Searching for cluster substructure using APM and ROSAT data

We present a detailed study of the morphological features of 22 rich galaxy clusters. Our sample is constructed from a cross-correlation of optical (Abell+APM) data with X-ray (0.1 - 2.4) keV ROSAT pointed observations. We systematically compare cluster images and morphological parameters in an attempt to reliably identify possible substructure in both optical and the X-ray images. To this end, we compute various moments of the optical and X-ray surface-brightness distribution such as the ellipticities, center-of-mass shifts and ellipsoidal orientations. We assess the significance of our results using Monte Carlo simulations. We find significant correlations between the optical and X-ray morphological parameters, indicating that in both parts of the spectrum it is possible to identify correctly the dynamical state of a cluster. Most of our clusters (17/22) have a good 1-to-1 correspondence between the optical and the X-ray images and about 10 appear to have strong indications of substructure. This corresponds to a minimum percentage of order 45 per cent which is in very good accordance with other similar analyses. Finally, 5 out of 22 systems seem to have distinct subclumps in the optical which are not verified in the X-ray images, and thus are suspect of being due to optical projection effects. These results will serve as a useful guide in interpreting subsequent analyses of large optical cluster catalogues.Comment: 15 pages, including 9 figures, MNRAS in press, revised versio

On the Complexity of Computing Sparse Equilibria and Lower Bounds for No-Regret Learning in Games

Characterizing the performance of no-regret dynamics in multi-player games is a foundational problem at the interface of online learning and game theory. Recent results have revealed that when all players adopt specific learning algorithms, it is possible to improve exponentially over what is predicted by the overly pessimistic no-regret framework in the traditional adversarial regime, thereby leading to faster convergence to the set of coarse correlated equilibria (CCE). Yet, despite considerable recent progress, the fundamental complexity barriers for learning in normal- and extensive-form games are poorly understood. In this paper, we make a step towards closing this gap by first showing that -- barring major complexity breakthroughs -- any polynomial-time learning algorithms in extensive-form games need at least $2^{\log^{1/2 - o(1)} |\mathcal{T}|}$ iterations for the average regret to reach below even an absolute constant, where $|\mathcal{T}|$ is the number of nodes in the game. This establishes a superpolynomial separation between no-regret learning in normal- and extensive-form games, as in the former class a logarithmic number of iterations suffices to achieve constant average regret. Furthermore, our results imply that algorithms such as multiplicative weights update, as well as its \emph{optimistic} counterpart, require at least $2^{(\log \log m)^{1/2 - o(1)}}$ iterations to attain an $O(1)$-CCE in $m$-action normal-form games. These are the first non-trivial -- and dimension-dependent -- lower bounds in that setting for the most well-studied algorithms in the literature. From a technical standpoint, we follow a beautiful connection recently made by Foster, Golowich, and Kakade (ICML '23) between sparse CCE and Nash equilibria in the context of Markov games. Consequently, our lower bounds rule out polynomial-time algorithms well beyond the traditional online learning framework.Comment: To appear at ITCS 202

Adaptation and validation of a test to measure Greek elementary students' basic cycling skills

The aim of this study was to adapt a previously developed test to gain a detailed insight into the cycling skills of Greek children and examine the impact that sex, age and cycling experience have on cycling skills. Students (n=80) from a local elementary school in Attica region (Greece) took the adapted cycling skills test consisting of 12 test stations. An exploratory factor analysis was conducted to investigate the factor structure of the cycling test. Descriptive statistics were performed on children’s cycling skill scores. Furthermore, independent sample t-tests and Pearson r correlations were executed to evaluate individual correlates of cycling skills. Two factors were extracted: the ‘during cycling skills’ and the ‘attention/handling cycling skills’ factor. Most children faced difficulties for skills that required more advanced attention skills and while cycling over obstacles. No significant differences in separate factors, as well as the overall cycling skill, were noted between boys and girls. Significant correlations were observed between years of cycling experience and cycling skills, while age was not correlated to these factors. The 12-item test battery adapted in the present study is suitable for the evaluation of cycling skills of Greek elementary students. Implications of the current research are further discussed

Η Επιρροή της Ευρωπαϊκής Ένωσης (ΕΕ) στην Εθνική Πολιτική Περιβάλλοντος: Θεσμικοί Μετασχηματισμοί, Διοικητικές Επιπτώσεις και Έλλειμμα Εφαρμογής

Ο στόχος της συγκεκριμένης διδακτορικής εργασίας είναι διττός: αφενός, εξετάζει την επιρροή της Ευρωπαϊκής Ένωσης (ΕΕ) στην εθνική πολιτική περιβάλλοντος και, αφετέρου, διερευνά τους κύριους αποτρεπτικούς παράγοντες που ευθύνονται για το έλλειμμα εφαρμογής. Η διαδικασία της τυπικής και πρακτικής υλοποίησης αποτελεί το κρισιμότερο τεστ για την αξιολόγηση του εξευρωπαϊσμού των δημόσιων πολιτικών. Εάν τα κράτη μέλη δεν συμμορφώνονται με τους ευρωπαϊκούς κανόνες, τότε δεν στοιχειοθετούνται μεταβολές στο εσωτερικό λόγω των υπερεθνικών πιέσεων. Αν και η ΕΕ, με όλους τους μηχανισμούς πίεσης που διαθέτει, έχει θετικό αντίκτυπο στο περιεχόμενο και τις δομές της πολιτικής, η Ελλάδα ταξινομείται στις «βραδυπορούσες» χώρες στον τομέα του περιβάλλοντος (Sbragia 1996). Έχοντας συμπληρώσει 35 χρόνια συμμετοχής στους ευρωπαϊκούς θεσμούς, η ανταπόκριση της περιβαλλοντικής διοίκησης παραμένει παθητική. Η προβληματική υλοποίηση και η κακή απόδοση αποτελούν συστημικά χαρακτηριστικά της ελληνικής περίπτωσης. Αρκετοί υποστηρίζουν ότι η απουσία πολιτικής βούλησης, εξαιτίας του κόστους εφαρμογής των οδηγιών, αποτελεί την κύρια επεξηγηματική αιτία. Ωστόσο, τι γίνεται όταν οι κυβερνήσεις είναι πρόθυμες ή εξαναγκάζονται να τηρήσουν τις ευρωπαϊκές δεσμεύσεις; Από την εξέταση των τριών μελετών περίπτωσης, καθίσταται εμφανές ότι η βούληση δεν αρκεί για την επίλυση των προβλημάτων. Η έρευνα, επικεντρώνοντας στο στάδιο της υλοποίησης, διακρίνει τρεις βασικές αιτίες για την ελλιπή εφαρμογή: α) χαμηλός βαθμός εμπιστοσύνης και συγκρουσιακή κουλτούρα β) διοικητική ανεπάρκεια και γ) θεσμικά προβλήματα. Μέσω της μελέτης της πολιτικής περιβάλλοντος, που είναι ένα από τα πιο ρυθμισμένα πεδία στην ΕΕ, προτείνεται ένα πλαίσιο ανάλυσης για την ερμηνεία της γενικότερης υστέρησης της χώρας στο ευρωπαϊκό γίγνεσθαι. Η μεγάλη εικόνα απεικονίζει μια διαρκή ένταση ανάμεσα στις υπερεθνικές δεσμεύσεις και τις παραδοσιακές πρακτικές. Η εθνική ανταπόκριση συναρτάται με αυτές τις δύο αντίρροπες δυνάμεις, καθώς οι περισσότεροι κυβερνητικοί δρώντες επιστρατεύουν την στρατηγική της «διγλωσσίας», ώστε να εξισορροπήσουν μεταξύ των δύο.The purpose of this thesis is twofold. On the one hand, it examines EU’s impact on the national environmental policy and on the other hand, it aims to shed some light on the deterrent factors which are responsible for its implementation deficit. The process of formal and actual implementation is the most critical test for the evaluation of Europeanization of public policies. If member states do not comply with EU regulations, changes at the national level cannot be justified as the outcome of external pressures. Although EU, with all its pressure mechanisms, has a positive impact on the member states’ policy content and structures, Greece has been classified as a ‘laggard’ country on environmental issues (Sbragia 1996). Although Greece is an EU member for more than 35 years, the response of the relevant national authorities remains quite passive. Inadequate implementation and poor performance constitute systemic characteristics of the Greek case. Many scholars argue that the lack of political intention, due to the high cost of the implementation of the directives, is the main factor lying behind the implementation deficit. What happens though when governments are willing or are being forced to comply with EU law? By examining three case studies, it is obvious that political will is not a sufficient condition for the solution of problems. Our study, by focusing on the lower level of implementation, has identified three main factors as the major reasons for the implementation deficit: a) lack of trust along with consensual political culture, b) administrative inefficiency and ineffectiveness and c) institutional problems. Through the current study on environmental policy, one of the most regulated fields in the EU, it is proposed a framework for analyzing the main reasons that keep Greece lagging behind on European policies in comparison to the majority of other EU countries. The wider picture reflects an ongoing tension between supranational commitments and national practices. The national response is closely related to these two competitive forces, given that the majority of governmental actors use an ambiguous strategy - acting in a different way at the national and the transnational level - in order to balance between the two forces

Transport Properties of Shale Gas in Relation to Kerogen Porosity

Kerogen is a micro-porous amorphous solid, which consist the major component of the organic matter scattered in the potentially lucrative shale formations hosting shale gas. Deeper understanding of the way kerogen porosity characteristics affect the transport properties of hosted gas is important for the optimal design of the extraction process. In this work, we employ molecular simulation techniques in order to investigate the role of porosity on the adsorption and transport behavior of shale gas in overmature type II kerogen found at many currently productive shales. To account for the wide range of porosity characteristics present in the real system, a large set of 60 kerogen structures that exhibit a diverse set of void space attributes was used. Grand Canonical Monte Carlo (GCMC) simulations were performed for the study of the adsorption of CH4, C2H6, n-C4H10 and CO2 at 298.15 K and 398.15 K and a variety of 2 pressures. The amount adsorbed is found to correlate linearly with the porosity of the kerogen. Furthermore, the adsorption of a quaternary mixture of CH4, C2H6, CO2 and N2 was investigated in the same conditions, indicating that the composition resembling that of the shale gas is achieved under higher temperature and pressure values, i.e. conditions closer to these prevailing in the hosting shale field. The diffusion of CH4, C2H6 and CO2, both as pure components and as components of the quaternary mixture, was investigated using equilibrium Molecular Dynamics (MD) simulations at temperatures of 298.15 and 398.15 K and pressures of 1 and 250 atm. In addition to the effect of temperature and pressure, the importance of limiting pore diameter (LPD), maximum pore diameter (MPD), accessible volume (Vacc) and accessible surface (Sacc) on the observed adsorbed amount and diffusion coefficient was revealed by qualitative relationships. The diffusion across the models was found to be anisotropic and the maximum component of the diffusion coefficient to correlate linearly with LPD, indicating that the controlling step of the transport process is the crossing of the limiting pore region. Finally, the transport behavior of the pure compounds was compared with their transport properties when in mixture and it was found that the diffusion coefficient of each compound in the mixture is similar to the corresponding one in pure. This observation agrees with earlier studies in different kerogen models comprising wider pores that have revealed negligible cross-correlation Onsager coefficients