First passage time exponent for higher-order random walks:Using Levy flights

We present a heuristic derivation of the first passage time exponent for the integral of a random walk [Y. G. Sinai, Theor. Math. Phys. {\bf 90}, 219 (1992)]. Building on this derivation, we construct an estimation scheme to understand the first passage time exponent for the integral of the integral of a random walk, which is numerically observed to be $0.220\pm0.001$. We discuss the implications of this estimation scheme for the $n{\rm th}$ integral of a random walk. For completeness, we also address the $n=\infty$ case. Finally, we explore an application of these processes to an extended, elastic object being pulled through a random potential by a uniform applied force. In so doing, we demonstrate a time reparameterization freedom in the Langevin equation that maps nonlinear stochastic processes into linear ones.Comment: 4 figures, submitted to PR

Models of plastic depinning of driven disordered systems

Two classes of models of driven disordered systems that exhibit history-dependent dynamics are discussed. The first class incorporates local inertia in the dynamics via nonmonotonic stress transfer between adjacent degrees of freedom. The second class allows for proliferation of topological defects due to the interplay of strong disorder and drive. In mean field theory both models exhibit a tricritical point as a function of disorder strength. At weak disorder depinning is continuous and the sliding state is unique. At strong disorder depinning is discontinuous and hysteretic.Comment: 3 figures, invited talk at StatPhys 2

Accumulation of Heavy Metals in Selected Vegetables, Their Availability and Correlation in Lithogenic and Nonlithogenic Fractions of Soils from Some Agricultural Areas in Malaysia

ABSTRACT Heavy metal content was determined in selected vegetables cultivated in some highland and lowland areas in Peninsular Malaysia. Leafy vegetables were represented by convolvulus (Ipomoea aquatica) and green mustard or sawi (Brassica rapa var. parachinensis), tubers and bulbs by sweet potato (Ipomoea batatas) and onion (Allium cepa), and fruity vegetables by chilly (Capsicum annuum), brinjal (Solanum melongena) and long bean (Vigna sinensis), respectively. Heavy metals from lithogenic and nonlithogenic soil fractions were studied at Cameron Highlands situated in the Pahang state and at lowland areas in Klang, Bangi, Gombak and Sepang districts in the Selangor state. The aim of the study was to investigate the availability of heavy metals and their potential uptake by vegetables in selected agricultural areas. The metals analysed were ferrum (Fe), zinc (Zn) cadmium (Cd), manganese (Mn), plumbum (Pb), copper (Cu) and chromium (Cr). Three soil samples were collected from each area and sampling was done at 1-30 cm depth. Extraction of heavy metals was carried out using sequential extraction and four fractions were produced comprising the easily leachable and ion exchange fraction, the acid reducible fraction, the oxidation organic fraction and the resistant fraction respectively. Heavy metal content in plant and soil samples were determined by atomic absorption spectrophotometry following standard methods (AOAC). Most metals were found at concentrations normally observed in vegetables grown in uncontaminated agricultural areas, with zinc (Zn) and manganese (Mn) content being highest, followed by copper (Cu), plumbum (Pb) and cadmium (Cd). However, the levels of potentially toxic metals such as Pb, Cd and Cr in the vegetables studied were found to be below the stipulated levels. Analysis of soil samples showed that the highest concentrations of heavy metals were obtained from the resistant fraction as compared to the other soil fractions. Concentration of Fe and Pb was found to be high in Sepang, whereas that of Cu was highest in Gombak and Cd levels were generally high in Sepang and Gombak. In contrast, the concentration of metals in the easily leachable and ion exchange fractions were low. Since differential uptake and accumulation of metals in the various plant parts are influenced by the availability of metals from the latter two fractions of the soils, the results indicate that availability of heavy metals to the cultivated plants (and thus, its consequent health risk to consumers) is also low. Based on the results obtained, the availability of heavy metals can be arranged as follows: Zn > Mn > Cd > Cu > Pb > Fe. The agricultural soils were found to contain high levels of Fe, Mn and Zn, whilst Cd and Cr were found in very low levels, well below the critical soil levels listed for arable land

Structural Principles in Robo Activation and Auto-Inhibition

This is the author accepted manuscript. The final version is available from Elsevier (Cell Press) via the DOI in this record.Proper brain function requires high-precision neuronal expansion and wiring, processes controlled by the transmembrane Roundabout (Robo) receptor family and their Slit ligands. Despite their great importance, the molecular mechanism by which Robos’ switch from “off” to “on” states remains unclear. Here, we report a 3.6 Å crystal structure of the intact human Robo2 ectodomain (domains D1–8). We demonstrate that Robo cis dimerization via D4 is conserved through hRobo1, 2, and 3 and the C. elegans homolog SAX-3 and is essential for SAX-3 function in vivo. The structure reveals two levels of auto-inhibition that prevent premature activation: (1) cis blocking of the D4 dimerization interface and (2) trans interactions between opposing Robo receptors that fasten the D4-blocked conformation. Complementary experiments in mouse primary neurons and C. elegans support the auto-inhibition model. These results suggest that Slit stimulation primarily drives the release of Robo auto-inhibition required for dimerization and activation.ICRFIS

Depinning and plasticity of driven disordered lattices

We review in these notes the dynamics of extended condensed matter systesm, such as vortex lattices in type-II superconductors and charge density waves in anisotropic metals, driven over quenched disorder. We focus in particular on the case of strong disorder, where topological defects are generated in the driven lattice. In this case the repsonse is plastic and the depinning transition may become discontinuous and hysteretic.Comment: 21 pages, 6 figures. Proceedings the XIX Sitges Conference on Jamming, Yielding, and Irreversible Deformations in Condensed Matter, Sitges, Barcelona, Spain, June 14-18, 200

Addressing robustness in time-critical, distributed, task allocation algorithms.

The aim of this work is to produce and test a robustness module (ROB-M) that can be generally applied to distributed, multi-agent task allocation algorithms, as robust versions of these are scarce and not well-documented in the literature. ROB-M is developed using the Performance Impact (PI) algorithm, as this has previously shown good results in deterministic trials. Different candidate versions of the module are thus bolted on to the PI algorithm and tested using two different task allocation problems under simulated uncertain conditions, and results are compared with baseline PI. It is shown that the baseline does not handle uncertainty well; the task-allocation success rate tends to decrease linearly as degree of uncertainty increases. However, when PI is run with one of the candidate robustness modules, the failure rate becomes very low for both problems, even under high simulated uncertainty, and so its architecture is adopted for ROB-M and also applied to MIT’s baseline Consensus Based Bundle Algorithm (CBBA) to demonstrate its flexibility. Strong evidence is provided to show that ROB-M can work effectively with CBBA to improve performance under simulated uncertain conditions, as long as the deterministic versions of the problems can be solved with baseline CBBA. Furthermore, the use of ROB-M does not appear to increase mean task completion time in either algorithm, and only 100 Monte Carlo samples are required compared to 10,000 in MIT’s robust version of the CBBA algorithm. PI with ROB-M is also tested directly against MIT’s robust algorithm and demonstrates clear superiority in terms of mean numbers of solved tasks.N/

Intrinsic activity in the fly brain gates visual information during behavioral choices

The small insect brain is often described as an input/output system that executes reflex-like behaviors. It can also initiate neural activity and behaviors intrinsically, seen as spontaneous behaviors, different arousal states and sleep. However, less is known about how intrinsic activity in neural circuits affects sensory information processing in the insect brain and variability in behavior. Here, by simultaneously monitoring Drosophila's behavioral choices and brain activity in a flight simulator system, we identify intrinsic activity that is associated with the act of selecting between visual stimuli. We recorded neural output (multiunit action potentials and local field potentials) in the left and right optic lobes of a tethered flying Drosophila, while its attempts to follow visual motion (yaw torque) were measured by a torque meter. We show that when facing competing motion stimuli on its left and right, Drosophila typically generate large torque responses that flip from side to side. The delayed onset (0.1-1 s) and spontaneous switch-like dynamics of these responses, and the fact that the flies sometimes oppose the stimuli by flying straight, make this behavior different from the classic steering reflexes. Drosophila, thus, seem to choose one stimulus at a time and attempt to rotate toward its direction. With this behavior, the neural output of the optic lobes alternates; being augmented on the side chosen for body rotation and suppressed on the opposite side, even though the visual input to the fly eyes stays the same. Thus, the flow of information from the fly eyes is gated intrinsically. Such modulation can be noise-induced or intentional; with one possibility being that the fly brain highlights chosen information while ignoring the irrelevant, similar to what we know to occur in higher animals