Self-Directed Learning and Its Relation to the VC-Dimension and to Teacher-Directed Learning

We study the self-directed (SD) learning model. In this model a learner chooses examples, guesses their classification and receives immediate feedback indicating the correctness of its guesses. We consider several fundamental questions concerning this model: the parameters of a task that determine the cost of learning, the computational complexity of a student, and the relationship between this model and the teacher-directed (TD) learning model. We answer the open problem of relating the cost of self-directed learning to the VC-dimension by showing that no such relation exists. Furthermore, we refute the conjecture that for the intersection-closed case, the cost of self-directed learning is bounded by the VC-dimension. We also show that the cost of SD learning may be arbitrarily higher that that of TD learning.Finally, we discuss the number of queries needed for learning in this model and its relationship to the number of mistakes the student incurs. We prove a trade-off formula showing that an algorithm that makes fewer queries throughout its learning process, necessarily suffers a higher number of mistakes.     

Short and long term toxicity of crude oil and oil dispersants to two representative coral species

Oil dispersants, the tool of choice for treating oil spills in tropical marine environments, is potentially harmful to marine life, including reef corals. In a previous study, we found that dispersed oil and oil dispersants are harmful to soft and hard coral species at early life stages. In this broader study, we employed a "nubbin assay" on more than 10 000 coral fragments to evaluate the short- and long-term impacts of dispersed oil fractions (DOFs) from six commercial dispersants, the dispersants and water-soluble-fractions (WSFs) of Egyptian crude oil, on two Indo Pacific branching coral species, Stylophora pistillata and Pocillopora damicornis. Survivorship and growth of nubbins were recorded for up to 50 days following a single, short (24 h) exposure to toxicants in various concentrations. Manufacturer-recommended dispersant concentrations proved to be highly toxic and resulted in mortality for all nubbins. The dispersed oil and the dispersants were significantly more toxic than crude oil WSFs. As corals are particularly susceptible to oil detergents and dispersed oil, the results of these assays rules out the use of any oil dispersant in coral reefs and in their vicinity. The ecotoxicological impacts of the various dispersants on the corals could be rated on a scale from the least to the most harmful agent, as follows: Slickgone > Petrotech > Inipol = Biorieco > Emulgal > Dispolen.     

一种验证电梯初装时测试的平衡系数的方法

电梯“溜梯”是指电梯在非电动运行的情况下,由于轿厢与对重之间的质量差造成曳引轮转矩超过制动力矩或者破坏曳引条件而引起轿厢上升或下降的现象。平衡系数的变化可能引起电梯“溜梯”事故。电梯在安装验收完成后,在使用中如果轿厢进行过度装饰或对重块发生变化,都会带来平衡系数的改变。因此,如何验证电梯初装时测试的平衡系数是否有效成了人们关注的一个问题。除了电梯自检记录和验收检验记录中测试的电流载荷曲线图可以为平衡系数的有效性提供证据之外,在本文中,笔者提出一种验证电梯初装时测试的平衡系数有效性的方法,与业界人士探讨。     

On Shear Rheology of Gel Propellants

Selected fuel, oxidizer and simulant gels were prepared and rheologically characterized using a rotational rheometer. For fuel gelation both organic and inorganic gellants were utilized, whereas oxidizers and simulants were gelled with addition of silica and polysaccharides, respectively. The generalized Herschel‐Bulkley constitutive model was found to most adequately represent the gels studied. Hydrazine‐based fuels, gelled with polysaccharides, were characterized as shear‐thinning pseudoplastic fluids with low shear yield stress, whereas inhibited red‐fuming nitric acid (IRFNA) and hydrogen peroxide oxidizers, gelled with silica, were characterized as yield thixotropic fluids with significant shear yield stress. Creep tests were conducted on two rheological types of gels with different gellant content and the results were fitted by Burgers‐Kelvin viscoelastic constitutive model. The effect of temperature on the rheological properties of gel propellant simulants was also investigated. A general rheological classification of gel propellants and simulants is proposed.     

Development of Fine Scale PZT Ceramic Fiber/Polymer Shell Composite Transducers

The relic processing technique was used to fabricate fine-scale piezoelectric lead zirconate titanate (PZT) ceramic fiber/polymer shell composites. In this technique sacrificial activated carbon fabrics were soaked in a PZT precursor solution, dried, and heat treated to form piezoceramic relics. Relics were embedded with polymer, which was allowed to cure, and the resulting composites were polished, electroded, and poled. Different facets of the composite- forming process were examined: structural modifications, soaking, firing, and polymer impregnation. The physical and electromechanical properties of the unique resulting composite were evaluated. Optimized PZT shell composites with 39 vol% ceramic exhibited the following property values: K ε200, tan δε5.5%, d ₃₃ε290 pC/N, d _hε100 pC/N, d _h g _hε6000 ± 10^-15 m²N, k _pε0.19, and k _tε0.28.     

Strength and Wear Behavior of Mg Alloy AE42 Reinforced with Carbon Short Fibers

In addition to the advantage of the lightweight of magnesium alloys, magnesium composites have moderate strength and elastic modulus. The proposed application of magnesium composites as diesel truck pistons makes it necessary to assess their wear performance. Little research data have been discussed on wear behavior of Mg alloy AE42 matrix and its composites. Thus, this paper reports wear behavior of magnesium alloy AE42(Mg–Al–Mn—RE; rare earth) and its composite AE42-C, which contains 23 vol% of randomly oriented carbon short fibers. Materials characterization, including density measurements, hardness testing, microstructures investigation, and compression testing at temperatures of 25, 150,and 300 °C, were conducted. Wear tests were performed under various loads and sliding distances. Wear mechanisms were also proposed based on the examination of worn surfaces using optical microscopy and scanning electron microscopy equipped with EDX(energy-dispersive X-ray spectrometry) analysis system. The hardness of AE42-23 vol% C composite is twice the hardness of the Mg matrix alloy AE42. Significant improvements to yield stress and compressive strength at temperatures of 25, 150, and 300 °C of the composite versus the AE42 alloy are achieved. Wear resistance of the composite is improved considerably versus that of the Mg alloy AE42 at the various sliding distances. Smearing of graphite on the worn surface produces a lubricating film that delays change from mild to severe wear of the composite, especially at high loads. EDX analysis of the worn surface shows oxidation of the matrix alloy at higher wear loads, and this mechanism decreases in the presence of carbon fibers under the same loads. Abrasive wear, oxidation, and plastic deformation are the dominant wear mechanisms for the alloy matrix AE42, whereas mainly abrasive wear is the wear mechanism of AE42-23 vol% C composite under the proposed testing conditions.     

MODIFIED CONDUCTION SHAPE FACTORS FOR ISOTHERMAL BODIES EMBEDDED IN A SEMI-INFINITE MEDIUM

Numerical finite-element method calculations were performed for isothermal surfaces embedded in a semi-infinite medium with convection boundary conditions. A modified shape factor was defined that combines the conduction heat transfer in the medium and the convection to the environment, with the geometrical characteristics of the problem. Infinite finite elements are used; these form the last layer of elements in the infinite directions. The modified shape factors for a sphere and for a long horizontal cylinder are presented in graphs and correlations, both using nondimensional parameters. The heat flux distribution on the surface of the body and the temperature and heat transfer on the surface of the medium are also presented.     

A Forward-Secure Public-Key Encryption Scheme

Cryptographic computations are often carried out on insecure devices for which the threat of key exposure represents a serious concern. Forward security allows one to mitigate the damage caused by exposure of secret keys. In a forward-secure scheme, secret keys are updated at regular periods of time; exposure of the secret key corresponding to a given time period does not enable an adversary to "break" the scheme (in the appropriate sense) for any prior time period. We present the first constructions of (non-interactive) forward-secure public-key encryption schemes. Our main construction achieves security against chosen-plaintext attacks in the standard model, and all parameters of the scheme are poly-logarithmic in the total number of time periods. Some variants and extensions of this scheme are also given. We also introduce the notion of binary tree encryption and construct a binary tree encryption scheme in the standard model. Our construction implies the first hierarchical identity-based encryption scheme in the standard model. (The notion of security we achieve, however, is slightly weaker than that achieved by some previous constructions in the random oracle model.)     

Polyharmonic smoothing splines and the multidimensional Wiener filtering of fractal-like signals.

Motivated by the fractal-like behavior of natural images, we develop a smoothing technique that uses a regularization functional which is a fractional iterate of the Laplacian. This type of functional was initially introduced by Duchon for the approximation of nonuniformily sampled, multidimensional data. He proved that the general solution is a smoothing spline that is represented by a linear combination of radial basis functions (RBFs). Unfortunately, this is tedious to implement for images because of the poor conditioning of RBFs and their lack of decay. Here, we present a much more efficient method for the special case of a uniform grid. The key idea is to express Duchon's solution in a fractional polyharmonic B-spline basis that spans the same space as the RBFs. This allows us to derive an algorithm where the smoothing is performed by filtering in the Fourier domain. Next, we prove that the above smoothing spline can be optimally tuned to provide the MMSE estimation of a fractional Brownian field corrupted by white noise. This is a strong result that not only yields the best linear filter (Wiener solution), but also the optimal interpolation space, which is not bandlimited. It also suggests a way of using the noisy data to identify the optimal parameters (order of the spline and smoothing strength), which yields a fully automatic smoothing procedure. We evaluate the performance of our algorithm by comparing it against an oracle Wiener filter, which requires the knowledge of the true noiseless power spectrum of the signal. We find that our approach performs almost as well as the oracle solution over a wide range of conditions.