High-order, Dispersionless "Fast-Hybrid" Wave Equation Solver. Part I: O(1)\mathcal{O}(1) Sampling Cost via Incident-Field Windowing and Recentering

This paper proposes a frequency/time hybrid integral-equation method for the time dependent wave equation in two and three-dimensional spatial domains. Relying on Fourier Transformation in time, the method utilizes a fixed (time-independent) number of frequency-domain integral-equation solutions to evaluate, with superalgebraically-small errors, time domain solutions for arbitrarily long times. The approach relies on two main elements, namely, 1) A smooth time-windowing methodology that enables accurate band-limited representations for arbitrarily-long time signals, and 2) A novel Fourier transform approach which, in a time-parallel manner and without causing spurious periodicity effects, delivers numerically dispersionless spectrally-accurate solutions. A similar hybrid technique can be obtained on the basis of Laplace transforms instead of Fourier transforms, but we do not consider the Laplace-based method in the present contribution. The algorithm can handle dispersive media, it can tackle complex physical structures, it enables parallelization in time in a straightforward manner, and it allows for time leaping---that is, solution sampling at any given time $T$ at $\mathcal{O}(1)$-bounded sampling cost, for arbitrarily large values of $T$, and without requirement of evaluation of the solution at intermediate times. The proposed frequency-time hybridization strategy, which generalizes to any linear partial differential equation in the time domain for which frequency-domain solutions can be obtained (including e.g. the time-domain Maxwell equations), and which is applicable in a wide range of scientific and engineering contexts, provides significant advantages over other available alternatives such as volumetric discretization, time-domain integral equations, and convolution-quadrature approaches.Comment: 33 pages, 8 figures, revised and extended manuscript (and now including direct comparisons to existing CQ and TDIE solver implementations) (Part I of II

Groups as a means or an end? Social capital and the promotion of cooperation in Ghana

In the past two decades there has been a growing emphasis within the international development industry on promoting group activity. This paper charts how interpretation of the loose concept of social capital has shaped donor and NGO discourses on, and their preoccupation with, groups. Donors are using blueprints of group cooperation in an asocial and aspatial manner that ignores local specificities of place, space and cultural context. An empirical case is examined that demonstrates how donor discourse is reinterpreted, translated and even rejected by players at different spatial scales. The reasons for the continued donor preoccupation with groups in the face of local resistances are explored

Social capital as culture? Promoting cooperative action in Ghana

In this chapter we focus on culture and its complex interconnections with the concept of social capital. Our study is set in the context of recent preoccupations of the World Bank (followed by other donors) with the concept of social capital itself and the related construct that it can be built in order to promote economic growth and development. The adoption of the social capital concept is perhaps the closest that the World Bank has come in recent years in its recognition of the potential linkages between local cultures (notably cultures of enterprise), economic growth and development. That is not to say that we agree that culture is social capital – or vice versa. Far from it, indeed! Our thesis is rather that the World Bank has taken up social capital in a highly essentialized form - as group cooperation per se - in its development initiatives, whilst at the same time congratulating itself on its adoption of a more culturally (as opposed to economistically) oriented development paradigm. The Bank’s conflation of social capital construction with group activities, and its consequent efforts to promote development through supporting group-based initiatives, far from illustrating a cultural turn in its development thinking, arguably reflect a continuing lack of sensitivity to cultural diversity and the specific geographical contexts within which diverse cultural registers (elite, popular and youth cultures, among others) evolve and interact (see also Nederveen Pieterse, this volume). Following a brief introduction to the links between concepts of social capital and culture, we review recent development problems and donor activities in Ghana. We then present two rural case studies of group-based development interventions in Ghana’s coastal savanna to illustrate our argument that while culture is complex, multi-faceted and inextricably linked with place and time dynamics, recent development interventions seemingly emanating out of donors’ desire to build social capital have been based on very poor conceptualizations of culture

Evolving institutions of trust: personalized and institutional bases of trust in Nigerian and Ghanaian food trading

This paper examines the processes of building cooperation in a context of sparse public-sector regulation. The Nigerian and Ghanaian food sectors are characterized by a highly dispersed and fragmented system of micro-entrepreneurs from diverse ethnic groups who both compete and cooperate in order to flourish. Drawing on ethnographic research, we consider the relationships and contracts that require an element of cross-cultural trust, how personal social relations and institutional forms are used to ensure trust, and the role of cultural norms. Our empirical findings indicate that individuals draw on both personalized social relations and institutional forms of trust that are underpinned by culture specific norms. Through personalized trust, traders have been able to operate across cultural boundaries, building common norms of behavior over centuries, and shaping these into what are perceived essentially as professional, albeit personalized, codes of conduct and semi formal institutional forms (such as associations) that function in parallel to the state

Exchange coupling between silicon donors: the crucial role of the central cell and mass anisotropy

Donors in silicon are now demonstrated as one of the leading candidates for implementing qubits and quantum information processing. Single qubit operations, measurements and long coherence times are firmly established, but progress on controlling two qubit interactions has been slower. One reason for this is that the inter donor exchange coupling has been predicted to oscillate with separation, making it hard to estimate in device designs. We present a multivalley effective mass theory of a donor pair in silicon, including both a central cell potential and the effective mass anisotropy intrinsic in the Si conduction band. We are able to accurately describe the single donor properties of valley-orbit coupling and the spatial extent of donor wave functions, highlighting the importance of fitting measured values of hyperfine coupling and the orbital energy of the $1s$ levels. Ours is a simple framework that can be applied flexibly to a range of experimental scenarios, but it is nonetheless able to provide fast and reliable predictions. We use it to estimate the exchange coupling between two donor electrons and we find a smoothing of its expected oscillations, and predict a monotonic dependence on separation if two donors are spaced precisely along the [100] direction.Comment: Published version. Corrected b and B values from previous versio

Developments in electromagnetic tomography instrumentation.

A new EMT sensor and instrumentation is described which combines the best features of previous systems and has a modular structure to allow for future system expansion and development

Assessing Distributed Leadership and the Successful Implementation of Evidence-Based Practices

Kelly and Halverson are to be congratulated on their contribution to the field of education. Their efforts in designing The Comprehensive Assessment of Leadership forLearning (CALL) represents a step forward inm the fomative assessment of distributed leadership in schools and their work is noteworthy in its rapid linking of survey assessment data to specific feedback and recommendations for users. Issues relevant to evidence-based practices, implementation, and professional common language are addressed in this commentary

Optical Systems Characterization and Analysis Research Project

The Optical Systems Characterization and Analysis Research (OSCAR) project was formed in 1994 to look at advanced optical modeling methods, utilizing phase retrieval, to model the optical point spread function of the Hubble Space Telescope (HST). OSCAR developed these techniques and accurately modeled the HST response and further developed maximum entropy deconvolution algorithms and mined the database and published seminal scientific manuscripts. OSCAR subsequently got involved in the Next Generation Space Telescope (NGST), now the James Webb Space Telescope (JWST), and in JWST testbed development. OSCAR has further expanded to include interferometric and coronagraphic systems such as the Stellar Imager (SI) and Terrestrial Planet Finder (TPF). This monograph details some of the work OSCAR is and has been involved in. OSCAR is science driven, thus requiring involvement, which gleams hard science, and does not function as an engineering support center