A Nearly Linear-Time PTAS for Explicit Fractional Packing and Covering Linear Programs

We give an approximation algorithm for packing and covering linear programs (linear programs with non-negative coefficients). Given a constraint matrix with n non-zeros, r rows, and c columns, the algorithm computes feasible primal and dual solutions whose costs are within a factor of 1+eps of the optimal cost in time O((r+c)log(n)/eps^2 + n).Comment: corrected version of FOCS 2007 paper: 10.1109/FOCS.2007.62. Accepted to Algorithmica, 201

Curved Tails in Polymerization-Based Bacterial Motility

The curved actin ``comet-tail'' of the bacterium Listeria monocytogenes is a visually striking signature of actin polymerization-based motility. Similar actin tails are associated with Shigella flexneri, spotted-fever Rickettsiae, the Vaccinia virus, and vesicles and microspheres in related in vitro systems. We show that the torque required to produce the curvature in the tail can arise from randomly placed actin filaments pushing the bacterium or particle. We find that the curvature magnitude determines the number of actively pushing filaments, independent of viscosity and of the molecular details of force generation. The variation of the curvature with time can be used to infer the dynamics of actin filaments at the bacterial surface.Comment: 8 pages, 2 figures, Latex2

Preschoolers' depression severity and behavior during dyadic interactions: The mediating role of parental support

ABSTRACT Objective: To investigate the relationship between preschool depression severity, observed behavior, and parental emotional support in a population of 3.0-to 5.6-year-olds and their mothers. Method: One hundred fifty preschoolers who underwent a comprehensive mental health assessment during which DSM-1V diagnoses were derived were included in this analysis. Child and parent behaviors during challenging structured dyadic tasks were systematically coded. Dyads with preschoolers in three diagnostic groups of interest were explored: depression, disruptive, and healthy. Depression severity sum scores were derived for children in all of the groups. Results: Depression severity accounted for a significant (p < .05) portion of the variance in preschoolers' persistence, compliance, and enthusiasm during dyadic tasks after controlling for the effects of age and gender. Depression severity was also significantly associated with parental emotional support, which was itself associated with all three preschool behaviors. When the effect of parental support was controlled for statistically, however, preschoolers' depression severity was no longer significantly assodiated with observed persistence or compliance, whereas the relationship between depression severity and enthusiasm remained significant