On the Propagation of Slip Fronts at Frictional Interfaces

The dynamic initiation of sliding at planar interfaces between deformable and rigid solids is studied with particular focus on the speed of the slip front. Recent experimental results showed a close relation between this speed and the local ratio of shear to normal stress measured before slip occurs (static stress ratio). Using a two-dimensional finite element model, we demonstrate, however, that fronts propagating in different directions do not have the same dynamics under similar stress conditions. A lack of correlation is also observed between accelerating and decelerating slip fronts. These effects cannot be entirely associated with static local stresses but call for a dynamic description. Considering a dynamic stress ratio (measured in front of the slip tip) instead of a static one reduces the above-mentioned inconsistencies. However, the effects of the direction and acceleration are still present. To overcome this we propose an energetic criterion that uniquely associates, independently on the direction of propagation and its acceleration, the slip front velocity with the relative rise of the energy density at the slip tip.Comment: 15 pages, 6 figure

A Cost-based Optimizer for Gradient Descent Optimization

As the use of machine learning (ML) permeates into diverse application domains, there is an urgent need to support a declarative framework for ML. Ideally, a user will specify an ML task in a high-level and easy-to-use language and the framework will invoke the appropriate algorithms and system configurations to execute it. An important observation towards designing such a framework is that many ML tasks can be expressed as mathematical optimization problems, which take a specific form. Furthermore, these optimization problems can be efficiently solved using variations of the gradient descent (GD) algorithm. Thus, to decouple a user specification of an ML task from its execution, a key component is a GD optimizer. We propose a cost-based GD optimizer that selects the best GD plan for a given ML task. To build our optimizer, we introduce a set of abstract operators for expressing GD algorithms and propose a novel approach to estimate the number of iterations a GD algorithm requires to converge. Extensive experiments on real and synthetic datasets show that our optimizer not only chooses the best GD plan but also allows for optimizations that achieve orders of magnitude performance speed-up.Comment: Accepted at SIGMOD 201

Control theory for principled heap sizing

We propose a new, principled approach to adaptive heap sizing based on control theory. We review current state-of-the-art heap sizing mechanisms, as deployed in Jikes RVM and HotSpot. We then formulate heap sizing as a control problem, apply and tune a standard controller algorithm, and evaluate its performance on a set of well-known benchmarks. We find our controller adapts the heap size more responsively than existing mechanisms. This responsiveness allows tighter virtual machine memory footprints while preserving target application throughput, which is ideal for both embedded and utility computing domains. In short, we argue that formal, systematic approaches to memory management should be replacing ad-hoc heuristics as the discipline matures. Control-theoretic heap sizing is one such systematic approach

Differential stabilization of reaction intermediates: specificity checkpoints for M.EcoRI revealed by transient fluorescence and fluorescence lifetime studies

M.EcoRI, a bacterial sequence-specific S-adenosyl-l-methionine-dependent DNA methyltransferase, relies on a complex conformational mechanism to achieve its remarkable specificity, including DNA bending, base flipping and intercalation into the DNA. Using transient fluorescence and fluorescence lifetime studies with cognate and noncognate DNA, we have characterized several reaction intermediates involving the WT enzyme. Similar studies with a bending-impaired, enhanced-specificity M.EcoRI mutant show minimal differences with the cognate DNA, but significant differences with noncognate DNA. These results provide a plausible explanation of the way in which destabilization of reaction intermediates can lead to changes in substrate specificity

Superheating and solid-liquid phase coexistence in nanoparticles with non-melting surfaces

We present a phenomenological model of melting in nanoparticles with facets that are only partially wet by their liquid phase. We show that in this model, as the solid nanoparticle seeks to avoid coexistence with the liquid, the microcanonical melting temperature can exceed the bulk melting point, and that the onset of coexistence is a first-order transition. We show that these results are consistent with molecular dynamics simulations of aluminum nanoparticles which remain solid above the bulk melting temperature.Comment: 8 pages, 5 figure

Terrorism Inclination and Self-Esteem Level of Secondary School Students in Nigeria

Incidences of terrorism and proclivity or penchant towards such acts has necessitated the need to examine if secondary school students are inclined towards terrorism, the gender that is most inclined to terrorist tendencies and whether self-esteem issues contribute to terrorism inclination. This is to assist in initiating general and gender specific educational campaign among secondary school students aimed at curbing terrorism in Nigeria. This study aimed to identify the specific gender which is more predisposed to terrorism and to manifesting self-esteem issues among secondary school students. In order to investigate gender differences in terrorism inclination and self-esteem issues among secondary school students in Lagos, 590 students from 8 secondary schools in Lagos State were administered the 10 item Terrorism Inclination Scale by Agbo and Ezeuduji (2010) and the Rosenberg’s self-esteem scale. The participants consisted of 249 males and 341 females aged 9 to 23 years (M = 14.44, SD = 1.58) in Junior and Senior Secondary Schools. The result of a multivariate analysis of variance (MANOVA) used to analyse gender differences in terrorism inclination and self-esteem issues among secondary school students in Nigeria showed a significant influence of gender in terrorism inclination among secondary school students, F (2, 587) = 4.876, p<= 0.01; Wilks’ Lambda = 0.984, partial η2 =0.02 but there was no gender difference in self-esteem level of secondary school students. Male secondary school students reported both higher terrorism inclination (X̅ = 24.56, SD = 8.11) and higher level of self-esteem (X̅ = 21.05, SD = 3.36) than female secondary school students. This study has clearly shown the existence of terrorism proclivity among secondary school students irrespective of their self-esteem level. The need for educators to focus on secondary school students in order to dissuade them from sympathising with terrorists so as to curb the spate of terrorism in Nigeria was discussed

Teknologi Litik di Situs Talimbue, Sulawesi Tenggara: Teknologi Berlanjut dari Masa Pleistosen Akhir Hingga Holosen.

Abstract.  The  Lithic  Technology  at  Talimbue  Site,  Southeast  Sulawesi:  Continuing  Technology from Late Pleistocene up to Holocene Periods. The Talimbue site at Southeast Sulawesi is packed with  lithic  and  these  offer  a  new  perspective  on  the  lithic  technology  of  Sulawesi.  The  absence  of information  on  the  prehistoric  lithic  technology  of  Southeast  Sulawesi  is  a  factor  of  interest  that makes  research  on  knowledge  of  the  Talimbue  site  necessary.  Lithic  artefacts  were  manufactured from  the  terminal  Pleistocene  to  the  Late  Holocene.  This  research  will  disentangle  the  details  of the lithic technology at the Talimbue Site. The analyzed flaked stone artefacts fall into 3 categories, which are retouched flakes, debitage and cores. For its part, debitage was classified into 3 categories, which are complete flakes, broken flakes and debris. The retouch index was also measured so as to provide a quantitative estimate of the level of retouch intensity of the retouched flakes. The results of  the  analysis  indicate  changes  in  the  stone  flake  technology  during  the  period  of  occupation  of the Talimbue Site. The change of technology occurs because the process of adaptation caused by a change of environment. Abstrak. Temuan  litik  yang  sangat  padat  di  Situs  Talimbue  di  Sulawesi  Tenggara  menunjukkan sebuah persepektif baru dalam kajian teknologi litik di Sulawesi. Kekosongan informasi teknologi litik masa prasejarah di wilayah Sulawesi Tenggara adalah hal yang menarik dikaji dalam penelitian di Situs Talimbue. Artefak litik digunakan dari masa Pleistosen Akhir hingga masa Holosen Akhir. Penelitian ini akan menguraikan secara detail bagaimana teknologi litik di Situs Talimbue. Artefak batu diserpih yang dianalisis menjadi 3 kategori, yaitu serpih diretus, serpihan dan batu inti. Serpihan kemudian  diklasifikasi  menjadi  3  kategori,  yaitu  serpih  utuh,  serpih  rusak  dan  tatal.  Pengukuran indeks retus juga dilakukan bertujuan untuk mengestimasi secara kuantitatif tingkat intensitas retus terhadap serpih yang telah diretus. Hasil penelitian menunjukkan perubahan teknologi artefak batu diserpih terjadi selama masa hunian di Situs Talimbue. Perubahan teknologi terjadi karena adanya proses adaptasi yang disebabkan oleh perubahan lingkungan