Accelerating Metropolis-Hastings algorithms: Delayed acceptance with prefetching

MCMC algorithms such as Metropolis-Hastings algorithms are slowed down by the computation of complex target distributions as exemplified by huge datasets. We offer in this paper an approach to reduce the computational costs of such algorithms by a simple and universal divide-and-conquer strategy. The idea behind the generic acceleration is to divide the acceptance step into several parts, aiming at a major reduction in computing time that outranks the corresponding reduction in acceptance probability. The division decomposes the "prior x likelihood" term into a product such that some of its components are much cheaper to compute than others. Each of the components can be sequentially compared with a uniform variate, the first rejection signalling that the proposed value is considered no further, This approach can in turn be accelerated as part of a prefetching algorithm taking advantage of the parallel abilities of the computer at hand. We illustrate those accelerating features on a series of toy and realistic examples.Comment: 20 pages, 12 figures, 2 tables, submitte

Myanmar: the beginning of reforms and the end of sanctions

On 23 April 2012, the European Union (EU) suspended its sanctions against Burma/ Myanmar in response to a series of domestic political reforms that have been enacted since Thein Sein became the new president in March 2011. Since March 2011, Burma/ Myanmar has witnessed a liberalization of the press, the release of political prisoners and the initiation of a political dialogue between the regime on the one hand and the opposition and ethnic groups on the other. The reforms culminated in by-elections on 1 April 2012, which in turn resulted in a landslide victory for Aung San Suu Kyi's National League for Democracy (NLD). Overall, political reforms in Burma/ Myanmar are being initiated from "above." They are elite-driven and stem from the president and progressive members of the military-dominated party, the Union Solidarity and Development Party (USDP). Political reforms in Myanmar are a regime reaction to both internal and external factors. Internally, the military felt secure enough to embark on the slow liberalization of the political system. Externally, the growing economic presence of China seems to have worried the generals. Progress is, however, slow, uneven and very fragile. Hard-liners within the regime still threaten the reform process. Moreover, the influence of the military within the political system and the economy is still pervasive. The EU has responded to the reforms by suspending almost all existing sanctions, with the exception of the arms embargo and the withdrawal of trade preferences, while starting to employ "carrots" such as development aid. By EU standards, agreement on the suspension of the virtual entirety of sanctions has been reached remarkably swiftly. The exclusion of the arms embargo from the suspension reflects the EU’s policies on armaments supply. The resumption of trade preferences will require a separate, lengthier legal process

Myanmar: the beginning of reforms and the end of sanctions

On 23 April 2012, the European Union (EU) suspended its sanctions against Burma/ Myanmar in response to a series of domestic political reforms that have been enacted since Thein Sein became the new president in March 2011. Since March 2011, Burma/ Myanmar has witnessed a liberalization of the press, the release of political prisoners and the initiation of a political dialogue between the regime on the one hand and the opposition and ethnic groups on the other. The reforms culminated in by-elections on 1 April 2012, which in turn resulted in a landslide victory for Aung San Suu Kyi's National League for Democracy (NLD). Overall, political reforms in Burma/ Myanmar are being initiated from "above." They are elite-driven and stem from the president and progressive members of the military-dominated party, the Union Solidarity and Development Party (USDP). Political reforms in Myanmar are a regime reaction to both internal and external factors. Internally, the military felt secure enough to embark on the slow liberalization of the political system. Externally, the growing economic presence of China seems to have worried the generals. Progress is, however, slow, uneven and very fragile. Hard-liners within the regime still threaten the reform process. Moreover, the influence of the military within the political system and the economy is still pervasive. The EU has responded to the reforms by suspending almost all existing sanctions, with the exception of the arms embargo and the withdrawal of trade preferences, while starting to employ "carrots" such as development aid. By EU standards, agreement on the suspension of the virtual entirety of sanctions has been reached remarkably swiftly. The exclusion of the arms embargo from the suspension reflects the EU’s policies on armaments supply. The resumption of trade preferences will require a separate, lengthier legal process

Topological volumes of fibrations: A note on open covers

We establish a straightforward estimate for the number of open sets with fundamental group constraints needed to cover the total space of fibrations. This leads to vanishing results for simplicial volume and minimal volume entropy, e.g., for certain mapping tori.Comment: 20 pages; minor revisions; To appear in Proc. Roy. Soc. Edinburgh Sect.

Sodium hydroxide pretreatment as an effective approach to reduce the dye/holes recombination reaction in P-Type DSCs

We report the synthesis of a novel squaraine dye (VG21-C12) and investigate its behavior as p-type sensitizer for p-type Dye-Sensitized Solar Cells. The results are compared with O4-C12, a well-known sensitizer for p-DSC, and sodium hydroxide pretreatment is described as an effective approach to reduce the dye/holes recombination. Various variable investigation such as dipping time, dye loading, photocurrent, and resulting cell efficiency are also reported. Electrochemical impedance spectroscopy (EIS) was utilized for investigating charge transport properties of the different photoelectrodes and the recombination phenomena that occur at the (un)modified electrode/electrolyte interface

Microstructure and shear strength evolution of a lime-treated clay for use in road construction

The results of a comprehensive experimental programme are presented for assessing the long-term microstructural modifications and evaluating the effects of microstructural rearrangement on the stress–strain behaviour of a lime-treated high-plasticity clay for road embankments. The stress–strain behaviour at different lime content and curing time was investigated by means of direct shear tests; microstructural analyses were carried out combining Scanning Electron Microscope observations and Mercury Intrusion Porosimetry tests. The results show that the stress–strain behaviour of treated clay is strongly dilatant with a high peak of strength, which increases with time and lime content. Furthermore, a hyperbolic function may be used for predicting the increase in strength at the end of the stabilisation process. Microstructural analysis shows that the treatment induces a redistribution of the porosity between macro- and micropores and an increase in matric suction, associated to a low reduction of water content and to aggregates shrinkage. This behaviour is affected by lime content and curing time since it results from the formation of the pozzolanic compounds on the surface of clay aggregates that induces both a bonding between hardened clay aggregates and an increase of their interlocking degree, with a consequent increase in the shear strength

Amenable category and complexity

Amenable category is a variant of the Lusternik-Schnirelman category, based on covers by amenable open subsets. We study the monotonicity problem for degree-one maps and amenable category and the relation between amenable category and topological complexity.Comment: 33 pages; to appear in AG

Analisi della microstruttura di un'argilla di alta plasticità trattata con calce

Nella nota si presentano i primi risultati di una indagine sperimentale volta a studiare l’effetto del trattamento con calce sulle caratteristiche microstrutturali di un’argilla di alta plasticità, proveniente dagli scavi eseguiti nel corso dei lavori di ammodernamento di una strada di grande comunicazione in Sicilia. I provini, compattati in laboratorio a diverse percentuali di calce, sono stati sottoposti a prove porosimetriche ad intrusione di mercurio MIP ed osservazioni al microscopio elettronico SEM. L’analisi dei risultati evidenzia che l’argilla trattata con calce assume una distribuzione dei pori tipica dei materiali a doppia porosità e che gli effetti sulla microstruttura dipendono dalla quantità di calce aggiunta e dal tempo di maturazione

Advanced turbulence models and boundary conditions for flows around different configurations of ground-mounted buildings

When dealing with Atmospheric Boundary Layer (ABL) simulations, commercial computational fluid dynamics (CFD) acquires a strategic resonance. Thanks to its good compromise between accuracy of results and calculation time, RANS still represents a valid alternative to more resource-demanding methods. However, focusing on the models’ performances in urban studies, LES generally outmatches RANS results, even if the former is at least one order of magnitude more expensive. Consequently, the present work aims to propose a variety of approaches meant to solve some of the major problems linked to RANS simulations and to further improve its accuracy in typical urban contexts. All of these models are capable of switching from an undisturbed flux formulation to a disturbed one through a local deviation or a marker function. For undisturbed flows, a comprehensive approach is adopted, solving the issue of the erroneous stream-wise gradients affecting the turbulent profiles. Around obstacles, Non-Linear Eddy-Viscosity closures are adopted, due to their prominent capability in capturing the anisotropy of turbulence. The purpose of this work is then to propose a new Building Influence Area concept and to offer more affordable alternatives to LES simulations without sacrificing a good grade of accuracy