximising the utility of enterprise millimetre-wave networks
No full textMillimetre-wave (mmWave) technology is a promising candidate for meeting the intensifying demand for ultra fast wireless connectivity, especially in high-end enterprise networks. Very narrow beam forming is mandatory to mitigate the severe attenuation specific to the extremely high frequency (EHF) bands exploited. Simultaneously, this greatly reduces interference, but generates problematic communication blockages. As a consequence, client association control and scheduling in scenarios with densely deployed mmWave access points become particularly challenging, while policies designed for traditional wireless networks remain inappropriate. In this paper we formulate and solve these tasks as utility maximisation problems under different traffic regimes, for the first time in the mmWave context. We specify a set of low-complexity algorithms that capture distinctive terminal deafness and user demand constraints, while providing near-optimal client associations and airtime allocations, despite the problems’ inherent NP-completeness. To evaluate our solutions, we develop an NS-3 implementation of the IEEE 802.11ad protocol, which we construct upon preliminary 60GHz channel measurements. Simulation results demonstrate that our schemes provide up to 60% higher throughput as compared to the commonly used signal strength based association policy for mmWave networks, and outperform recently proposed load-balancing oriented solutions, as we accommodate the demand of 33% more clients in both static and mobile scenarios
One GPU to Snoop Them All: A Full-Band Bluetooth Low Energy Sniffer
No full textSniffing Bluetooth data sessions is considered a difficult task, because of the frequency-hopping channel access scheme this technology implements. In this paper we present a novel open-source sniffer that can monitor Bluetooth Low Energy (BLE) traffic on all channels in real time. The sniffer builds on a Software-Defined Radio (SDR) framework to capture the entire BLE spectrum and exploits Graphics Processing Unit (GPU) capabilities to channelize and process BLE traffic in real time. We show that our sniffer can easily and reliably detect active BLE connections, and infer their properties, including Access Address, CRC values and hopping sequences. From a general standpoint, we show that tracking many BLE data sessions at the same time becomes feasible even with relatively inexpensive equipment, as we are able to discover up to 24 simultaneous sessions within 80 ms on average
Dead on arrival: An empirical study of the Bluetooth 5.1 positioning system
No full textThe recently released Bluetooth 5.1 specification introduces fine-grained positioning capabilities in this wireless technology, which is deemed essential to context-/location-based Internet of Things (IoT) applications. In this paper, we evaluate experimentally, for the first time, the accuracy of a positioning system based on the Angle of Arrival (AoA) mechanism adopted by the Bluetooth standard. We first scrutinize the fidelity of angular detection and then assess the feasibility of using angle information from multiple fixed receivers to determine the position of a device. Our results reveal that angular detection is limited to a restricted range. On the other hand, even in a simple deployment with only two antennas per receiver, the AoA-based positioning technique can achieve sub-meter accuracy; yet attaining localization within a few centimeters remains a difficult endeavor. We then demonstrate that a malicious device may be able to easily alter the truthfulness of the measured AoA, by tampering with the packet structure. To counter this protocol weakness, we propose simple remedies that are missing in the standard, but which can be adopted with little effort by manufacturers, to secure the Bluetooth 5.1 positioning system
Symmetril Moulds, Generic Group Schemes, Resummation of MZVs
No full textThe present article deals with various generating series and group schemes (not necessarily affine ones) associated with MZVs. Our developments are motivated by Ecalle’s mould calculus approach to the latter. We propose in particular a Hopf algebra–type encoding of symmetril moulds and introduce a new resummation process for MZVs
Monitoring of a methane-seeping pockmark by cabled benthic observatory (Patras Gulf, Greece)
No full textA new seafloor observatory, the gas monitoring
module (GMM), has been developed for continuous and
long-term measurements of methane and hydrogen sulphide
concentrations in seawater, integrated with temperature (T),
pressure (P) and conductivity data at the seafloor. GMM
was deployed in April 2004 within an active gas-bearing
pockmark in the Gulf of Patras (Greece), at a water depth of
42 m. Through a submarine cable linked to an onshore
station, it was possible to remotely check, via direct phone
connection, GMM functioning and to receive data in nearreal
time. Recordings were carried out in two consecutive
campaigns over the periods April–July 2004, and September
2004–January 2005, amounting to a combined dataset
of ca. 6.5 months. This represents the first long-term
monitoring ever done on gas leakage from pockmarks by
means of CH4+H2S+T+P sensors. The results show
frequent T and P drops associated with gas peaks, more
than 60 events in 6.5 months, likely due to intermittent,
pulsation-like seepage. Decreases in temperature in the
order of 0.1–1°C (up to 1.7°C) below an ambient T of ca.
17°C (annual average) were associated with short-lived
pulses (10–60 min) of increased CH4+H2S concentrations.
This seepage “pulsation” can either be an active process
driven by pressure build-up in the pockmark sediments, or a
passive fluid release due to hydrostatic pressure drops
induced by bottom currents cascading into the pockmark
depression. Redundancy and comparison of data from
different sensors were fundamental to interpret subtle proxy
signals of temperature and pressure which would not be
understood using only one sensor.Published297-302JCR Journalreserve
The impact of methanotrophic activity on methane emissions through the soils of geothermal areas
No full textMethane plays an important role in the Earth’s atmospheric chemistry and radiative balance being the
most important greenhouse gas after carbon dioxide. It has recently been established that geogenic
gases contribute significantly to the natural CH4 flux to the atmosphere (Etiope et al., 2008).
Volcanic/geothermal areas contribute to this flux, being the site of widespread diffuse degassing of
endogenous gases (Chiodini et al., 2005). In such an environment soils are a source rather than a sink
for atmospheric CH4 (Cardellini et al., 2003; Castaldi and Tedesco, 2005; D’Alessandro et al., 2009;
2011; 2013). Due to the fact that methane soil flux measurements are laboratory intensive, very few
data have been collected until now in these areas. Preliminary studies (Etiope et al., 2007) estimated
a total CH4 emission from European geothermal and volcanic systems in the range 4-16 kt a-1. This
estimate was obtained indirectly from CO2 or H2O output data and from CO2/CH4 or H2O/CH4 values
measured in the main gaseous manifestations. Such methods, although acceptable to obtain
order-of-magnitude estimates, completely disregard possible methanotrophic activity within the soil.
At the global scale, microbial oxidation in soils contributes for about 3-9% to the total removal of
methane from the atmosphere. But the importance of methanotrophic organisms is even larger
because they oxidise the greatest part of the methane produced in the soil and in the subsoil before its
emission to the atmosphere. Environmental conditions in the soils of volcanic/geothermal areas (i.e.
low oxygen content, high temperature and proton activity, etc.) have been considered inadequate for
methanotrophic microrganisms. But recently, it has been demonstrated that methanotrophic
consumption in soils occurs also under such harsh conditions due to the presence of acidophilic and
thermophilic Verrucomicrobia. These organisms were found in Italy at the Solfatara di Pozzuoli (Pol et
al., 2007), in New Zealand at Hell’s Gate (Dunfield et al., 2007) and in Kamchatka, Russia (Islam et al.,
2008).
Both the Italian and the Hellenic territories are geodynamically very active with many active
volcanic and geothermal areas. Here we report on methane flux measurements made at Pantelleria
(Italy) and at Sousaki and Nisyros (Greece). The total methane output of these three systems is about
10, 19 and 1 t a-1, respectively (D’Alessandro et al., 2009; 2011; 2013). The total emissions obtained
from methane flux measurements are up to one order of magnitude lower than those obtained through
indirect estimations. Clues of methanotrophic activity within the soils of these areas can be found in
the CH4/CO2 ratio of the flux measurements which is always lower than that of the respective fumarolic
manifestations, indicating a loss of CH4 during the travel of the gases towards earth’s surface.
Furthermore laboratory methane consumption experiments made on soils collected at Pantelleria
and Sousaki revealed, for most samples, CH4 consumption rates up to 9.50 μg h-1 and 0.52 μg h-1
respectively for each gram of soil (dry weight). Only few soil samples displayed no methane
2
consumption activity.
Finally, microbiological and molecular investigations allowed us to identify the presence of
methanotrophic bacteria belonging to the Verrucomicrobia and to the Alpha- and
Gamma-Proteobacteria in the soils of the geothermal area of Favara Grande at Pantelleria. While the
presence of the former was not unexpected due to the fact that they include acidophilic and
thermophilic organisms that were previously found in other geothermal environments, the latter are
generally considered not adapted to live in harsh geothermal environments. Their presence in the soils
of Pantelleria could be explained by the fact that these soils do not have extremely low pH values (>5).
Indeed thermotollerant methanotrophic Gamma-proteobacteria, have been previously found in the
sediments of thermal springs in Kamchatka (Kizilova et al., 2012). Such species could find their niches
in the shallowest part of the soils of Favara Grande were the temperatures are not so high and they
thrive on the abundant upraising hydrothermal methane.
References:
Cardellini C., Chiodini G., Frondini F., Granieri D., Lewicki J., Peruzzi L., 2003. Accumulation chamber
measurements of methane fluxes: application to volcanic–geothermal areas and landfills. Appl.
Geochem. 18, 45–54.
Castaldi S., Tedesco D., 2005. Methane production and consumption in an active volcanic
environment of Southern Italy. Chemosphere 58, 131–139.
Chiodini G., Granieri D., Avino R., Caliro S., Costa A., 2005. Carbon dioxide diffuse degassing and
estimation of heat release from volcanic and hydrothermal systems. J. Geophys. Res. 110, B08204.
D’Alessandro W., Bellomo S., Brusca L., Fiebig J., Longo M., Martelli M., Pecoraino G., Salerno F.,
2009. Hydrothermal methane fluxes from the soil at Pantelleria island (Italy). J. Volcanol. Geotherm.
Res. 187, 147–157.
D’Alessandro W., Brusca L., Kyriakopoulos K., Martelli M., Michas G., Papadakis G., Salerno F., 2011.
Diffuse hydrothermal methane output and evidence of methanotrophic activity within the soils at
Sousaki (Greece). Geofluids 11, 97–107
D’Alessandro W., Gagliano A.L., Kyriakopoulos K., Parello F., 2013. Hydrothermal methane fluxes
from the soil at Lakki plain (Nisyros island, Greece). Bull. Geol. Soc. Greece, vol. XLVII Proc. of the
13th International Congress, Chania, Sept. 2013
Dunfield P.F., Yuryev A., Senin P., Smirnova A.V., Stott M.B., Hou S., Ly B., Saw J.H., Zhou Z., Ren Y,
Wang J., Mountain B.W., Crowe M.A., Weatherby T.M., Bodelier P.L.E., Liesack W., Feng L., Wang L.,
Alam M., 2007. Methane oxidation by an extremely acidophilic bacterium of the phylum
Verrucomicrobia. Nature, 450, 879–882.
Etiope G., Fridriksson T., Italiano F., Winiwarter W., Theloke J., 2007. Natural emissions of methane
from geothermal and volcanic sources in Europe. J. Volcanol. Geotherm. Res. 165, 76–86.
Etiope G., Lassey K.R., Klusman R.W., Boschi E., 2008. Reappraisal of the fossil methane budget and
related emission from geologic sources. Geophys. Res. Lett. 35, L09307.
Islam T., Jensen S., Reigstad L.J., Larsen Ø., Birkeland N.K., 2008. Methane oxidation at 55°C and
pH 2 by a thermoacidophilic bacterium belonging to the Verrucomicrobia phylum. Proc. Natl. Acad. Sci.
105, 300–304.
Kizilova A.K., Dvoryanchikova E.N., Sukhacheva M.V., Kravchenko I.K., Gal’chenko V.F., 2012.
Investigation of the communities of the Hot Springs of the Uzon Caldera, Kamchatka, by Molecular
Ecological Techniques. Microbiology, 81, 606-613.
Pol A., Heijmans K., Harhangi H.R., Tedesco D., Jetten M.S.M., Op den Camp H.J.M., 2007.
Methanotrophy below pH 1 by a new Verrucomicrobia species. Nature, 450, 874–878
- …
