Synchronous Digital Circuits as Functional Programs

Functional programming techniques have been used to describe synchronous digital circuits since the early 1980s and have proven successful at describing certain types of designs. Here we survey the systems and formal underpinnings that constitute this tradition. We situate these techniques with respect to other formal methods for hardware design and discuss the work yet to be done

Psychological contracts in a business school context

Over the last three decades the UK higher education system has operated under an ideological approach sometimes referred to as New Managerialism (Deem, 2004). The psychological contract of the individual actor within this altered environment was the subject of the research in this study. The psychological contract has been defined as an individual’s beliefs regarding the terms and conditions of a reciprocal informal exchange agreement between themselves and their organisations (Rousseau, 1989). The thesis focused on the psychological contracts of higher education lecturers in a post-92 University Business School in the United Kingdom. The study considered the construction of the psychological contract, the appropriateness of the initial contract, perceived influences on the contract, and behavioural consequences of contract breach and/or violation. The research was focussed on the role of the lecturer in interpreting and unpacking his/her perceptions and understandings. The research questions required data that was personal and experiential. Interviews were undertaken which allowed participants to provide life history accounts that described and theorised about their actions in the social world over time. The approach used had a number of limitations which were identified and considered within the thesis. Notwithstanding the limitations of the research approach, the data suggested that each individual had analysed the extent to which a new employment context would deliver transactional, relational, and ideological reward. However, ideology was less relevant in making the decision to accept higher education employment than either transactional or relational elements. Post-entry, sensemaking acted as a confirmation mechanism in respect of the expectations of what the job would entail and the pecuniary and non-pecuniary benefits that would be received. Initial contracts were relatively accurate in their conceptualisation of the work involved in being a higher education academic. Within the Business School examined in this study, management decisions impacted on participants from both an economic and socio-economic perspective. Employees described how individual work contexts were altered by management decisions. Reaction to decisions depended on individual circumstances at any given juncture based on the influences from multiple contexts both internal and external to the workplace. Context was not homogenous and wide-ranging individual differences were apparent. These contexts played a part in defining to what extent changed work environments would be accepted or not. Participants were continuously active and involved in the evaluation of the multiple contexts that were relevant to them. The capacity to manipulate managers and influence decisions to counteract context change was also evident. The ability to thwart changes to work context varied between individuals and over time. This study identified how participants were able to create and shape their own work environment to satisfy their needs and wants during their careers within a structure that remained predominantly organic in nature despite a changing higher education environment. The goal of the employee was to create the idiosyncratic deal, the specific individually tailored work environment that would deliver the satisfaction required from higher education employment. The psychological contracts were self-focussed and self-oriented but this did not necessarily mean that employees were not also actively involved in assisting the organisation to achieve its ambitions. The notion that a managerial agenda had resulted in the erosion of individualism in higher education was not supported. There was evidence that the psychological contract was unilaterally changed and altered by the employee whenever he or she chose, rather than a negotiated change to a binding agreement. Alteration was intrinsically a private determination and often not communicated

Arrows for knowledge-based circuits

Knowledge-based programs (KBPs) are a formalism for directly relating agents' knowledge and behaviour in a way that has proven useful for specifying distributed systems. Here we present a scheme for compiling KBPs to executable automata in finite environments with a proof of correctness in Isabelle/HOL. We use Arrows, a functional programming abstraction, to structure a prototype domain-specific synchronous language embedded in Haskell. By adapting our compilation scheme to use symbolic representations we can apply it to several examples of reasonable size

Characterising the Gravitational Instability in Cooling Accretion Discs

We perform numerical analyses of the structure induced by gravitational instabilities in cooling gaseous accretion discs. For low enough cooling rates a quasi-steady configuration is reached, with the instability saturating at a finite amplitude in a marginally stable disc. We find that the saturation amplitude scales with the inverse square root of the cooling parameter beta = t_cool / t_dyn, which indicates that the heating rate induced by the instability is proportional to the energy density of the induced density waves. We find that at saturation the energy dissipated per dynamical time by weak shocks due is of the order of 20 per cent of the wave energy. From Fourier analysis of the disc structure we find that while the azimuthal wavenumber is roughly constant with radius, the mean radial wavenumber increases with radius, with the dominant mode corresponding to the locally most unstable wavelength. We demonstrate that the density waves excited in relatively low mass discs are always close to co-rotation, deviating from it by approximately 10 per cent. This can be understood in terms of the flow Doppler-shifted phase Mach number -- the pattern speed self-adjusts so that the flow into spiral arms is always sonic. This has profound effects on the degree to which transport through self-gravity can be modelled as a viscous process. Our results thus provide (a) a detailed description of how the self-regulation mechanism is established for low cooling rates, (b) a clarification of the conditions required for describing the transport induced by self-gravity through an effective viscosity, (c) an estimate of the maximum amplitude of the density perturbation before fragmentation occurs, and (d) a simple recipe to estimate the density perturbation in different thermal regimes.Comment: 16 pages, 22 figures. Accepted for publication in MNRAS 11 November 200

Resolved images of self-gravitating circumstellar discs with ALMA

In this paper we present simulated observations of massive self-gravitating circumstellar discs using the Atacama Large Millimetre/sub-millimetre Array (ALMA). Using a smoothed particle hydrodynamics model of a $0.2M_{\odot}$ disc orbiting a $1M_{\odot}$ protostar, with a cooling model appropriate for discs at temperatures below $\sim 160$K and representative dust opacities, we have constructed maps of the expected emission at sub-mm wavelengths. We have then used the CASA ALMA simulator to generate simulated images and visibilities with various array configurations and observation frequencies, taking into account the expected thermal noise and atmospheric opacities. We find that at 345 GHz (870 $\mu$m) spiral structures at a resolution of a few AU should be readily detectable in approximately face-on discs out to distances of the Taurus-Auriga star-forming complex.Comment: 8 pages, 6 figures, accepted to MNRAS. Figure quality degraded. Full paper with higher quality figures available at http://www2.fisica.unimi.it/lodato/CossinsLodatoTesti1.pd

The Nature of Angular Momentum Transport in Radiative Self-Gravitating Protostellar Discs

Semi-analytic models of self-gravitating discs often approximate the angular momentum transport generated by the gravitational instability using the phenomenology of viscosity. This allows the employment of the standard viscous evolution equations, and gives promising results. It is, however, still not clear when such an approximation is appropriate. This paper tests this approximation using high resolution 3D smoothed particle hydrodynamics (SPH) simulations of self-gravitating protostellar discs with radiative transfer. The nature of angular momentum transport associated with the gravitational instability is characterised as a function of both the stellar mass and the disc-to-star mass ratio. The effective viscosity is calculated from the Reynolds and gravitational stresses in the disc. This is then compared to what would be expected if the effective viscosity were determined by assuming local thermodynamic equilibrium or, equivalently, that the local dissipation rate matches the local cooling rate. In general, all the discs considered here settle into a self-regulated state where the heating generated by the gravitational instability is modulated by the local radiative cooling. It is found that low-mass discs can indeed be represented by a local "alpha-parametrisation", provided that the disc aspect ratio is small (H/R < 0.1) which is generally the case when the disc-to-star mass ratio q <0.5. However, this result does not extend to discs with masses approaching that of the central object. These are subject to transient burst events and global wave transport, and the effective viscosity is not well modelled by assuming local thermodynamic equilibrium. In spite of these effects, it is shown that massive (compact) discs can remain stable and not fragment, evolving rapidly to reduce their disc-to-star mass ratios through stellar accretion and radial spreading.Comment: 13 pages, 44 figures, accepted for publication in MNRA

Comparison of prestellar core elongations and large-scale molecular cloud structures in the Lupus 1 region

Turbulence and magnetic fields are expected to be important for regulating molecular cloud formation and evolution. However, their effects on sub-parsec to 100 parsec scales, leading to the formation of starless cores, are not well understood. We investigate the prestellar core structure morphologies obtained from analysis of the Herschel-SPIRE 350 mum maps of the Lupus I cloud. This distribution is first compared on a statistical basis to the large-scale shape of the main filament. We find the distribution of the elongation position angle of the cores to be consistent with a random distribution, which means no specific orientation of the morphology of the cores is observed with respect to the mean orientation of the large-scale filament in Lupus I, nor relative to a large-scale bent filament model. This distribution is also compared to the mean orientation of the large-scale magnetic fields probed at 350 mum with the Balloon-borne Large Aperture Telescope for Polarimetry during its 2010 campaign. Here again we do not find any correlation between the core morphology distribution and the average orientation of the magnetic fields on parsec scales. Our main conclusion is that the local filament dynamics---including secondary filaments that often run orthogonally to the primary filament---and possibly small-scale variations in the local magnetic field direction, could be the dominant factors for explaining the final orientation of each core