Geometric flow description of minimal surfaces

We introduce a description of a minimal surface in a space with boundary, as the world hypersurface that the entangling surface traces. It does so by evolving from the boundary to the interior of the bulk under an appropriate geometric flow, whose parameter is the holographic coordinate. We specify this geometric flow for arbitrary bulk geometry. In the case of pure AdS spaces, we implement a perturbative approach for the solution of the flow equation around the boundary. We systematically study both the form of the perturbative solution as well as its dependence on the boundary conditions. This expansion is sufficient for the determination of all the divergent terms of the holographic entanglement entropy, including the logarithmic universal terms in odd spacetime bulk dimensions, for an arbitrary entangling surface, in terms of the extrinsic geometry of the latter. © 2020 author

Salient features of dressed elliptic string solutions on R× S 2

We study several physical aspects of the dressed elliptic strings propagating on R× S 2 and of their counterparts in the Pohlmeyer reduced theory, i.e. the sine-Gordon equation. The solutions are divided into two wide classes; kinks which propagate on top of elliptic backgrounds and non-localised periodic disturbances of the latter. The former class of solutions obey a specific equation of state that is in principle experimentally verifiable in systems which realize the sine-Gordon equation. Among both of these classes, there appears to be a particular class of interest the closed dressed strings. They in turn form four distinct subclasses of solutions. One of those realizes instabilities of the seed elliptic solutions. The existence of such solutions depends on whether a superluminal kink with a specific velocity can propagate on the corresponding elliptic sine-Gordon solution. Unlike the elliptic strings, the dressed ones exhibit interactions among their spikes. These interactions preserve an appropriately defined turning number, which can be associated to the topological charge of the sine-Gordon counterpart. Finally, the dispersion relations of the dressed strings are studied. A qualitative difference between the two wide classes of dressed strings is discovered. © 2019, The Author(s)

The dressing method as non linear superposition in sigma models

We apply the dressing method on the Non Linear Sigma Model (NLSM), which describes the propagation of strings on ℝ × S2, for an arbitrary seed. We obtain a formal solution of the corresponding auxiliary system, which is expressed in terms of the solutions of the NLSM that have the same Pohlmeyer counterpart as the seed. Accordingly, we show that the dressing method can be applied without solving any differential equations. In this context a superposition principle emerges: the dressed solution is expressed as a non-linear superposition of the seed with solutions of the NLSM with the same Pohlmeyer counterpart as the seed. © 2021, The Author(s)

Elliptic string solutions on R× S 2 and their pohlmeyer reduction

We study classical string solutions on R× S 2 that correspond to elliptic solutions of the sine-Gordon equation. In this work, these solutions are systematically derived by inverting the Pohlmeyer reduction. A mapping of the physical properties of the string solutions to those of their Pohlmeyer counterparts is established. An interesting element of this mapping is the association of the number of spikes of the string to the topological charge in the sine-Gordon theory. Finally, the adopted parametrization of the solutions facilitates the identification of a dense subset of the moduli space of solutions, where the dispersion relation can be expressed in a closed form, arbitrarily far from the infinite size limit. © 2018, The Author(s)

Dressed minimal surfaces in AdS4

We apply an arbitrary number of dressing transformations to a static minimal surface in AdS4. Interestingly, a single dressing transformation, with the simplest dressing factor, interrelates the latter to solutions of the Euclidean non linear sigma model in dS3. We present an expression for the area element of the dressed minimal surface in terms of that of the initial one and comment on the boundary region of the dressed surface. Finally, we apply the above formalism to the elliptic minimal surfaces and obtain new ones. © 2020, The Author(s)

Idiopathic posterior laryngeal web on an adult patient

Posterior laryngeal webs are uncommon pathologies that are usually acquired at some point in adult life. Prior and prolonged intubation is the leading cause for developing such lesions. In certain rare cases of posterior laryngeal webs, no identifiable cause can be associated with the development of this pathology. We present a case with such an idiopathic lesion. Surgery is the treatment of choice. Several techniques and modifications can be used, to achieve maximum airway release and, at the same time, ensure that restenosis will be avoided. Our patient recurred following initial treatment and a more complex revision surgery was necessary. Special attention should be given in differentiating these lesions from abductor vocal cord paralysis. Furthermore, careful investigation of the cause and individualisation of patient treatment are crucial. © 2021 BMJ Publishing Group Limited. Published by BMJ

Cartographical parameters of navigation systems: Landmarks

206 σ.Μεταπτυχιακή Εργασία- - Εθνικό Μετσόβιο Πολυτεχνείο. Διεπιστημονικό - Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) "Γεωπληροφορική"Το κεντρικό ζήτημα με το οποίο καταπιάνεται η παρούσα διπλωματική εργασία είναι αν τα ορόσημα μπορούν να αποκτήσουν πρωταγωνιστικό ρόλο στην πλοήγηση επαναδιαμορφώνοντας την αρχιτεκτονική των navigation systems. Στην παρούσα εργασία διαπιστώνεται η χρησιμότητα τους στην πλοήγηση, προσδιορίζεται η βαρύτητα των ορόσημων ως προς τον προσανατολισμό, και αναλύεται η «χαρτογραφική» όψη τους.The main issue of this study is the role of landmarks in modern navigation systems: Throughout this study, the author estimates and justifies their importance and analyses their cartographical aspect.Αναστάσης-Αυγερινός Γ. Κατσίνη