43 research outputs found
The anatomy of urban social networks and its implications in the searchability problem
The appearance of large geolocated communication datasets has recently
increased our understanding of how social networks relate to their physical
space. However, many recurrently reported properties, such as the spatial
clustering of network communities, have not yet been systematically tested at
different scales. In this work we analyze the social network structure of over
25 million phone users from three countries at three different scales: country,
provinces and cities. We consistently find that this last urban scenario
presents significant differences to common knowledge about social networks.
First, the emergence of a giant component in the network seems to be controlled
by whether or not the network spans over the entire urban border, almost
independently of the population or geographic extension of the city. Second,
urban communities are much less geographically clustered than expected. These
two findings shed new light on the widely-studied searchability in
self-organized networks. By exhaustive simulation of decentralized search
strategies we conclude that urban networks are searchable not through
geographical proximity as their country-wide counterparts, but through an
homophily-driven community structure
The elliptic model for communication fluxes
In this paper, a model (called the elliptic model) is proposed to estimate the number of social ties between two locations using population data in a similar manner to how transportation research deals with trips. To overcome the asymmetry of transportation models, the new model considers that the number of relationships between two locations is inversely proportional to the population in the ellipse whose foci are in these two locations. The elliptic model is evaluated by considering the anonymous communications patterns of 25 million users from three different countries, where a location has been assigned to each user based on their most used phone tower or billing zip code. With this information, spatial social networks are built at three levels of resolution: tower, city and region for each of the three countries. The elliptic model achieves a similar performance when predicting communication fluxes as transportation models do when predicting trips. This shows that human relationships are influenced at least as much by geography as is human mobility
Steep sharp-crested gravity waves on deep water
A new type of steady steep two-dimensional irrotational symmetric periodic
gravity waves on inviscid incompressible fluid of infinite depth is revealed.
We demonstrate that these waves have sharper crests in comparison with the
Stokes waves of the same wavelength and steepness. The speed of a fluid
particle at the crest of new waves is greater than their phase speed.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Fault Detection and isolationscheme in continuous time nonlinear stochastic dynamical system
Study of neurovascular contact in essential hemifacial spasm: an example of CISS sequence and magnetic resonance angiography
Background and purpose: The purpose of this article is to assess the validity of the magnetic resonance imaging (MRI) CISS 3D sequence associated with 3D time of flight (TOF) angiographic sequence in order to detect neurovascular contact (NVC) between the facial nerve and neighbouring arteries in patients with essential hemifacial spasm (HFS) and to determine the relationship between HFS symptoms and NVC and NVC image features (type, number and site). Materials and methods: We prospectively enrolled 120 cerebellopontine angle (CPA) MRI studies, 44 cases with HFS symptoms and 76 which were asymptomatic (controls), using axial T2-weighted (CISS) and axial 3D TOF series with associated Maximal intensity (MIP) reconstructions. Prior TOF angiographic studies were available for 56 cases without associated CISS images and the results obtained from that study were compared with the results of the current study. Results: The diagnostic values obtained significantly favoured the protocol used in this study, which demonstrated a sensitivity of 77.27% and a specificity of 75%. There was a statistically significant relationship between the presence of NVC and HFS symptoms (p < 0.0001). Only one statistically significant relationship was found between facial nerve displacement (in type of NVC) and HFS symptoms (p = 0.019). Conclusions: The proposed MRI protocol is sensitive and valid for detecting NVC in patients with HFS. The results of our study support a relationship between NVC and HFS symptoms. It is not a simple relationship, however. It may be influenced by other factors, such as displacement of the facial nerve due to NVC. Resumen: Objetivos: Evaluar la validez de la secuencia CISS 3D de resonancia magnética (RM) en relación con secuencias angiográficas TOF 3D para la detección del contacto neurovascular (CNV) entre el nervio facial y las arterias vecinas en pacientes con espasmo hemifacial esencial (EHE) y determinar la asociación entre la presencia de sÃntomas de EHE y el CNV y las caracterÃsticas de imagen del CNV (tipo, número y localización). Material y métodos: Se han estudiado prospectivamente 120 ángulos pontocerebelosos (APC), 44 con clÃnica de EHE y 76 asintomáticos (controles), mediante el empleo de una serie axial potenciada en T2 (CISS) y una serie axial con técnica angiográfica TOF 3D con reconstrucciones MIP; 56 de estos APC tenÃan un estudio angiográfico TOF previo y se compararon sus resultados con los del estudio actual. Resultados: Los valores diagnósticos obtenidos fueron significativos a favor del protocolo del presente trabajo que mostró una sensibilidad del 77,27% y una especificidad del 75%. Se demostró una asociación estadÃsticamente significativa entre la presencia de CNV y la clÃnica de EHE (p < 0,0001). Entre las caracterÃsticas del CNV estudiadas, únicamente se halló una asociación estadÃsticamente significativa entre el desplazamiento del nervio facial (dentro de tipo de CNV) y la clÃnica de EHE (p = 0,019). Conclusiones: El protocolo de RM propuesto es sensible y válido para detectar CNV en pacientes con EHE. Los resultados de nuestro estudio apoyan la asociación entre el CNV y la clÃnica de EHE, si bien no se trata de una asociación simple, sino que puede estar influida por otros factores, como el desplazamiento del nervio facial producido por el CNV. Keywords: Constructive interference at steady state, Neurovascular contact, Essential hemifacial spasm, Magnetic resonance imaging, Time of flight, Palabras clave: Constructive interference at steady state, Contacto neurovascular, Espasmo hemifacial esencial, Resonancia magnética, Time of fligh
Estudio por resonancia magnética del contacto neurovascular en el espasmo hemifacial esencial: empleo de secuencia CISS y angiografÃa por resonancia magnética
Resumen: Objetivos: Evaluar la validez de la secuencia CISS 3D de resonancia magnética (RM) en relación con secuencias angiográficas TOF 3D para la detección del contacto neurovascular (CNV) entre el nervio facial y las arterias vecinas en pacientes con espasmo hemifacial esencial (EHE) y determinar la asociación entre la presencia de sÃntomas de EHE y el CNV y las caracterÃsticas de imagen del CNV (tipo, número y localización). Material y métodos: Se han estudiado prospectivamente 120 ángulos pontocerebelosos (APC), 44 con clÃnica de EHE y 76 asintomáticos (controles), mediante el empleo de una serie axial potenciada en T2 (CISS) y una serie axial con técnica angiográfica TOF 3D con reconstrucciones MIP; 56 de estos APC tenÃan un estudio angiográfico TOF previo y se compararon sus resultados con los del estudio actual. Resultados: Los valores diagnósticos obtenidos fueron significativos a favor del protocolo del presente trabajo que mostró una sensibilidad del 77,27% y una especificidad del 75%. Se demostró una asociación estadÃsticamente significativa entre la presencia de CNV y la clÃnica de EHE (p < 0,0001). Entre las caracterÃsticas del CNV estudiadas, únicamente se halló una asociación estadÃsticamente significativa entre el desplazamiento del nervio facial (dentro de tipo de CNV) y la clÃnica de EHE (p = 0,019). Conclusiones: El protocolo de RM propuesto es sensible y válido para detectar CNV en pacientes con EHE. Los resultados de nuestro estudio apoyan la asociación entre el CNV y la clÃnica de EHE, si bien no se trata de una asociación simple, sino que puede estar influida por otros factores, como el desplazamiento del nervio facial producido por el CNV. Abstract: Background and purpose: The purpose of this article is to assess the validity of the magnetic resonance imaging (MRI) CISS 3D sequence associated with 3D time of flight (TOF) angiographic sequence in order to detect neurovascular contact (NVC) between the facial nerve and neighbouring arteries in patients with essential hemifacial spasm (HFS) and to determine the relationship between HFS symptoms and NVC and NVC image features (type, number and site). Materials and methods: We prospectively enrolled 120 cerebellopontine angle (CPA) MRI studies, 44 cases with HFS symptoms and 76 which were asymptomatic (controls), using axial T2-weighted (CISS) and axial 3D TOF series with associated Maximal intensity (MIP) reconstructions. Prior TOF angiographic studies were available for 56 cases without associated CISS images and the results obtained from that study were compared with the results of the current study. Results: The diagnostic values obtained significantly favoured the protocol used in this study, which demonstrated a sensitivity of 77.27% and a specificity of 75%. There was a statistically significant relationship between the presence of NVC and HFS symptoms (p < 0,0001). Only one statistically significant relationship was found between facial nerve displacement (in type of NVC) and HFS symptoms (p = 0.019). Conclusions: The proposed MRI protocol is sensitive and valid for detecting NVC in patients with HFS. The results of our study support a relationship between NVC and HFS symptoms. It is not a simple relationship, however. It may be influenced by other factors, such as displacement of the facial nerve due to NVC. Palabras clave: Constructive interference at steady state, Contacto neurovascular, Espasmo hemifacial esencial, Resonancia magnética, Time of flight, Keywords: Constructive interference at steady state, Neurovascular contact, Essential hemifacial spasm, Magnetic resonance imaging, Time of fligh