Bidirectional transport and pulsing states in a multi-lane ASEP model

In this paper, we introduce an ASEP-like transport model for bidirectional motion of particles on a multi-lane lattice. The model is motivated by {\em in vivo} experiments on organelle motility along a microtubule (MT), consisting of thirteen protofilaments, where particles are propelled by molecular motors (dynein and kinesin). In the model, organelles (particles) can switch directions of motion due to "tug-of-war" events between counteracting motors. Collisions of particles on the same lane can be cleared by switching to adjacent protofilaments (lane changes). We analyze transport properties of the model with no-flux boundary conditions at one end of a MT ("plus-end" or tip). We show that the ability of lane changes can affect the transport efficiency and the particle-direction change rate obtained from experiments is close to optimal in order to achieve efficient motor and organelle transport in a living cell. In particular, we find a nonlinear scaling of the mean {\em tip size} (the number of particles accumulated at the tip) with injection rate and an associated phase transition leading to {\em pulsing states} characterized by periodic filling and emptying of the system.Comment: 11 figure

From Grants to Loans and Fees: The Demand for Post-Compulsory Education in England and Wales from 1955 to 2008

The UK has progressively moved from a Higher Education (HE) system which is funded at the tax payers' expense to one which is funded by individual participants (and their parents) by scrapping student grants, introducing student loans and charging tuition fees. The purpose of this paper is to identify the impact of these changes on the demand for HE using time-series data for England and Wales over the period 1955 to 2008. We use a Seemingly Unrelated Regressions model of three indicators of demand for post-compulsory education allowing for structural breaks. Tests show that most of the breaks occurred in line with several important policy changes. We find that less generous student financial support arrangements have had a significant negative impact on university enrolment. We simulate the impact of raising tuition fees to £9,000 pa and find that this will reduce demand for HE from boys by 7.51 percentage points and from girls by 4.92 percentage points.post compulsory education, student finance, structural change

A Search for Temporal Variations in Station Terms in Southern California from 1984 to 2002

We use relative arrival times and locations for similar earthquake pairs that are found using a cross-correlation method to analyze the time dependence of P and S station terms in southern California from 1984 to 2002. We examine 494 similar event clusters recorded by Southern California Seismic Network (SCSN) stations and compute absolute arrival-time variations from the differential arrival-time residuals obtained following event relocation. We compute station terms from the robust means of the absolute arrival-time residuals from all events recorded by each station at 3-month intervals. We observe nine stations with abrupt offsets in timing of 20–70 msec, which are likely caused by equipment changes during our study period. Taking these changes into account could improve the relative location accuracy for some of the event clusters. For other stations, we generally do not see systematic temporal variations greater than about 10 msec. Analysis of residuals along individual ray paths does not reveal any clear localized regions of apparent velocity changes at depth. These results limit large-scale, long-lasting temporal variations in P and S velocities across southern California during this time period to less than about ±0.2%. However, there is an increased fraction of individual travel-time residuals exceeding 20 msec immediately following major earthquakes from source regions near the mainshock rupture