New proof for the undecidability of the circular PCP

We give simpler proof for the undecidability of the circular Post Correspondence Problem that was originally proved undecidable by Ruohonen (Acta Informatica 19:357–367, 1983). The key feature of our proof is the undecidability of the word problem of special semi-Thue systems where derivations are deterministic and reversible for words containing a single occurrence of a letter from a given set.     

Stratified Sampling for Stochastic Transparency

The traditional method of rendering semi‐transparent surfaces using alpha blending requires sorting the surfaces in depth order. There are several techniques for order‐independent transparency, but most require either unbounded storage or can be fragile due to forced compaction of information during rendering. Stochastic transparency works in a fixed amount of storage and produces results with the correct expected value. However, carelessly chosen sampling strategies easily result in high variance of the final pixel colors, showing as noise in the image. In this paper, we describe a series of improvements to stochastic transparency that enable stratified sampling in both spatial and alpha domains. As a result, the amount of noise in the image is significantly reduced, while the result remains unbiased.     

Building Distributed Context-Aware Applications

Context-aware applications gather information from sensors about their users and operating environment. Sensor handling is a complicated issue that makes it hard and time-consuming to develop context-aware applications. This paper shows how dynamic discovery protocols can be employed to deal with the physical distribution of sensors and the need to share sensors between many applications. We report on our experiences from building a position-aware application using the dynamic discovery protocol that is part of the Jini software infrastructure for distributed systems.     

Evaluation of green buildings in Sweden

This paper reports on experiences from the Swedish participation in Green Building Challenge '98 and ongoing similar work in Sweden. It argues against very simplified environmental assessment methods and recommends, as far as possible, calculation of environmental effects caused by buildings during their life cycle. Participation in the development of the GBC tool provided a broad view of the issues, clarified different opinions in a new field and provided additional stimulation while developing our national assessment methodology, EcoEffect. The environmental assessment of buildings is discussed in relation to the strength and weaknesses of the GBC system. The design and characteristics of 'green buildings' in Sweden is addressed, including ten poster presentations and two case studies presented at GBC '98. Cet article rend compte des experiences liees a la participation de la Suede au projet Green Building Challenge '98 et a des travaux similaires en cours d'execution en Suede. Il procede a l'analyse critique de methodes d'evaluation de l'environnement tres simplifiees et recommande que, dans la mesure du possible, on calcule les incidences sur l'environnement imputables aux batiments pendant leur cycle de vie. La contribution aux travaux de developpement du GBC Tool a permis d'arriver a une vision plus large des problemes, de faire la lumiere sur des opinions differentes dans un domaine nouveau et est a l'origine d'une stimulation nouvelle tout en developpant notre methodologie d'evaluation nationale, EcoEffect. L'evaluation environnementale de batiments est passee en revue sur fond de points forts et de points faibles du systeme GBC. L'article etudie la conception et les caracteristiques des 'batiments verts' en Suede; ce sujet fait egalement l'objet de 10 presentations par affiches et de 2 etudes de cas presentees a la conference GBC '98.     

Back-trajectory analyses of fine particulate matter measured at Big Bend National Park in the historical database and the 1996 scoping study

Analyses of the sources of fine particles associated with visibility reduction at Big Bend National Park during a 10-year period from 1989-1998 and from a regional visibility scoping study conducted during September and October 1996 at 19 sites in Texas and Mexico are summarized and compared. Fine sulfate particles are the largest fraction of the fine mass, and scattering by sulfates is estimated to be nearly half of the non-Rayleigh light extinction at Big Bend. Fine particulate sulfur concentrations are seasonal, with the highest values occurring during the summer and fall when back trajectory analyses show that air masses are most likely to arrive at Big Bend from the southeast after passing over Mexico or from areas to the northeast including east Texas. Episodically, high concentrations of fine mass and high light extinction values can be due to other species such as fine organic carbon or blowing soil dust. Organic carbon concentrations are often extremely high during the spring, especially during May. A combination of back trajectory analyses and the coincidence of high organic carbon and high non-soil potassium concentrations leads to the hypothesis that these concentrations are due to fires, primarily seasonal agricultural burning in Mexico and Central America. Fine soil concentrations often reach values that are twice the annual mean during July. These concentrations also frequently have high Al/Ca ratios, indicative of Saharan dust. Back trajectories associated with these events show air masses arriving from the southeast and are consistent with the hypothesis of transport of air masses from Africa during July. There is a high frequency of transport of air masses from Mexico to Big Bend, especially during the summer when fine mass concentrations and light extinction are highest. Therefore, sources and potential sources of sulfates and other fine particles in Mexico, particularly in areas southeast of the park have a high likelihood of contributing to visibility degradation at the park. Source areas to the northeast of the park, in east Texas and farther upwind also contribute to high fine sulfate concentrations.     

Potential of all‐cellulose composites in corrugated board applications: Comparison of chemical pulp raw materials

All‐cellulose composites (ACCs) were produced using various commercially available chemical pulps by partial dissolution method using an aqueous zinc chloride (ZnCl₂) solvent. Characterization methods used for defining material performance, keeping especially corrugated board products in mind, were as follows: scanning electron microscopy, tensile strength, 2‐point bending stiffness, concora medium test, and short‐span crush test. Hardwood (bleached eucalyptus), softwood (bleached spruce, bleached and unbleached pine), speciality softwood pulps (sulphite dissolving with 530 and 398 mL/g intrinsic viscosities), and annual plant pulp (bleached abaca) were investigated to give a broad overview of the potential for making ACCs. Softwood pulps, especially bleached, showed highest increase in mechanical properties across the board. Hardwood pulp showed relatively good results, and the selected annual plant pulp (abaca) responded partially negatively to treatment. Comparing unbleached and bleached softwood ACCs, it seems that bleaching is beneficial for tensile properties. However, when material comes under compressive or bending loads, unbleached pulp performs very well. Comparing sulphite dissolved pulps, it could be proven that pulps with higher intrinsic viscosities, thus higher polymerization grade, respond better to partial dissolution. It was proven that a wide set of pulps, also of lesser cellulose purity, used in the paper industry are viable for ACC production. Furthermore, results from short‐span crush test and concora medium tests show high potential of ACCs for corrugated board applications.     

Arsenic Removal from Mine Waters with Sorption Techniques

Potential low-cost sorption materials (mostly industrial by-products) were screened for removal of arsenic from mine effluent water. First, the maximum adsorption capacities were determined in batch tests with various liquid to solid ratios. The highest arsenic sorption capacity, 46 mg As/g of sorption material, was measured for cast iron chips. The most promising materials were also studied in batch tests that assessed the reaction kinetics and in kinetic column tests for their behavior in a filter or reactive barrier application. The column tests revealed the cast iron chips caused clogging in the percolation column when operating with real mine water. A commercial ferric oxi-hydroxide sorption material developed for As removal for drinking water showed good As removal in the column tests. Around 10,000 bed volumes of mine process water containing 2 mg/L of arsenic was treated with this material, and treated water concentrations ranged between 0 and 0.05 mg/L before breakthrough. The measured adsorption capacity for the commercial ferric oxi-hydroxide sorption material was 8.3 mg As/g.     

Degree of polarization in tightly focused optical fields

We analyze the degree of polarization of random, statistically stationary electromagnetic fields in the focal region of a high-numerical-aperture imaging system. The Richards-Wolf theory for focusing is employed to compute the full 3 x 3 electric coherence matrix, from which the degree of polarization is obtained by using a recent definition for general three-dimensional electromagnetic waves. Significant changes in the state of partial polarization, compared with that of the incident illumination, are observed. For example, a wave consisting of two orthogonal and uncorrelated incident-electric-field components produces rings of full polarization in the focal plane. These effects are explained by considering the distribution of the spectral densities of the three electric field components as well as the correlations between them.     

Simultaneous use of time-resolved fluorescence and anti-stokes photoluminescence in a bioaffinity assay

A bioaffinity assay is described where anti-Stokes photoluminescence of inorganic lanthanide phosphors and time-resolved fluorescence of lanthanide chelates are measured from a single microtitration well without any disturbance from these label technologies to each other. Up-converting phosphor (UPC-phosphor) bioconjugate was produced by grinding the commercial, micrometer-sized UPC-phosphors to colloidal, submicrometer-sized phosphor particles and by attaching these phosphors to biomolecules. Experiments were carried out in standard 96-well microtitration plates to determine detection limits, linearity, and cross-talk of UPC-phosphor and europium chelate. In numbers of molecules the lower limits of detection for UPC-phosphor were roughly 3 x 10(3) particles in solution and 1 x 10(4) particles in solid phase, and for europium label same values were 9 x 10(6) and 9 x 10(7) molecules. Linearity of detection was for UPC-phosphor 5 orders of magnitude in solution and over 4 orders of magnitude in solid phase and for europium label over 5 orders of magnitude in solution and over 4 orders of magnitude in solid phase. The cross-talk between the two labels was practically nonexistent. In this study we show that up-converting anti-Stokes photoluminescent phosphors could be employed in bioaffinity assays as very potential labels with significant advantages either alone or together with long-lifetime lanthanide chelates.