7,465 research outputs found
Join-Reachability Problems in Directed Graphs
For a given collection G of directed graphs we define the join-reachability
graph of G, denoted by J(G), as the directed graph that, for any pair of
vertices a and b, contains a path from a to b if and only if such a path exists
in all graphs of G. Our goal is to compute an efficient representation of J(G).
In particular, we consider two versions of this problem. In the explicit
version we wish to construct the smallest join-reachability graph for G. In the
implicit version we wish to build an efficient data structure (in terms of
space and query time) such that we can report fast the set of vertices that
reach a query vertex in all graphs of G. This problem is related to the
well-studied reachability problem and is motivated by emerging applications of
graph-structured databases and graph algorithms. We consider the construction
of join-reachability structures for two graphs and develop techniques that can
be applied to both the explicit and the implicit problem. First we present
optimal and near-optimal structures for paths and trees. Then, based on these
results, we provide efficient structures for planar graphs and general directed
graphs
Monte Carlo algorithm based on internal bridging moves for the atomistic simulation of thiophene oligomers and polymers
We introduce a powerful Monte Carlo (MC) algorithm for the atomistic
simulation of bulk models of oligo- and poly-thiophenes by redesigning MC moves
originally developed for considerably simpler polymer structures and
architectures, such as linear and branched polyethylene, to account for the
ring structure of the thiophene monomer. Elementary MC moves implemented
include bias reptation of an end thiophene ring, flip of an internal thiophene
ring, rotation of an end thiophene ring, concerted rotation of three thiophene
rings, rigid translation of an entire molecule, rotation of an entire molecule
and volume fluctuation. In the implementation of all moves we assume that
thiophene ring atoms remain rigid and strictly co-planar; on the other hand,
inter-ring torsion and bond bending angles remain fully flexible subject to
suitable potential energy functions. Test simulations with the new algorithm of
an important thiophene oligomer, {\alpha}-sexithiophene ({\alpha}-6T), at a
high enough temperature (above its isotropic-to-nematic phase transition) using
a new united atom model specifically developed for the purpose of this work
provide predictions for the volumetric, conformational and structural
properties that are remarkably close to those obtained from detailed atomistic
Molecular Dynamics (MD) simulations using an all-atom model. The new algorithm
is particularly promising for exploring the rich (and largely unexplored) phase
behavior and nanoscale ordering of very long (also more complex)
thiophene-based polymers which cannot be addressed by conventional MD methods
due to the extremely long relaxation times characterizing chain dynamics in
these systems
A case of an accessory testicular artery
Dissection of an adult male cadaver revealed the presence of an accessory left
testicular artery in addition to the normal right and left testicular arteries. In this
case the accessory left testicular artery originated from the ventrolateral wall of
the descending aorta. The origin was located between the superior mesenteric
artery and the left renal vein. The accessory artery continued to course from the
aorta laterally toward the superior ventral portion of the left kidney and then
passed ventrally to the kidney on its course inferiorly to the pelvic region. Communication
was observed between the accessory left testicular artery and the
left renal artery. This variation of gonadal vasculature is of interest from the
point of view of its embryogenesis, and possible clinical significance
Evaluation of Compton scattering sequence reconstruction algorithms for a portable position sensitive radioactivity detector based on pixelated Cd(Zn)Te crystals
We present extensive simulation studies on the performance of algorithms for
the Compton sequence reconstruction used for the development of a portable
spectroscopic instrument (COCAE), with the capability to localize and identify
radioactive sources, by exploiting the Compton scattering imaging. Various
Compton Sequence reconstruction algorithms have been compared using a large
number of simulated events. These algorithms are based on Compton kinematics,
as well as on statistical test criteria that exploit the redundant information
of events having two or more photon interactions in the active detector's
volume. The efficiency of the best performing technique is estimated for a wide
range of incident gamma-ray photons emitted from point-like gamma sources.Comment: 16 pages, 17 figure
Μαθηματική Προτυποποίηση του νευρομοντέλου Hodgkin - Huxley
68 σ.Το περιεχόμενο αυτής της διπλωματικής εργασίας είναι η μαθηματική κατασκευή και παρουσίαση του μοντέλου Hodgkin – Huxley που αφορά στη διάδοση του δυναμικού δράσης στο γιγαντιαίο άξονα του καλαμαριού, καθώς και η αριθμητική επίλυση του. Τα δυναμικά δράσης αποτελούν το μηχανισμό μετάδοσης της πληροφορίας από κύτταρο σε κύτταρο. Έτσι περιγράφοντας μαθηματικά το μοντέλο Hodgkin – Huxley γίνεται κατανοητός ο τρόπος με τον οποίο λειτουργεί ο οργανισμός μας όταν μεταδίδεται κάποια πληροφορία στα νευρικά κύτταρα.
Στην εισαγωγή δίνεται το απαραίτητο βιολογικό υπόβαθρο και οι αναγκαίες έννοιες που χρειάζονται για την διαδικασία παραγωγής του μοντέλου.
Στο δεύτερο κεφάλαιο «Θεμέλια του μοντέλου Hodgkin - Huxley» παρουσιάζονται αναγκαίες εξισώσεις πάνω στις οποίες στηρίζεται η παραγωγή του μοντέλου Hodgkin - Huxley.
Στο τρίτο κεφάλαιο «Κατασκευή του μοντέλου Hodgkin - Huxley» γίνεται η κατασκευή του μοντέλου Hodgkin – Huxley από καθαρά μαθηματική σκοπιά, καταλήγοντας σε ένα σύστημα τεσσάρων μη γραμμικών διαφορικών εξισώσεων που περιγράφουν την απόκριση των συναρτήσεων των πυλών και της συνάρτησης του δυναμικού της μεμβράνης συναρτήσει του χρόνου όταν πυροδοτείται ένα δυναμικό δράσης.
Στο τέταρτο και τελευταίο κεφάλαιο «Αριθμητικές μέθοδοι για την επίλυση του μοντέλου Hodgkin - Huxley» επιλύεται αριθμητικά το μοντέλο Hodgkin – Huxley με γνωστές αριθμητικές μεθόδους και με τη βοήθεια της γλώσσας προγραμματισμού Matlab γίνονται οι γραφικές παραστάσεις των συναρτήσεων του μοντέλου συναρτήσει του χρόνου.Content of this thesis is the mathematical structure and presentation of the Hodgkin - Huxley model on the propagation of action potentials in the giant squid axon and the numerical solution of it. Action potentials are the transmission mechanism of information from cell to cell. So describing the Hodgkin - Huxley model we understand the way our body works when transmitting any information on nerve cells.
In the introduction the necessary biological background and concepts needed are given for the production process of this model.
In the second chapter, "Foundations of the Hodgkin – Huxley model» necessary equations are presented underpinning the production of the model.
The third chapter "Construction of the Hodgkin – Huxley model» presents the construction of Hodgkin - Huxley model from a purely mathematical point of view, resulting in a system of four nonlinear differential equations describing the response of the gate functions and the function of membrane potential versus time when an action potential is triggered.
In the fourth and final chapter "Numerical methods for solving the Hodgkin – Huxley model» the Hodgkin – Huxley model is numerically solved with known numerical methods and function graphs are plotted using Matlab programming languageΖάγκος Δ. Λουκά
Anatomical variations of the superficial and deep palmar arches
The use of radial arteries as an arterial bypass conduit is an invasive procedure
which is becoming popular among various medical centres. The greatest risk
associated with harvesting the radial artery is ischaemia of the soft tissues of the
hand. In this study we dissected 200 hands derived from 100 formalin-fixed
cadavers in order to identify arterial patterns that will allow safe removal of the
radial artery for use in bypass procedures. A complete superficial palmar arch
(SPA) was found in 90% of the cases and divided into 5 types, while the remaining
10% possessed an incomplete palmar arch. Types of SPA are designated by
the letter S. In type S-I (40%), the SPA is formed by anastomosis of the superficial
volar branch of the radial artery to the ulnar artery. Type S-II (35%) is formed
entirely of the ulnar artery. Type S-III (15%) is formed by anastomosis of the
ulnar and median arteries. Type S-IV (6%) is formed by anastomosis of the ulnar,
radial, and median arteries and Type S-V (4%) is formed by a branch of the deep
palmar arch (DPA) communicating with the SPA.DPA was identified in all specimens
and classified into three types, all designated by the letter D. Type D-I
(60%) is formed by anastomosis of the deep volar branch of the radial artery and
the inferior deep branch of the ulnar branch. Type D-II (30%) is formed by anastomosis
of the deep volar branch of the radial artery and the superior deep
branch of the ulnar artery. Type D-III (10%) is formed by anastomosis of the
deep volar branch of the radial artery with both deep branches of the ulnar
artery. This data could provide an important source of information for vascular
surgeons harvesting radial arteries
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