17,168 research outputs found
A combinatorial result with applications to self-interacting random walks
We give a series of combinatorial results that can be obtained from any two
collections (both indexed by ) of left and right pointing arrows
that satisfy some natural relationship. When applied to certain
self-interacting random walk couplings, these allow us to reprove some known
transience and recurrence results for some simple models. We also obtain new
results for one-dimensional multi-excited random walks and for random walks in
random environments in all dimensions
Demonstration of a Heat-Stable Cyclic GMP Phosphodiesterase in the Medium of Physarum flavicomum
Wear observations applied to Lifeboat Slipway Launches
It is necessary to use an inclined slipway to launch a large lifeboat in locations where there is no natural harbour or where there is a large tidal range. Slipway stations consist of an initial section where the boat is held on rollers followed by an inclined keelway of nickel/chromium coated steel, the lifeboat is released from the top of the slipway and proceeds under its own weight into the water. The lifeboat is subsequently recovered to the top of the slipway using a winch line. With the introduction of the new, larger Tamar class lifeboat existing boathouses are being upgraded and existing low friction coated steel slipway lining materials replaced with a low-friction jute fibre/phenolic resin composite, which is designed to operate with unlubricated conditions. This has led to problems of high wear on slipway panels, particularly where the lifeboat mounts the slipway for recovery.
This paper describes a method for assessing slipway lining materials and lubricants. The selection of an appropriate test machine, the TE92 rotary tribometer, and design of a modified ring on disc arrangement incorporating panel interfaces and attaching holes effects is described. An experimental methodology is developed using programmed running intervals to simulate dwell effects.
Experimental data is thus presented to establish slipway panel wear rates for a range of lubricants and contact pressures. Experimental results are incorporated into real-world slipway surveys to develop the wear scenario. Results and implications of this research for future lifeboat slipway design are discussed
Synchronization of electrically coupled resonate-and-fire neurons
Electrical coupling between neurons is broadly present across brain areas and
is typically assumed to synchronize network activity. However, intrinsic
properties of the coupled cells can complicate this simple picture. Many cell
types with strong electrical coupling have been shown to exhibit resonant
properties, and the subthreshold fluctuations arising from resonance are
transmitted through electrical synapses in addition to action potentials. Using
the theory of weakly coupled oscillators, we explore the effect of both
subthreshold and spike-mediated coupling on synchrony in small networks of
electrically coupled resonate-and-fire neurons, a hybrid neuron model with
linear subthreshold dynamics and discrete post-spike reset. We calculate the
phase response curve using an extension of the adjoint method that accounts for
the discontinuity in the dynamics. We find that both spikes and resonant
subthreshold fluctuations can jointly promote synchronization. The subthreshold
contribution is strongest when the voltage exhibits a significant post-spike
elevation in voltage, or plateau. Additionally, we show that the geometry of
trajectories approaching the spiking threshold causes a "reset-induced shear"
effect that can oppose synchrony in the presence of network asymmetry, despite
having no effect on the phase-locking of symmetrically coupled pairs
Extraordinary Sex Ratios: Cultural Effects on Ecological Consequences
We model sex-structured population dynamics to analyze pairwise competition
between groups differing both genetically and culturally. A sex-ratio allele is
expressed in the heterogametic sex only, so that assumptions of Fisher's
analysis do not apply. Sex-ratio evolution drives cultural evolution of a
group-associated trait governing mortality in the homogametic sex. The two-sex
dynamics under resource limitation induces a strong Allee effect that depends
on both sex ratio and cultural trait values. We describe the resulting
threshold, separating extinction from positive growth, as a function of female
and male densities. When initial conditions avoid extinction due to the Allee
effect, different sex ratios cannot coexist; in our model, greater female
allocation always invades and excludes a lesser allocation. But the culturally
transmitted trait interacts with the sex ratio to determine the ecological
consequences of successful invasion. The invading female allocation may permit
population persistence at self-regulated equilibrium. For this case, the
resident culture may be excluded, or may coexist with the invader culture. That
is, a single sex-ratio allele in females and a cultural dimorphism in male
mortality can persist; a low-mortality resident trait is maintained by
father-to-son cultural transmission. Otherwise, the successfully invading
female allocation excludes the resident allele and culture, and then drives the
population to extinction via a shortage of males. Finally, we show that the
results obtained under homogeneous mixing hold, with caveats, in a spatially
explicit model with local mating and diffusive dispersal in both sexes.Comment: final version, reflecting changes in response to referees' comment
Configuration control of seven-degree-of-freedom arms
A seven degree of freedom robot arm with a six degree of freedom end effector is controlled by a processor employing a 6 by 7 Jacobian matrix for defining location and orientation of the end effector in terms of the rotation angles of the joints, a 1 (or more) by 7 Jacobian matrix for defining 1 (or more) user specified kinematic functions constraining location or movement of selected portions of the arm in terms of the joint angles, the processor combining the two Jacobian matrices to produce an augmented 7 (or more) by 7 Jacobian matrix, the processor effecting control by computing in accordance with forward kinematics from the augmented 7 by 7 Jacobian matrix and from the seven joint angles of the arm a set of seven desired joint angles for transmittal to the joint servo loops of the arm. One of the kinematic functions constraints the orientation of the elbow plane of the arm. Another one of the kinematic functions minimizes a sum of gravitational torques on the joints. Still another kinematic function constrains the location of the arm to perform collision avoidance. Generically, one kinematic function minimizes a sum of selected mechanical parameters of at least some of the joints associated with weighting coefficients which may be changed during arm movement. The mechanical parameters may be velocity errors or gravity torques associated with individual joints
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