Approximation Algorithms for Quickest Spanning Tree Problems

Let $G=(V,E)$ be an undirected multigraph with a special vertex ${\it root} \in V$, and where each edge $e \in E$ is endowed with a length $l(e) \geq 0$ and a capacity $c(e) > 0$. For a path $P$ that connects $u$ and $v$, the {\it transmission time} of $P$ is defined as $t(P)=\mbox{\large$\Sigma$}_{e \in P} l(e) + \max_{e \in P}\!{(1 / c(e))}$. For a spanning tree $T$, let $P_{u,v}^T$ be the unique $u$--$v$ path in $T$. The {\sc quickest radius spanning tree problem} is to find a spanning tree $T$ of $G$ such that $\max _{v \in V} t(P^T_{root,v})$ is minimized. In this paper we present a 2-approximation algorithm for this problem, and show that unless $P =NP$, there is no approximation algorithm with a performance guarantee of $2 - \epsilon$ for any $\epsilon >0$. The {\sc quickest diameter spanning tree problem} is to find a spanning tree $T$ of $G$ such that $\max_{u,v \in V} t(P^T_{u,v})$ is minimized. We present a ${3 \over 2}$-approximation to this problem, and prove that unless $P=NP$ there is no approximation algorithm with a performance guarantee of ${3 \over 2}-\epsilon$ for any $\epsilon >0$.     

Effects of traumatic brain injury on working memory-related brain activation in adolescents.

Eight adolescents (ages 13-18 years) who sustained traumatic brain injury (TBI) and eight gender- and age-matched typically developing (TD) adolescents underwent event-related functional MRI (fMRI) while performing a Sternberg letter recognition task. Encoding, maintenance, and retrieval were examined with memory loads of one or four items during imaging. Both groups performed above a 70% accuracy criterion and did not differ in performance. TD adolescents showed greater increase in frontal and parietal activation during high-load relative to low-load maintenance than the TBI group. The TBI patients showed greater increase in activation during high-load relative to low-load encoding and retrieval than the TD group. Results from this preliminary study suggest that the capability to differentially allocate neural resources according to memory load is disrupted by TBI for the maintenance subcomponent of working memory. The overrecruitment of frontal and extrafrontal regions during encoding and retrieval following TBI may represent a compensatory process. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Investigation of microstructure and mechanical properties of multi-layer Cr/Cr2O3 coatings

Single and multi-layer Cr/Cr₂O₃ coatings were deposited by reactive magnetron sputtering with the total thickness of 7 μm on steel substrates. X-ray diffraction analysis showed that single and multi-layer Cr/Cr₂O₃ coatings have different preferred crystal orientations. Columnar microstructure was detected by transmission electron microscopy both in metal chromium and ceramic chromium oxide layers. Grain size increased with the coating thickness. The value of single and multi-layer coating's fracture toughness is between 4 and 6 MPa·m^1/2 measured with the Berkovich tip indentation, and it is between 2.8 and 3.9 MPa·m^1/2 when measured with the Vickers indenter. The adhesion is about 192.1 and 246.7 J/m² for single and multi-layer coatings, respectively.     

Oxidative Damage in Sporadic Colorectal Cancer: Molecular Mapping of Base Excision Repair Glycosylases MUTYH and hOGG1 in Colorectal Cancer Patients

Oxidative stress, oxidative DNA damage and resulting mutations play a role in colorectal carcinogenesis. Impaired equilibrium between DNA damage formation, antioxidant status, and DNA repair capacity is responsible for the accumulation of genetic mutations and genomic instability. The lesion-specific DNA glycosylases, e.g., hOGG1 and MUTYH, initiate the repair of oxidative DNA damage. Hereditary syndromes (MUTYH-associated polyposis, NTHL1-associated tumor syndrome) with germline mutations causing a loss-of-function in base excision repair glycosylases, serve as straight forward evidence on the role of oxidative DNA damage and its repair. Altered or inhibited function of above glycosylases result in an accumulation of oxidative DNA damage and contribute to the adenoma-adenocarcinoma transition. Oxidative DNA damage, unless repaired, often gives rise G:C > T:A mutations in tumor suppressor genes and proto-oncogenes with subsequent occurrence of chromosomal copy-neutral loss of heterozygosity. For instance, G>T transversions in position c.34 of a KRAS gene serves as a pre-screening tool for MUTYH-associated polyposis diagnosis. Since sporadic colorectal cancer represents more complex and heterogenous disease, the situation is more complicated. In the present study we focused on the roles of base excision repair glycosylases (hOGG1, MUTYH) in colorectal cancer patients by investigating tumor and adjacent mucosa tissues. Although we found downregulation of both glycosylases and significantly lower expression of hOGG1 in tumor tissues, accompanied with G>T mutations in KRAS gene, oxidative DNA damage and its repair cannot solely explain the onset of sporadic colorectal cancer. In this respect, other factors (especially microenvironment) per se or in combination with oxidative DNA damage warrant further attention. Base excision repair characteristics determined in colorectal cancer tissues and their association with disease prognosis have been discussed as well.     

Microcavity-integrated graphene photodetector

There is an increasing interest in using graphene (1, 2) for optoelectronic applications. (3-19) However, because graphene is an inherently weak optical absorber (only ≈2.3% absorption), novel concepts need to be developed to increase the absorption and take full advantage of its unique optical properties. We demonstrate that by monolithically integrating graphene with a Fabry-Pérot microcavity, the optical absorption is 26-fold enhanced, reaching values >60%. We present a graphene-based microcavity photodetector with responsivity of 21 mA/W. Our approach can be applied to a variety of other graphene devices, such as electro-absorption modulators, variable optical attenuators, or light emitters, and provides a new route to graphene photonics with the potential for applications in communications, security, sensing and spectroscopy.     

Molecular Mechanisms Underlying Intensive Care Unit-Acquired Weakness and Sarcopenia

Skeletal muscle is a highly adaptable organ, and its amount declines under catabolic conditions such as critical illness. Aging is accompanied by a gradual loss of muscle, especially when physical activity decreases. Intensive care unit-acquired weakness is a common and highly serious neuromuscular complication in critically ill patients. It is a consequence of critical illness and is characterized by a systemic inflammatory response, leading to metabolic stress, that causes the development of multiple organ dysfunction. Muscle dysfunction is an important component of this syndrome, and the degree of catabolism corresponds to the severity of the condition. The population of critically ill is aging; thus, we face another negative effect—sarcopenia—the age-related decline of skeletal muscle mass and function. Low-grade inflammation gradually accumulates over time, inhibits proteosynthesis, worsens anabolic resistance, and increases insulin resistance. The cumulative consequence is a gradual decline in muscle recovery and muscle mass. The clinical manifestation for both of the above conditions is skeletal muscle weakness, with macromolecular damage, and a common mechanism—mitochondrial dysfunction. In this review, we compare the molecular mechanisms underlying the two types of muscle atrophy, and address questions regarding possible shared molecular mechanisms, and whether critical illness accelerates the aging process.     

Introduction to the Special Issue on Meta-Learning

Recent advances in meta-learning are providing the foundations to construct meta-learning assistants and task-adaptive learners. The goal of this special issue is to foster an interest in meta-learning by compiling representative work in the field. The contributions to this special issue provide strong insights into the construction of future meta-learning tools. In this introduction we present a common frame of reference to address work in meta-learning through the concept of meta-knowledge. We show how meta-learning can be simply defined as the process of exploiting knowledge about learning that enables us to understand and improve the performance of learning algorithms.     

Thermoelectric Effect Spectroscopy and Photoluminescence of High-Resistivity CdTe:In

Thermoelectric effect spectroscopy and photoluminescence techniques were used to study the defect levels in samples from three crystals of CdTe:In grown by the vertical gradient freeze method. The main goal of the investigation was to study defects, which strongly trap charge carriers or act as recombination centers in order to eliminate them from the technological process. The main difference among detecting and non-detecting samples was the absence of electron traps with a very high capture cross-section and energy 0.6 eV to 0.7 eV, which act as lifetime killers even at low concentrations. Recently published ab initio calculations show a complex of Te antisite and Cd vacancy within this energy range.     

Hematites Precipitated in Alkaline Precursors: Comparison of Structural and Textural Properties for Methane Oxidation

Hematite (α-Fe₂O₃) catalysts prepared using the precipitation methods was found to be highly effective, and therefore, it was studied with methane (CH₄), showing an excellent stable performance below 500 °C. This study investigates hematite nanoparticles (NPs) obtained by precipitation in water from the precursor of ferric chloride hexahydrate using precipitating agents NaOH or NH₄OH at maintained pH 11 and calcined up to 500 °C for the catalytic oxidation of low concentrations of CH₄ (5% by volume in air) at 500 °C to compare their structural state in a CH₄ reducing environment. The conversion (%) of CH₄ values decreasing with time was discussed according to the course of different transformation of goethite and hydrohematites NPs precursors to magnetite and the structural state of the calcined hydrohematites. The phase composition, the size and morphology of nanocrystallites, thermal transformation of precipitates and the specific surface area of the NPs were characterized in detail by X-ray powder diffraction, transmission electron microscopy, infrared spectroscopy, thermal TG/DTA analysis and nitrogen physisorption measurements. The results support the finding that after goethite dehydration, transformation to hydrohematite due to structurally incorporated water and vacancies is different from hydrohematite α-Fe₂O₃. The surface area SBET of Fe₂O₃_NH-70 precipitate composed of protohematite was larger by about 53 m²/g in comparison with Fe₂O₃_Na-70 precipitate composed of goethite. The oxidation of methane was positively influenced by the hydrohematites of the smaller particle size and the largest lattice volume containing structurally incorporated water and vacancies.