Threshold-induced phase transitions in perceptrons

Error rates of a Boolean perceptron with threshold and either spherical or Ising constraint on the weight vector are calculated for storing patterns from biased input and output distributions derived within a one-step replica symmetry breaking (RSB) treatment. For unbiased output distribution and non-zero stability of the patterns, we find a critical load, α p, above which two solutions to the saddlepoint equations appear; one with higher free energy and zero threshold and a dominant solution with non-zero threshold. We examine this second-order phase transition and the dependence of α p on the required pattern stability, κ, for both one-step RSB and replica symmetry (RS) in the spherical case and for one-step RSB in the Ising case.

21-residue peptide's dynamics at and between elementary structural transitions

Elementary conformational changes of the backbone of a 21-residue peptide A5(A3RA)3A are studied using molecular dynamics simulations in explicit water. The processes of the conformational transitions and the regimes of stationary fluctuations between them are investigated using minimal perturbations of the system. The perturbations consist of a few degrees rotation of the velocity of one of the systems' atoms and keep the system on the same energy surface. It is found that (i) the system dynamics is insignificantly changed by the perturbations in the regimes between the transitions; (ii) it is very sensitive to the perturbations just before the transitions that prevents the peptide from making the transitions; and (iii) the perturbation of any atom of the system, including distant water molecules is equally effective in preventing the transition. The latter implies strongly collective dynamics of the peptide and water during the transitions.

The political shaping of energy technology:combined heat and power in Britain

There has been negligible adoption of combined heat and power (CHP) for district heating (DH) in Britain, despite continued advocacy. This thesis constructs an account of the treatment of the option, and devises a framework for explanation. Analysis of technological development and adoption, it is argued, should be similar to that of other social processes, and be subject to the same requirements and criticisms. They will, however, show features peculiar to the institutions developing and selecting technologies, their relation to different social groups, and the forms of knowledge in and about technology. Conventional approaches - organisation and interorganisation theories, and analyses of policy-making - give useful insights but have common limitations. Elements of an analytical framework situating detailed issues and outcomes in a structured historical context are derived from convergent radical critiques. Thus activity on CHP/DH is essentially shaped by the development and relations of energy sector institutions: central and local government, nationalised industries and particularly the electricity industry. Analysis of them is related to the specific character of the British state. A few CHP and DH installations were tried before 1940. During postwar reconstruction, extensive plans for several cities were abandoned or curtailed. In the 1960s and 70s, many small non-CHP DH schemes were installed on housing estates. From the mid-70s, the national potential of CHP/DH has been reappraised, with widespread support and favourable evaluations, but little practical progress. Significant CHP/DH adoption is shown to have been systematically excluded ultimately by the structure of energy provision; centralised production interests dominate and co-ordination is weak. Marginal economics and political commitment have allowed limited development in exceptional circumstances. Periods of upheaval provided greater opportunity and incentive for CHP/DH but restructuring eventually obstructed it. Explanation of these outcomes is shown to require analysis at several levels, from broad context to detailed action.

Postcolonial transitions in Africa:Decolonization in West Africa and present day South Africa

Black Economic Empowerment is a highly debated issue in contemporary South Africa. Yet few South Africans realize that they are following a postcolonial trajectory already experienced by other countries. This paper presents a case study of British firms during decolonization in Ghana and Nigeria in the 1950s and 1960s, which saw a parallel development in business and society to that which occurred in South Africa in the 1990s and 2000s. Despite fundamental differences between these states, all have had to empower a majority of black citizens who had previously suffered discrimination on the basis of race. The paper employs concepts from social capital theory to show that the process of postcolonial transition in African economies has been more politically and socially disruptive than empowerment in Western countries. Historical research contributes to our understanding of the nature of institutional shocks in emerging economies.

Supply market uncertainty:exploring consequences and responses within sustainability transitions

Often it is commercial, not technological, factors which hinder the adoption of potentially valuable innovations. In energy policy, much attention is given to analysing and incentivising consumer demand for renewable energy, but new technologies may also need new supply markets, to provide products and services to build, operate and maintain the innovative technology. This paper addresses the impact of supply constraints on the long-term viability of sustainability related innovations, using the case of bioenergy from organic waste. Uncertainties in the pricing and availability of feedstock (i.e. waste) may generate market deadlock and deter potential investors. We draw on prior research to conceptualise the problem, and identify what steps might be taken to address it. We propose a research agenda aimed at purchasing and supply scholars and centred on the need to understand better the interplay between market evolution and supply uncertainty and 'market shaping' - how stakeholders can legitimately influence supply market evolution - to support the adoption of sustainability related innovation.

Gain dependent pulse regimes transitions in a dissipative dispersion-managed fibre laser

For the first time, we demonstrate the possibility to switch between three distinct pulse regimes in a dissipative dispersion-managed (DM) fibre laser by solely controlling the gain saturation energy. Nonlinear Schrödinger equation based simulations show the transitions between hyper-Gaussian similaritons, parabolic similaritons, and dissipative solitons in the same laser cavity. It is also shown that such transitions exist in a wide dispersion range from all-normal to slightly net-normal dispersion. This work demonstrates that besides dispersion and filter managements gain saturation energy can be a new degree of freedom to manage pulse regimes in DM fibre lasers, which offers flexibility in designing ultrafast fibre lasers. Also, the result indicates that in contrast to conservative soliton lasers whose intensity profiles are unique, dissipative DM lasers show diversity in pulse shapes. The findings not only give a better understanding of pulse shaping mechanisms in mode-locked lasers, but also provide insight into dissipative systems.

New modular structure DC-DC converter without electrolytic capacitors for renewable energy applications

Electrolytic capacitors are extensively used in power converters but they are bulky, unreliable, and have short lifetimes. This paper proposes a new capacitor-free high step-up dc-dc converter design for renewable energy applications such as photovoltaics (PVs) and fuel cells. The primary side of the converter includes three interleaved inductors, three main switches, and an active clamp circuit. As a result, the input current ripple is greatly reduced, eliminating the necessity for an input capacitor. In addition, zero voltage switching (ZVS) is achieved during switching transitions for all active switches, so that switching losses can be greatly reduced. Furthermore, a three-phase modular structure and six pulse rectifiers are employed to reduce the output voltage ripple. Since magnetic energy stored in the leakage inductance is recovered, the reverse-recovery issue of the diodes is effectively solved. The proposed converter is justified by simulation and experimental tests on a 1-kW prototype.