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

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 síntomas de EHE y el CNV y las características de imagen del CNV (tipo, número y localización). Material y métodos: Se han estudiado prospectivamente 120 ángulos pontocerebelosos (APC), 44 con clínica 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 tenían 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 estadísticamente significativa entre la presencia de CNV y la clínica de EHE (p < 0,0001). Entre las características del CNV estudiadas, únicamente se halló una asociación estadísticamente significativa entre el desplazamiento del nervio facial (dentro de tipo de CNV) y la clínica 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 clínica 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 angiografía 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 síntomas de EHE y el CNV y las características de imagen del CNV (tipo, número y localización). Material y métodos: Se han estudiado prospectivamente 120 ángulos pontocerebelosos (APC), 44 con clínica 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 tenían 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 estadísticamente significativa entre la presencia de CNV y la clínica de EHE (p < 0,0001). Entre las características del CNV estudiadas, únicamente se halló una asociación estadísticamente significativa entre el desplazamiento del nervio facial (dentro de tipo de CNV) y la clínica 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 clínica 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