Sharp upper bounds on the second largest eigenvalues of connected graphs

AbstractLet λ2 be the second largest eigenvalue of a graph. Powers (1988) [4] gave some upper bounds of λ2 for general graphs and bipartite graphs, respectively. Considering that these bounds are not always attainable for connected graphs, we present sharp upper bounds of λ2 for connected graphs and connected bipartite graphs in this paper. Moreover, the extremal graphs are completely characterized

Analysis of the Fungal Virulence of Cryptococcus and Exploration of Novel Antifungals Against Cryptococcosis

Cryptococcosis is one of the leading causes of the deaths among AIDS patients. The high mortality rates of cryptococcosis are mainly due to inadequate information of its major causative agent Cryptococcus neoformans and the limitations of current therapies. Cryptococcus neoformans is an unconventional dimorphic fungus that can grow either as a yeast or in a filamentous form. To study this dimorphism that is critical to the pathogenicity of many fungi, we constructed the congenic a and α strains for XL280(α), a strain with robust ability to undergo the yeast-hyphal transition. We compared the congenic strains in different in vivo models and found they are equivalent in virulence. Furthermore, deletion or overexpression of a known transcription factor Znf2 in XL280 abolished or enhanced filamentation and biofilm formation, consistent with its established role. Therefore, the congenic strains provide a new resource for the study of morphogenesis and the related virulence of Cryptococcus. Meanwhile, we searched for novel antifungals by screening of a clinical compound library. Two hits from the screen, the antibiotic polymyxin B and the antidepressant sertraline are all potently fungicidal against Cryptococcus. Polymyxin B works synergistically with the azoles both in vitro and in vivo, thus it may serve as an adjunctive therapy with fluconazole in clinic. Our investigation on sertraline has implicated its unique advantage in treating cryptococcal infections since it is able to traverse the blood brain barrier and reduce the fungal burden in brains. We further examined the fungal target of sertraline and found that this compound inhibits the fungal protein synthesis. Taken together, the studies in this thesis facilitate further research on the pathogenesis of Cryptococcus and provide new antifungal drug candidates with clinical value

Optimal Variable Speed Limit Control Strategy on Freeway Segments under Fog Conditions

Fog is a critical external factor that threatens traffic safety on freeways. Variable speed limit (VSL) control can effectively harmonize vehicle speed and improve safety. However, most existing weather-related VSL controllers are limited to adapt to the dynamic traffic environment. This study developed optimal VSL control strategy under fog conditions with fully consideration of factors that affect traffic safety risks. The crash risk under fog conditions was estimated using a crash risk prediction model based on Bayesian logistic regression. The traffic flow with VSL control was simulated by a modified cell transmission model (MCTM). The optimal factors of VSL control were obtained by solving an optimization problem that coordinated safety and mobility with the help of the genetic algorithm. An example of I-405 in California, USA was designed to simulate and evaluate the effects of the proposed VSL control strategy. The optimal VSL control factors under fog conditions were compared with sunny conditions, and different placements of VSL signs were evaluated. Results showed that the optimal VSL control strategy under fog conditions changed the speed limit more cautiously. The VSL control under fog conditions in this study effectively reduced crash risks without significantly increasing travel time, which is up to 37.15% reduction of risks and only 0.48% increase of total travel time. The proposed VSL control strategy is expected to be of great use in the development of VSL systems to enhance freeway safety under fog conditions

Mesoscovic magnetic/semiconductor heterostructures

We report the experimental results of Fe and Fe3O4 nanostructures on GaAs(100) surfaces and hybrid Ferromagnetic/Semiconductor/Ferromagnetic (FM/SC/FM) spintronic devices. Element specific x-ray magnetic circular dichroism (XMCD) measurements have shown directly that Fe atoms on the GaAs(100)-4 x 6 surface are ferromagnetic. Within coverages of 2.5 to 4.8 ML superparamagnetic nanoclusters are formed and exhibiting strong uniaxial anisotropy, of the order of 6.0 x 10(5) erg/cm(3). The coercivities of epitaxial Fe dot arrays films grown on GaAs(100) were observed to be dependent on the separation and size of the dots indicating that interdot dipolar coupling affects the magnetization processes in these dots. In addition Fe3O4 films grown on deformed GaAs(100) substrates have been observed to form nanostripes following the topography of the substrate and magneto-optical Kerr effect (MOKE) measurements showed that these nanostripes have uniaxial magnetic anisotropy with easy axis perpendicular to the length of the nanostripes. Meanwhile the FM/SC/FM vertical device has exhibited a biasing current dependent on MR characteristics, with a maximum change of 12% in the MR observed, indicating for the first time a large room temperature spin injection and detection