Discrete-time H ∞ and H 2 control with structured disturbances and probability-relaxed requirements

This paper presents a method that reduces the conservatism inherent in the disturbance representation in standard H _∞ theory. It addresses several discrete-time control synthesis problems where the disturbances acting on the system are structured by a convex family of linear filters (‘signal polytope’) and where the system disturbance attenuation level attained over the signal polytope is ensured to (just) a prescribed probability. This setup enables realistic multi-feature disturbance delimiting while accounting for uncertainty in the disturbance model itself by allowing probability waivers. The core of the probability aspects of the proposed solutions is the search for a truncated signal polytope which provides both the required probability and the best robust disturbance attenuation level. Many examples are given, including one of an aircraft output-feedback control with a polytope of low-pass filters representing different wind phenomena. The examples demonstrate that addressing realistic disturbances results in better control designs (hence better performance) and that a small certainty waiver can yield a large performance gain.     

H∞ feedback‐control theory in biochemical systems

In this paper we study the possible optimality of biochemical pathways in the H_∞ sense. We start by presenting simple linearized models of single enzymatic reaction systems, where we apply classical and modern tools of feedback‐control theory. We then apply the results obtained by our analysis to a linearly unbranched enzyme pathway system, where we explore the effect of a negative feedback loop internally exerted on the system by a self‐product of the pathway. We then probe the sensitivity of the enzymatic system to variations in certain variables and we deal with the problem of assessing the optimality of the static‐output feedback control, in the H_∞ sense, inherent to the closed‐loop system. In this point we demonstrate the applicability of our results via a theoretical example that provides an open‐loop and closed‐loop analysis of a four‐block enzymatic system. We then apply the various tools we developed to the optimal analysis of the Threonine synthesis pathway which is regulated by three feedback loops. We demonstrate that this pathway is optimal in the H_∞ sense, in the face of considerable uncertainties in the various enzyme concentrations of the pathway. Copyright © 2007 John Wiley & Sons, Ltd.     

Adult age differences in memory performance: Tests of an associative deficit hypothesis.

An associative hypothesis to explain and predict older adults' deficient explicit episodic memory performance was outlined and tested. The hypothesis attributes a substantial part of older adults' deficient memory performance to their difficulty in merging unrelated attributes-units of an episode into a cohesive unit. Although each of the components can be memorized to a reasonable degree, the associations that tie the attributes-units to each other grow weaker in old age. Four experiments are reported that provide (a) a converging validity to the hypothesis by demonstrating this associative deficit for both interitem relationships and intraitem relationships and (b) a discriminant validity to the hypothesis by contrasting and testing competing predictions made by the associative hypothesis and by alternative hypotheses. The implications of these results to older adults' episodic memory performance are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Nanometer‐Scale Mapping of Elastic Modules in Biogenic Composites: The Nacre of Mollusk Shells

In this study, a newly developed nanoscale modulus mapping is applied in order to visualize the 2D‐distribution of mechanical characteristics in the aragonitic nacre layer of Perna canaliculus (green mussel) shells. Modulus maps provide lateral resolution of about 10 nm. They allow the aragonitic mineral (CaCO₃) tablets and the interfaces between them to be clearly resolved, which are filled by an organic substance (mainly beta‐chitin). The experimental data are compared with finite element simulations that also take into account the tip radius of curvature and the thickness of organic layers, as measured by means of scanning electron microscopy with back‐scattered electrons. Based on this comparison, the Young modulus of beta‐chitin is extracted. The obtained number, E_β = 40 GPa, is higher than previously evaluated. The collected maps reveal that the elastic modules in the nacre layer change gradually across the ceramic/organic interfaces within a spatial range four times wider than the thickness of the organic layers. This is possibly due to inhomogeneous distribution of organic macromolecules within ceramic tablets. According to the data, the concentration of macromolecules gradually increases when approaching the organic/ceramic interfaces. A behavior of this type is unique to biogenic materials and distinguishes them from synthetic composite materials. Finally, three possible mechanisms that attempt to explain why gradual changes of elastic modules significantly enhance the overall resistance to fracture of the nacre layer are briefly discussed. The experimental findings support the idea that individual ceramic tablets, comprising the nacre, are built of the compositionally and functionally graded ceramic material. This sheds additional light on the origin of the superior mechanical properties of biogenic composites.     

Hydrogen effects on the spall strength and fracture characteristics of amorphous Fe-Si-B alloy at very high strain rates

A novel approach is suggested, using laser-induced shock wave measurements to estimate the effects of cathodic hydrogen charging on the mechanical properties and fracture characteristics of materials. This approach is applied to (1) determine the dominant mechanism of hydrogen embrittlement (HE) in an amorphous Fe₈₀B₁₁Si₉ alloy; and (2) estimate the effects of the high pressures involved in cathodic charging. The dynamic spall strength of an amorphous Fe₈₀B₁₁Si₉ alloy shocked before and after hydrogenation by a high-power laser to very high pressures (tens of giga Pascals) is measured. The dynamic spall strength of crystalline iron is measured as well for comparison. An optically recording velocity interferometer system (ORVIS) is used to measure the profile of the free surface velocity in time. The spall strength and the strain rate are calculated from the measurement of the free surface velocity as a function of time. Fracture characteristics are studied by scanning electron microscopy (SEM). The main conclusions are (1) the most reasonable mechanism of HE in the amorphous Fe-Si-B alloy is the high-pressure bubble formation; (2) the high pressures involved in cathodic hydrogen charging or laser-induced shock waves measurements may have similar effects on fracture characteristics; and (3) at very high strain rates, the spall strength is determined mainly by the interatomic bonds.     

Sabbatical leave: Who gains and how much?

A rigorous quasi-experiment tested the ameliorative effects of a sabbatical leave, a special case of respite from routine work. We hypothesized that (a) respite increases resource level and well-being and (b) individual differences and respite features moderate respite effects. A sample of 129 faculty members on sabbatical and 129 matched controls completed measures of resource gain, resource loss, and well-being before, during, and after the sabbatical. Among the sabbatees, resource loss declined and resource gain and well-being rose during the sabbatical. The comparison group showed no change. Moderation analysis revealed that those who reported higher respite self-efficacy and greater control, were more detached, had a more positive sabbatical experience, and spent their sabbatical outside their home country enjoyed more enhanced well-being than others. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

On some questions regarding k -regular and k -context-free sequences

We answer two questions of Allouche and Shallit regarding k-regular sequences and k-context-free sequences. One of these has been solved independently in another way by Bell. We also provide a partial solution to another question of Allouche and Shallit regarding the subword complexity of k-context-free sequences.     

Foreword.

Two years ago, the editors reasoned that the diverse and fundamental character of religious experience would promote an equally varied special issue devoted to the topic of psychotherapy and religion. We have not been disappointed. The papers composing this issue vary greatly in terms of literary style, research methodology, theoretical persuasion, and religious topics. There remains a need to clarify the extent to which religious issues or material that emerge during therapy need to be dealt with via "new" techniques (involving major modifications of classical techniques and ideologies) or via the classical methodologies, informed, perhaps, by expanded insights and perspectives. This issue is meant to be both current and historically representative. These two goals have not always been in agreement. The day of monolithic theories, presumed to address all matters of religious and psychotherapeutic importance, has passed, but not without leaving trails along which the present studies follow. Contemporary religious pluralism is more closely matched with the varied beliefs and problems of those seeking psychotherapy. This change has entailed a growing refinement in the psychological study of therapy and religious practice; it also implies a more astute and psychologically informed understanding of both the healthy and pathological aspects of religious experience. (PsycINFO Database Record (c) 2010 APA, all rights reserved)