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Search Results: 1 - 10 of 747 matches for " Athanasios Sfetsos "
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Time Series Forecasting of Hourly PM10 Using Localized Linear Models  [PDF]
Athanasios Sfetsos, Diamando Vlachogiannis
Journal of Software Engineering and Applications (JSEA) , 2010, DOI: 10.4236/jsea.2010.34042
Abstract: The present paper discusses the application of localized linear models for the prediction of hourly PM10 concentration values. The advantages of the proposed approach lies in the clustering of the data based on a common property and the utilization of the target variable during this process, which enables the development of more coherent models. Two alternative localized linear modelling approaches are developed and compared against benchmark models, one in which data are clustered based on their spatial proximity on the embedding space and one novel approach in which grouped data are described by the same linear model. Since the target variable is unknown during the prediction stage, a complimentary pattern recognition approach is developed to account for this lack of information. The application of the developed approach on several PM10 data sets from the Greater Athens Area, Helsinki and London monitoring networks returned a significant reduction of the prediction error under all examined metrics against conventional forecasting schemes such as the linear regression and the neural networks.
An Investigation of the Factors Affecting the Ozone Concentrations in an Urban Environment  [PDF]
Athanasios Sfetsos, Diamando Vlachogiannis, Nikolaos Gounaris
Atmospheric and Climate Sciences (ACS) , 2013, DOI: 10.4236/acs.2013.31003

Adjoint sensitivity analysis allows to assess the areas that have the largest impact on a given receptor site. The adjoint version of the Community Multiscale Air Quality (CMAQ v4.5) model was employed to perform a sensitivity analysis of ground level ozone for the episodic event of June 24, 2003, in the city of Athens assuming as a receptor site that of Agia Paraskevi Station. The 3-dimensional meteorology fields calculated using the Mesoscale Model 5 (MM5, Penn State University version 3.7.2) were used to produce high resolution daily air emissions inventories for the main anthropogenic and biogenic pollutants with 1-hour time step by an in-house built processor named EMISLAB. The meteorological prediction fields in combination with the emissions inventories were consequently fed as inputs to the CMAQ model. The ozone sensitivities were obtained with respect to pollutant concentrations and emissions. The distribution of the sensitivities in the computational domain for different times delineated the regions where perturbations in some concentrations would result in significant changes in the ozone concentrations in the area of interest (Agia Paraskevi, in this case) at the final time. The investigation yielded that the most significant influences were the transported O3 and NOx concentrations from the industrial area in the northern parts of the city and the road traffic from the city centre.

Producing a Double Dividend for the EU-27 and USA with the Macro-Economic E4M-GAIA Model: Meeting G8 80% Emissions Reduction Target Leads to Economic Growth  [PDF]
Athanasios Dagoumas
Low Carbon Economy (LCE) , 2013, DOI: 10.4236/lce.2013.42006

The international negotiations concerning climate change, taken place during the UNFCCC conference in Durban at the end of year 2011, have failed to establish a new global agreement to reduce global emissions. Therefore, the G8 commitments on 80% reduction by 2050 seems to be the most realistic climate change mitigation framework for the time being, enhanced by the political will of the EU and USA administrations. For the needs of this paper, the G8 80% target is further extended to cover the whole EU-27 region, where the reduction commitments of the EU-27 member states are allocated based on the relevant allocation weights considered for the Kyoto Protocol obligations. This paper examines the implementation of the EU-27 and USA 80% emissions reduction target using a macro-economic hybrid model E4M-GAIA of the global economy, standing for Energy-Economy-Environment-Engineering Model of the Earth. The E4M-GAIA model, which adopts similar theoretical background with the “New Economics” school depicted mainly in the well-established Cambridge University’ E3 models, is used to implement this target and to compare it with a reference scenario, where no reduction target is pursued. Both scenarios consider that impact of the financial crisis, with updated information to the end of 2010. This paper aims to provide evidence that the proper direction of a portfolio of policies including: regulation, behavioral shift, revenue recycling, energy investments, energy and carbon pricing, can lead to double dividend, namely meeting a deep reduction target and providing gains for the economy.

A Representation of the Maximal Set in Choice Problems Where Information Is Incomplete  [PDF]
Andrikopoulos Athanasios
Theoretical Economics Letters (TEL) , 2018, DOI: 10.4236/tel.2018.811167
Abstract: Banerjee and Pattanaik [1] proved that the maximal set generated by a quasi-ordering is equal to the union of the sets of best elements of its ordering extensions. Suzumura and Xu [2] extended Banerjee and Pattanaik’s result by relaxing the axiom of transitivity to the axiom that Suzumura calls consistency. Arló Costa in [3] pointed out that in general, an optimizing model cannot require the transitivity of the binary relation used in an optimizing model. In this paper, by using two important ideas of John Duggan [4], I extend the above mentioned results to arbitrary binary relations whose extensions are complete and not necessarily transitive.
Exact string backgrounds from WZW models based on non-semi-simple groups
Konstadinos Sfetsos
Physics , 1993, DOI: 10.1142/S0217751X94001916
Abstract: We formulate WZW models based on a centrally extended version of the Euclidean group in $d$-dimensions. We obtain string backgrounds corresponding to conformal $\s$-models in $D=d^2$ space-time dimensions with exact central charge $c=d^2$ and $d(d-1)/2$ null Killing vectors. By identifying the corresponding conformal field theory we show that the one loop results coincide with the exact ones up to a shifting of a parameter.
Gauging a Non-Semi-Simple WZW Model
Konstadinos Sfetsos
Physics , 1993, DOI: 10.1016/0370-2693(94)90203-8
Abstract: We consider gauged WZW models based on a four dimensional non-semi-simple group. We obtain conformal $\s$-models in $D=3$ spacetime dimensions (with exact central charge $c=3$) by axially and vectorially gauging a one-dimensional subgroup. The model obtained in the axial gauging is related to the $3D$ black string after a correlated limit is taken in the latter model. By identifying the CFT corresponding to these $\s$-models we compute the exact expressions for the metric and dilaton fields. All of our models can be mapped to flat spacetimes with zero antisymmetric tensor and dilaton fields via duality transformations.
Non--Abelian Duality, Parafermions and Supersymmetry
Konstadinos Sfetsos
Physics , 1996, DOI: 10.1103/PhysRevD.54.1682
Abstract: Non--Abelian duality in relation to supersymmetry is examined. When the action of the isometry group on the complex structures is non--trivial, extended supersymmetry is realized non--locally after duality, using path ordered Wilson lines. Prototype examples considered in detail are, hyper--Kahler metrics with SO(3) isometry and supersymmetric WZW models. For the latter, the natural objects in the non--local realizations of supersymmetry arising after duality are the classical non--Abelian parafermions. The canonical equivalence of WZW models and their non--Abelian duals with respect to a vector subgroup is also established.
Duality-invariant class of two-dimensional field theories
Konstadinos Sfetsos
Physics , 1999, DOI: 10.1016/S0550-3213(99)00485-X
Abstract: We construct a new class of two-dimensional field theories with target spaces that are finite multiparameter deformations of the usual coset G/H-spaces. They arise naturally, when certain models, related by Poisson-Lie T-duality, develop a local gauge invariance at specific points of their classical moduli space. We show that canonical equivalences in this context can be formulated in loop space in terms of parafermionic-type algebras with a central extension. We find that the corresponding generating functionals are non-polynomial in the derivatives of the fields with respect to the space-like variable. After constructing models with three- and two-dimensional targets, we study renormalization group flows in this context. In the ultraviolet, in some cases, the target space of the theory reduces to a coset space or there is a fixed point where the theory becomes free.
Stochastic Tachyon Fluctuations, Marginal Deformations and Shock Waves in String Theory
Konstadinos Sfetsos
Physics , 1994, DOI: 10.1103/PhysRevD.52.2323
Abstract: Starting with exact solutions to string theory on curved spacetimes we obtain deformations that represent gravitational shock waves. These may exist in the presence or absence of sources. Sources are effectively induced by a tachyon field that randomly fluctuates around a zero condensate value. It is shown that at the level of the underlying conformal field theory (CFT) these deformations are marginal and moreover all \a'-corrections are taken into account. Explicit results are given when the original undeformed 4-dimensional backgrounds correspond to tensor products of combinations of 2-dimensional CFT's, for instance SL(2,R)/R \times SU(2)/U(1).
Coset Models and Differential Geometry
Konstadinos Sfetsos
Physics , 1996,
Abstract: String propagation on a curved background defines an embedding problem of surfaces in differential geometry. Using this, we show that in a wide class of backgrounds the classical dynamics of the physical degrees of freedom of the string involves 2-dim sigma-models corresponding to coset conformal field theories.
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