Fourth order nematic elasticity and modulated nematic phases: a poor man's approach

We propose an extension of Frank-Oseen's elastic energy for bulk nematic liquid crystals which is based on the hypothesis that the fundamental deformations allowed in nematic liquid crystals are splay, twist and bend. The extended elastic energy is a fourth order form in the fundamental deformations. The existence of bulk spontaneous modulated or deformed nematic liquid crystal ground states is investigated. The analysis is limited to bulk nematic liquid crystals in the absence of limiting surfaces and/or external fields. The non deformed ground state is stable only when Frank-Oseen's elastic constants are positive. In case where at least one of them is negative, the ground state becomes deformed. The analysis of the stability of the deformed states in the space of the elastic parameters allows to characterize different types of deformed nematic phases. Some of them are new nematic phases, for instance a twist -- splay nematic phase is predicted. Inequalities between second order elastic constants which govern the stability of the twist--bend and splay--bend state are obtained

Improved Delay Estimates for a Queueing Model for Random Linear Coding for Unicast

Consider a lossy communication channel for unicast with zero-delay feedback. For this communication scenario, a simple retransmission scheme is optimum with respect to delay. An alternative approach is to use random linear coding in automatic repeat-request (ARQ) mode. We extend the work of Shrader and Ephremides, by deriving an expression for the delay of random linear coding over field of infinite size. Simulation results for various field sizes are also provided.Comment: 5 pages, 3 figures, accepted at the 2009 IEEE International Symposium on Information Theor

Windsurf: trying to stay afloat in water and in life

Desde una óptica biográfico-narrativa, este artículo gira en torno a Moisés, un profesional del campo de la Actividad Física Educativa, Recreativa y Deportiva (AFERD) que, a pesar de sus limitadas habilidades natatorias, ha dedicado buena parte de su vida al windsurf. Los objetivos se ciñen, primero, a reconstruir su trayectoria vital en relación con la natación y el windsurf y, segundo, a interpretar su biografía a partir del contexto y condicionantes sociales en que se desarrolla. Metodológicamente hemos utilizado, entre otros, la entrevistas narrativa, la semi-estructurada y el relato autobiográfico por encargo. Tras desglosar la biografía de Moisés se argumenta y concluye una mejor comprensión a partir de ciertos conceptos (habitus, identidad deportiva y espacio-cultura-estilo-de-vida distintivo) que permiten relacionar lo ‘personal’ con lo ‘social’. Moisés asume todo lo que aquí se dice y, además, nos ha sugerido el título del artículo.From a biographical narrative perspective, this paper focuses on Moses, a professional of the field of Physical Activity in Education, Leisure and Sports (AFERD) who, despite his very limited swimming skills, has devoted much of his life to windsurfing. The two objectives are: first, the reconstruction of Moses’ biography in relation to swimming and windsurfing and, second, the interpretation of such biography taking into account its context and conditions of existence. Methods used, among others, are: narrative interviews, semi-structured interviews and assigned autobiographical essays. Having analysed Moses’ biography, it’s argued that it is best understood by using some concepts (habitus, athletic identity and distinctive space-culture-and-life-style) that allows us to relate the ‘personal’ to the ‘social’. Moses accepts our explanations and, furthermore, he suggests the paper’s title

Nanometric pitch in modulated structures of twist-bend nematic liquid crystals

The extended Frank elastic energy density is used to investigate the existence of a stable periodically modulate structure that appears as a ground state exhibiting a twist-bend molecular arrangement. For an unbounded sample, we show that the twist-bend nematic phase $N_{TB}$ is characterized by a heliconical structure with a pitch in the nano-metric range, in agreement with experimental results. For a sample of finite thickness, we show that the wave vector of the stable periodic structure depends not only on the elastic parameters but also on the anchoring energy, easy axis direction, and the thickness of the sample.Comment: 11 page

Regional Positioning Services as economic and construction activity indicators: the case study of Andalusian Positioning Network (Southern Spain)

The Andalusian Positioning Network (RAP) is a regional GNSS permanent network in southern Spain that offers two public services for precise positioning: real-time (RAP-IP) and post processing (RAP-FTP). The most of registered users are linked to the construction sector, so the collapse of the housing bubble and the Spanish economic crisis influenced the use of RAP services from 2008. The behavior of these services has differed somewhat, although analysis for the years 2008 to 2013 reveals a general decline. Since 2009 the RAP-FTP service demand fell by 50%, but the RAP-IP service was stable until October 2011, when it began a steep decline in December 2012. Analyzing the temporal demand and the geospatial connections of RAP-IP with economic impact indicators, we found a high linear correlation between this service and jobs in the construction sector (0.98) and permit building (0.96). The real time and associated geoinformation are its main advantages