Το κύριο αντικείμενο της διατριβής συνίσταται στη μελέτη της γεωμετρίας των τρισδιάστατων H-μετρικών πολλαπλοτήτων επαφής, ή, ισοδύναμα, των μετρικών πολλαπλοτήτων επαφής για τις οποίες το διανυσματικό πεδίο ξ είναι πεδίο ιδιοδιανυσμάτων του τελεστή Ricci Q. Συγκεκριμένα, αποδεικνύεται ότι μια τρισδιάστατη H-μετρική πολλαπλότητα επαφής [Μ, (η, ξ, φ, g)] χαρακτηρίζεται γεωμετρικά από μια συνθήκη που εμπλέκει τον τανυστή καμπυλότητας της Μ και τρεις διαφορίσιμες συναρτήσεις κ, μ και ν της Μ. Η συνθήκη αυτή οδηγεί στην εισαγωγή μιας νέας κλάσης μετρικών πολλαπλοτήτων επαφής: τις (κ, μ, ν)-πολλαπλότητες επαφής. Το ενδιαφέρον με τις (κ, μ, ν)-πολλαπλότητες επαφής είναι ότι για διάσταση μεγαλύτερη του τρία εκφυλίζονται στις (κ, μ)-πολλαπλότητες επαφής, δηλαδή, οι συναρτήσεις κ, μ είναι σταθερές και η συνάρτηση ν είναι η μηδενική συνάρτηση. Αντιθέτως, αποδεικνύεται ότι τέτοιες μετρικές πολλαπλότητες επαφής υπάρχουν στη διάσταση τρία. Ένα άλλο από τα προβλήματα που εξετάζονται σ' αυτή τη διατριβή είναι ο χαρακτηρισμός των διαρμονικών καμπυλών του Legendre και των αντι-αναλλοίωτων επιφανειών εμβυθισμένων σε τρισδιάστατες (κ, μ, ν)-πολλαπλότητες επαφής. Συγκεκριμένα, αποδεικνύεται ότι οι διαρμονικές καμπύλες του Legendre είναι οι γεωδαισιακές αυτών των χώρων. Επιπλέον, αποδεικνύεται ότι οι διαρμονικές και χωρίς ελαχιστικά σημεία αντι-αναλλοίωτες επιφάνειες που είναι εμβυθισμένες σε τρισδιάστατες γενικευμένες (κ, μ)-πολλαπλότητες επαφής και των οποίων το μέτρο του διανυσματικού πεδίου της μέσης καμπυλότητας είναι σταθερό, είναι τοπικά Ευκλείδειες. / The main object of this Doctoral Thesis is the study of the geometry of 3-dimensional H-contact metric manifolds, or, equivalently, the contact metric manifolds whose the vector field ξ is an eigenvector of the Ricci operator Q. More precisely, it is proved that 3-dimensional H-contact metric manifolds [M, (η, ξ, φ, g)] are geometrically characterized by a specific curvature condition and three differentiable functions κ, μ and ν of M. This condition leads to the introduction of a new class of contact metric manifolds: the (κ, μ, ν)-contact metric manifolds. It is remarkable that for dimension greater than three, such manifolds are reduced to (κ, μ)-contact metric manifolds, i.e. the functions κ, μ are constants and the function ν is the zero function. On the contrary, in three dimension (κ,μ,ν)-contact metric manifolds exist. Another problem which is studied is the classification of biharmonic Legendre curves and anti-invariant surfaces immersed in 3-dimensional (κ, μ, ν)-contact metric manifolds. It is proved that biharmonic Legendre curves in 3-dimensional (κ, μ, ν)-contact metric manifolds are necessarily geodesics. Furthermore, it is proved that biharmonic and without minimal points anti-invariant surfaces immersed in 3-dimensional generalized (κ, μ)-contact metric manifolds with constant norm of the mean curvature vector field, are locally flat.

Στη μεταπτυχιακή αυτή διπλωματική εργασία, αρχικά εισάγουμε τις έννοιες των μετρικών πολλαπλοτήτων σχεδόν επαφής και των μετρικών πολλαπλοτήτων επαφής, δίνοντας και μερικά παραδείγματα από κάθε κατηγορία. Στη συνέχεια, αναφέρουμε και αποδεικνύουμε λεπτομερώς μερικές βασικές γεωμετρικές ιδιότητες που χαρακτηρίζουν τις μετρικές πολλαπλότητες επαφής και, οι οποίες, εμπλέκουν τον τανυστή καμπυλό- τητας. Τέλος, δίνεται έμφαση σε ειδικές κατηγορίες μετρικών πολλαπλοτήτων επαφής που παρουσιάζουν ιδιαίτερο γεωμετρικό ενδιαφέρον και, κυρίως, είναι: πολλαπλότητες K- επαφής, πολλαπλότητες του Sasaki, (κ, μ) – πολλαπλότητες επαφής και μετρικές πολλαπλότητες Η – επαφής. / In this Master Thesis, we initially introduce the notions of almost contact metric manifolds and contact metric manifolds, giving some examples from each category. In sequel, we mention and prove explicitly some basic geometric properties of contact metric manifolds, which involve the curvature tensor. Summarizing, we focus on special classes of contact metric manifolds which have particular geometric interest and, mainly, are: K – contact manifolds, Sasakian manifolds, (κ, μ) – contact manifolds and H – contact metric manifolds.

Almost contact metric submersions constitute a class of Riemannian submersions whose total space is an almost contact metric manifold. Regarding the base space, two types are studied. Submersions of type I are those whose base space is an almost contact metric manifold while, when the base space is an almost Hermitian manifold, then the submersion is said to be of type II. After recalling the known notions and fundamental properties to be used in the sequel, relationships between the structure of the fibres with that of the total space are established. When the fibres are almost Hermitian manifolds, which occur in the case of a type I submersions, we determine the classes of submersions whose fibres are Kählerian, almost Kählerian, nearly Kählerian, quasi Kählerian, locally conformal (almost) Kählerian, Gi-manifolds and so on. This can be viewed as a classification of submersions of type I based upon the structure of the fibres. Concerning the fibres of a type II submersions, which are almost contact metric manifolds, we discuss how they inherit the structure of the total space. Considering the curvature property on the total space, we determine its corresponding on the fibres in the case of a type I submersions. For instance, the cosymplectic curvature property on the total space corresponds to the Kähler identity on the fibres. Similar results are obtained for Sasakian and Kenmotsu curvature properties. After producing the classes of submersions with minimal, superminimal or umbilical fibres, their impacts on the total or the base space are established. The minimality of the fibres facilitates the transference of the structure from the total to the base space. Similarly, the superminimality of the fibres facilitates the transference of the structure from the base to the total space. Also, it is shown to be a way to study the integrability of the horizontal distribution. Totally contact umbilicity of the fibres leads to the asymptotic directions on the total space. Submersions of contact CR-submanifolds of quasi-K-cosymplectic and quasi-Kenmotsu manifolds are studied. Certain distributions of the under consideration submersions induce the CR-product on the total space. / Mathematical Sciences / D. Phil. (Mathematics)

Almost contact metric submersions constitute a class of Riemannian submersions whose total space is an almost contact metric manifold. Regarding the base space, two types are studied. Submersions of type I are those whose base space is an almost contact metric manifold while, when the base space is an almost Hermitian manifold, then the submersion is said to be of type II. After recalling the known notions and fundamental properties to be used in the sequel, relationships between the structure of the fibres with that of the total space are established. When the fibres are almost Hermitian manifolds, which occur in the case of a type I submersions, we determine the classes of submersions whose fibres are Kählerian, almost Kählerian, nearly Kählerian, quasi Kählerian, locally conformal (almost) Kählerian, Gi-manifolds and so on. This can be viewed as a classification of submersions of type I based upon the structure of the fibres. Concerning the fibres of a type II submersions, which are almost contact metric manifolds, we discuss how they inherit the structure of the total space. Considering the curvature property on the total space, we determine its corresponding on the fibres in the case of a type I submersions. For instance, the cosymplectic curvature property on the total space corresponds to the Kähler identity on the fibres. Similar results are obtained for Sasakian and Kenmotsu curvature properties. After producing the classes of submersions with minimal, superminimal or umbilical fibres, their impacts on the total or the base space are established. The minimality of the fibres facilitates the transference of the structure from the total to the base space. Similarly, the superminimality of the fibres facilitates the transference of the structure from the base to the total space. Also, it is shown to be a way to study the integrability of the horizontal distribution. Totally contact umbilicity of the fibres leads to the asymptotic directions on the total space. Submersions of contact CR-submanifolds of quasi-K-cosymplectic and quasi-Kenmotsu manifolds are studied. Certain distributions of the under consideration submersions induce the CR-product on the total space. / Mathematical Sciences / D. Phil. (Mathematics)