142 research outputs found
Boost invariant marginally trapped surfaces in Minkowski 4-space
The extremal and partly marginally trapped surfaces in Minkowski 4-space,
which are invariant under the group of boost isometries, are classified.
Moreover, it is shown that there do not exist extremal surfaces of this kind
with constant Gaussian curvature. A procedure is given in order to construct a
partly marginally trapped surface by gluing two marginally trapped surfaces
which are invariant under the group of boost isometries. As an application, a
proper star-surface is constructed.Comment: 13 pages, comment added in section
Note on (conformally) semi-symmetric spacetimes
We provide a simple proof that conformally semi-symmetric spacetimes are
actually semi-symmetric. We also present a complete refined classification of
the semi-symmetric spacetimes.Comment: 5 pages, no figure
Timelike surfaces with zero mean curvature in Minkowski 4-space
On any timelike surface with zero mean curvature in the four-dimensional
Minkowski space we introduce special geometric (canonical) parameters and prove
that the Gauss curvature and the normal curvature of the surface satisfy a
system of two natural partial differential equations. Conversely, any two
solutions to this system determine a unique (up to a motion) timelike surface
with zero mean curvature so that the given parameters are canonical. We find
all timelike surfaces with zero mean curvature in the class of rotational
surfaces of Moore type. These examples give rise to a one-parameter family of
solutions to the system of natural partial differential equations describing
timelike surfaces with zero mean curvature.Comment: 15 page
Ideally embedded space-times
Due to the growing interest in embeddings of space-time in higher-dimensional
spaces we consider a specific type of embedding. After proving an inequality
between intrinsically defined curvature invariants and the squared mean
curvature, we extend the notion of ideal embeddings from Riemannian geometry to
the indefinite case. Ideal embeddings are such that the embedded manifold
receives the least amount of tension from the surrounding space. Then it is
shown that the de Sitter spaces, a Robertson-Walker space-time and some
anisotropic perfect fluid metrics can be ideally embedded in a five-dimensional
pseudo-Euclidean space.Comment: layout changed and typos corrected; uses revtex
Formal modelling as a component of user centred design
User centred design approaches typically focus understanding on context and producing sketch designs. These sketches are often non functional (e.g., paper) prototypes. They provide a means of exploring candidate design possibilities using techniques such as cooperative evaluation. This paper describes a further step in the process using formal analysis techniques. The sketch design of a device is enhanced into a specification that is then analysed using formal techniques, thus providing a systematic approach to checking plausibility and consistency during early design stages. Once analysed, a further prototype is constructed using an executable form of the specification, providing the next candidate for evaluation with potential users. The technique is illustrated through an example based on a pill dispenser.We are grateful to Nuno Rodrigues, João Vilaça and Nuno Dias from IPCA (Polytechnic Institute of Cavado and Ave) who developed the first prototypeof the pill dispenser. José C. Campos, Paolo Masci and Michael Harrison werefunded by project NORTE-01-0145-FEDER-000016, financed by the North Por-tugal Regional Operational Programme (NORTE 2020), under the PORTUGAL2020 Partnership Agreement, and through the European Regional DevelopmentFund (ERDF)
Modified Brans-Dicke theory of gravity from five-dimensional vacuum
We investigate, in the context of five-dimensional (5D) Brans-Dicke theory of
gravity, the idea that macroscopic matter configurations can be generated from
pure vacuum in five dimensions, an approach first proposed in the framework of
general relativity. We show that the 5D Brans-Dicke vacuum equations when
reduced to four dimensions lead to a modified version of Brans-Dicke theory in
four dimensions (4D). As an application of the formalism, we obtain two
five-dimensional extensions of four-dimensional O'Hanlon and Tupper vacuum
solution and show that they lead two different cosmological scenarios in 4D.Comment: 9 page
The relationship between sensory sensitivity and autistic traits in the general population.
Individuals with Autism Spectrum Disorders (ASDs) tend to have sensory processing difficulties (Baranek et al. in J Child Psychol Psychiatry 47:591–601, 2006). These difficulties include over- and under-responsiveness to sensory stimuli, and problems modulating sensory input (Ben-Sasson et al. in J Autism Dev Disorders 39:1–11, 2009). As those with ASD exist at the extreme end of a continuum of autistic traits that is also evident in the general population, we investigated the link between ASD and sensory sensitivity in the general population by administering two questionnaires online to 212 adult participants. Results showed a highly significant positive correlation (r = .775, p < .001) between number of autistic traits and the frequency of sensory processing problems. These data suggest a strong link between sensory processing and autistic traits in the general population, which in turn potentially implicates sensory processing problems in social interaction difficulties
Visual Methods for the Design of Shape-Changing Interfaces
Shape-changing interfaces use physical change in shape as input and/or output. As the field matures, it will move from technology-driven design toward more formal processes. However, this is challenging: end-users are not aware of the capabilities of shape-change, devices are difficult to demonstrate, and presenting single systems can ‘trap’ user-thinking into particular forms. It is crucial to ensure this technology is developed with requirements in mind to ensure successful end-user experiences. To address this challenge, we developed and tested (n = 50) an approach that combines low-fidelity white-box prototypes and high-fidelity video footage with end-user diagram and scenario sketching to design context dependent devices. We analysed the outputs of our test process and identified themes in device design requirements, and from this constructed a shape-change stack model to support practitioners in developing, classifying, and synthesising end-user requirements for this novel technology
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