The Biodiversity of the Mediterranean Sea: Estimates, Patterns, and Threats

The Mediterranean Sea is a marine biodiversity hot spot. Here we combined an extensive literature analysis with expert opinions to update publicly available estimates of major taxa in this marine ecosystem and to revise and update several species lists. We also assessed overall spatial and temporal patterns of species diversity and identified major changes and threats. Our results listed approximately 17,000 marine species occurring in the Mediterranean Sea. However, our estimates of marine diversity are still incomplete as yet—undescribed species will be added in the future. Diversity for microbes is substantially underestimated, and the deep-sea areas and portions of the southern and eastern region are still poorly known. In addition, the invasion of alien species is a crucial factor that will continue to change the biodiversity of the Mediterranean, mainly in its eastern basin that can spread rapidly northwards and westwards due to the warming of the Mediterranean Sea. Spatial patterns showed a general decrease in biodiversity from northwestern to southeastern regions following a gradient of production, with some exceptions and caution due to gaps in our knowledge of the biota along the southern and eastern rims. Biodiversity was also generally higher in coastal areas and continental shelves, and decreases with depth. Temporal trends indicated that overexploitation and habitat loss have been the main human drivers of historical changes in biodiversity. At present, habitat loss and degradation, followed by fishing impacts, pollution, climate change, eutrophication, and the establishment of alien species are the most important threats and affect the greatest number of taxonomic groups. All these impacts are expected to grow in importance in the future, especially climate change and habitat degradation. The spatial identification of hot spots highlighted the ecological importance of most of the western Mediterranean shelves (and in particular, the Strait of Gibraltar and the adjacent Alboran Sea), western African coast, the Adriatic, and the Aegean Sea, which show high concentrations of endangered, threatened, or vulnerable species. The Levantine Basin, severely impacted by the invasion of species, is endangered as well

Visions of aromaticity: from boron clusters to conjugated polycyclic hydrocarbons

The aim of this PhD dissertation is to explore the phenomenon of aromaticity in chemical systems. The dissertation will be concerned with the connection between electron counts and stabilities of molecules with specific shape (spherical clusters, sheet-like structures) and to find a relationship between aromaticity and molecular topology.Results will be predicted and verified and characterized by state-of-the-art quantum chemical computations.status: publishe

Semigroups of composition operators on Hardy spaces of the half-plane

a

Directed Graphs and Induced Magnetic Multipoles in Polycyclic Hydrocarbons

Time-odd interactions in molecules may give rise to a current along the bonds connecting the atoms. In a graph representation of a molecule the analogue of these currents corresponds to directed edges. We consider the distribution of currents in polycyclic molecules, using a minimal cycle basis. The irreducible representations of this basis in the automorphism group of the molecular graph delineate specific magnetic multipoles. This is illustrated for the case of corazulene with fourfold symmetry which may sustain a magnetic octopole. It is further shown how breaking of the fourfold symmetry can give rise to the induction of an octopolar magnetic moment by a homogeneous magnetic field. Two conjugated polycyclic hydrocarbon systems showing an induced octopolar moment are investigated by ab initio current density calculations.status: publishe

Composition operators and semigroups on spaces of analytic functions on the half plane

In the first chapter we give the definitions of the spaces, functions and operators that we use along with main concepts and properties. In the second chapter we consider the topological space of bounded composition operators on the Hardy space of the half plane with inner product, in the operator norm topology. The study of such spaces initiated on Hardy spaces of the disk by Berkson, Shapiro, Sundberg, MacCluer and still continues on various spaces of functions. However this is the first time we have results on spaces of analytic functions on the half plane. We relate the isolation of a composition operator with the way the boundary values of the function that induce it approach the boundary. We show that if two composition operators belong to the same component, then the functions that induce them have the same nontangential limits and the same angular derivatives on the boundary, and that the converse is not always true. In the third chapter we study semigroups of bounded composition operators on the Hardy spaces of the half plane which induced by semigroups of analytic functions from the half plane to the half plane. First Berkson and Porta at 1978 gave results on such semigroups of analytic functions on the half plane and on the disk along with results on the semigroups of composition operators that induce on Hardy spaces of the disk but not of the half plane. Here we continue their work. First we identify the semigroups of bounded composition operators on the Hardy spaces of the half plane along with their norms. We prove that any such semigroup is strongly continuous and we identify the generator and its point spectrum. As an application we find the norm, the point spectrum and the spectrum of a Volterra-type operator. Motivation for the fourth chapter was a result of Hardy on the arithmetic means of the Fourier coefficients of p-integrable functions on the unit circle. Considering as a starting point the corresponding question for p-integrable functions of the real axis, we study the Cesaro operator and its adjoint on Hardy spaces of the half plane. We identify them as resolvents for appropriate semigroups of composition operators and we find their norm and spectrum. Finally, their relation with the corresponding Cesaro operators on spaces of p-integrable functions of the real axis is also discussed, from which follows the corresponding result with that of Hardy’s for such functions.Στο πρώτο κεφάλαιο δίνουμε τους ορισμούς των χώρων, των συναρτήσεων και των τελεστών που χρησιμοποιούμε μαζί με βασικές έννοιες και ιδιότητες. Στο δεύτερο κεφάλαιο θεωρούμε τον τοπολογικό χώρο των φραγμένων τελεστών σύνθεσης στο χώρο Hardy του ημιεπιπέδου με εσωτερικό γινόμενο, με τοπολογία αυτή της νόρμα τελεστών. Η μελέτη τέτοιων χώρων ξεκίνησε στους χώρους Hardy του δίσκου από τους Berkson, Shapiro, Sundberg, MacCluer και συνεχίζεται σε διάφορους χώρους συναρτήσεων. Αυτή όμως είναι η πρώτη φορά που παρουσιάζονται αποτελέσματα σε χώρους αναλυτικών συναρτήσεων του ημιεπιπέδου. Συνδέουμε την απομόνωση ενός τελεστή σύνθεσης με τον τρόπο που οι συνοριακές τιμές της συνάρτησης που τον επάγει πλησιάζουν το σύνορο. Δείχνουμε ότι αν δυο τελεστές σύνθεσης ανήκουν στην ίδια συνεκτική συνιστώσα, τότε οι συναρτήσεις που τους επάγουν έχουν ίδια μη εφαπτομενικά όρια και ίσες γωνιακές παραγώγους στο σύνορο, με το αντίστροφο να μην ισχύει πάντα. Στο τρίτο κεφάλαιο μελετάμε ημιομάδες φραγμένων τελεστών σύνθεσης στους χώρους Hardy του ημιεπιπέδου που επάγονται από ημιομάδες αναλυτικών συναρτήσεων του ημιεπιπέδου στο ημιεπίπεδο. Πρώτοι οι Berkson και Porta το 1978 έδωσαν αποτελέσματα για τέτοιες ημιομάδες συναρτήσεων στο ημιεπίπεδο και στο δίσκο καθώς και για τις ημιομάδες τελεστών σύνθεσης που επάγουν στους χώρους Hardy του δίσκου αλλά όχι του ημιεπιπέδου. Εδώ συνεχίζουμε το έργο τους. Αρχικά καθορίζουμε τις ημιομάδες φραγμένων τελεστών σύνθεσης στους Hardy του ημιεπιπέδου και βρίσκουμε τις νόρμες τους. Αποδεικνύουμε ότι μια τέτοια ημιομάδα είναι ισχυρά συνεχής προσδιορίζοντας επίσης το γεννήτορα αυτής και το σημειακό του φάσμα. Ως εφαρμογή βρίσκουμε τη νόρμα, το σημειακό φάσμα και το φάσμα ενός μη συμπαγούς τελεστή τύπου Volterra. Έναυσμα για το τέταρτο κεφάλαιο ήταν ένα αποτέλεσμα του Hardy σχετικά με τους αριθμητικούς μέσους όρους των συντελεστών Fourier μιας p-ολοκληρώσιμης συνάρτησης στο μοναδιαίο κύκλο. Με αφετηρία το αντίστοιχο ερώτημα για p-ολοκληρώσιμες συναρτήσεις του πραγματικού άξονα, μελετάμε τον τελεστή Cesaro και το συζυγή του σε χώρους Hardy του ημιεπιπέδου. Δείχνουμε ότι οι τελεστές αυτοί μπορούν να προκύψουν ως επιλύοντες τελεστές κατάλληλων ημιομάδων τελεστών σύνθεσης και βρίσκουμε τη νόρμα και το φάσμα τους. Τέλος, εξετάζουμε τη σχέση αυτών με τους αντίστοιχους Cesaro σε χώρους p-ολοκληρώσιμων συναρτήσεων του πραγματικού άξονα, από την οποία έπεται το αντίστοιχο αποτέλεσμα με αυτό του Hardy για τέτοιες συναρτήσεις

Directed graphs and induced magnetic multipoles in polycyclic hydrocarbons

Time-odd interactions in molecules may give rise to a current along the bonds connecting the atoms. In a graph representation of a molecule the analogue of these currents corresponds to directed edges. We consider the distribution of currents in polycyclic molecules, using a minimal cycle basis. The irreducible representations of this basis in the automorphism group of the molecular graph delineate specific magnetic multipoles. This is illustrated for the case of corazulene with fourfold symmetry which may sustain a magnetic octopole. It is further shown how breaking of the fourfold symmetry can give rise to the induction of an octopolar magnetic moment by a homogeneous magnetic field. Two conjugated polycyclic hydrocarbon systems showing an induced octopolar moment are investigated by ab initio current density calculations

Optimized Data-Driven Models for Short-Term Electricity Price Forecasting Based on Signal Decomposition and Clustering Techniques

In recent decades, the traditional monopolistic energy exchange market has been replaced by deregulated, competitive marketplaces in which electricity may be purchased and sold at market prices like any other commodity. As a result, the deregulation of the electricity industry has produced a demand for wholesale organized marketplaces. Price predictions, which are primarily meant to establish the market clearing price, have become a significant factor to an energy company’s decision making and strategic development. Recently, the fast development of deep learning algorithms, as well as the deployment of front-end metaheuristic optimization approaches, have resulted in the efficient development of enhanced prediction models that are used for electricity price forecasting. In this paper, the development of six highly accurate, robust and optimized data-driven forecasting models in conjunction with an optimized Variational Mode Decomposition method and the K-Means clustering algorithm for short-term electricity price forecasting is proposed. In this work, we also establish an Inverted and Discrete Particle Swarm Optimization approach that is implemented for the optimization of the Variational Mode Decomposition method. The prediction of the day-ahead electricity prices is based on historical weather and price data of the deregulated Greek electricity market. The resulting forecasting outcomes are thoroughly compared in order to address which of the two proposed divide-and-conquer preprocessing approaches results in more accuracy concerning the issue of short-term electricity price forecasting. Finally, the proposed technique that produces the smallest error in the electricity price forecasting is based on Variational Mode Decomposition, which is optimized through the proposed variation of Particle Swarm Optimization, with a mean absolute percentage error value of 6.15%