Agent-based models, an emerging paradigm of simulation of complex systems, appear very suitable to parallel processing. However, during the parallelization of a simulator of financial markets, we found that some features of these codes highlight non-trivial issues of the present hardware/software platforms for parallel processing. Here we present the results of a series of tests, on different platforms, of simplified codes that reproduce such problems and can be used as a starting point in the search of a possible solution.

We consider the phase ordering process of a system quenched into a lamellar phase in the presence of a shear flow. By studying the continuum model based on the Brazowskii free energy in a self-consistent approximation, we analyse the effects of a weak anisotropy in the quartic coupling constant, finding that it radically changes the evolution of the system.

The kinetics of domain growth of fluid mixtures quenched from a disordered to a lamellar phase has been studied in three dimensions. We use a numerical approach based on the lattice Boltzmann method (LBM). A novel implementation for LBM which "fuses" the collision and streaming steps is used in order to reduce memory and bandwidth requirements. We find that extended defects between stacks of lamellae with different orientation dominate the late time dynamics.

We review the problem of network mobility and internetworking between heterogeneous data networks and present an approach to the integration of WLAN and cellular networks based on loose coupling and the use of emerging mobility protocols. The handoff performance of such approach is studied, at network and transport level, in a realistic scenario along with the impact on global performance of transport protocols. Finally, a method for eliminating any packet loss at the network layer during the handoff is presented and evaluated.

Vaccination protocols designed to elicit anti-cancer immune responses have, many times, failed in producing tumor eradication and in prolonging patient survival. Usually in cancer vaccination, epitopes from one organism are included in the genome or linked with some protein of another in the hope that the immunogenic properties of the latter will boost an immune response to the former. However, recent results have demonstrated that injections of two different vectors encoding the same recombinant antigen generate high levels of speci c immunity. Systematic comparison of the ef cacy of different vaccination protocols has been hampered by technical limitations, and clear evidence that the use of multiple vectors has advantages over single carrier injections is lacking. We used a computational model to investigate the dynamics of the immune response to different anti-cancer vaccines based on randomly generated antigen/carrier compounds. The computer model was adapted for simulations to this new area in immunology research and carefully validated to the purpose. As a matter of fact, it reproduces a relevant number of experimental observations. The model shows that when priming and boosting with the same construct, competition rather than cooperation develops amongst T cell clones of different speci cities. Moreover, from the simulations, it appears that the sequential use of multiple carriers may generate more robust anti-tumor immune responses and may lead to effective tumor eradication in a higher percentage of cases. Our results provide a rational background for the design of novel strategies for the achievement of immune control of cancer. r 2005 Elsevier Ltd. All rights reserved.

We present an in silico model that simulates the immune system responses to tumor cells in naive and vaccinated mice. We have demonstrated the ability of this model to accurately reproduce the experimental results. Motivation: In vivo experiments on HER-2/neu mice have shown the effectiveness of Triplex vaccine in protection of mice from mammary carcinoma. Full protection was conferred using Chronic (prophylactic) vaccination protocol while therapeutic vaccination was less ef cient. Our in silico model was able to closely reproduce the effects of various vaccination protocols. This model is the rst step towards the development of in silico experiments searching for optimal vaccination protocols. Results: In silico experiments carried out on two large statistical samples of virtual mice showed very good agreements with in vivo experiments for all experimental vaccination protocols. They also show, as supported by in vivo experiments, that the humoral response is fundamental in controlling the tumor growth and therefore suggest the selection and timing of experiments for measuring the activity of T cells.

Mathematical and computational models are designed to improve our understanding of biological phenomena, to confirm/reject hypotheses, and to find points of intervention by altering the behavior of the studied systems. Here we describe the role of mathematical/computational models of the immune system. In particular, we analyze some examples of how mathematical modeling can contribute to finding optimal vaccination strategies. Indeed, computational modeling offers an intriguing opportunity from the theoretical point of view, and it will be of interest for clinically oriented investigators who wish to find optimal therapeutic strategies and for pharmaceutical industries that want to produce effective and successful drugs.

Cellular signaling (or signal transduction) is the process by which extracellular signals are converted into cellular responses. Mathematical models of signaling pathways may help understanding their general regulatory principles, as well as the roles of the different components often involved in more than one pathway. The aim of the present work is to describe a discrete model in time and space to study the dynamics of the population of molecules involved in a specific pathway. To this purpose, we use a multi-compartment stochastic cellular automaton to simulate one of the best understood cell signaling pathways: the GPCR-pathway. We then show the effect on the signal-carrying ef ciency of condensing a molecular species, which is pivotal to the reaction pathway, in a closed region of the cytosol. We nd that localization increases the robustness of the translocation mechanism only above a density threshold. For homogeneous settings, by increasing the concentration above a critical value, the translocation is obstructed by mere physical occupation constraints of the signal-carrying molecule. This conclusion is in line with previous findings, according to which in highly dense regions of the cytosol, convection by autocatalytic activation of adjacent molecules might be more ef cient than diffusion as a main signal propagation mechanism.

We are interested in developing a simple model to investigate blood-vessel interactions in a finite arterial segment of the cardiovascular tree. For this purpose, we developed a continuum model for a vascular segment, and we coupled it with a discrete model for the remaining systemic circulation. In working out the modeling, we addressed some main issues, such as the nonlinearity of blood flow, the compliance of the vessel and the prestress state of the artery walls, that is always present aside from the filling of blood. Moreover, we set a discrete model capable of providing appropriate boundary conditions to the continuum model, by reproducing the proper waveforms entering the vessel and avoiding spurious reflections.

We present a numerical study of mixing and reaction efficiency in closed domains. In particular, we focus our attention on laminar flows. In the case of inert transport the mixing properties of the flows strongly depend on the details of the Lagrangian transport. We also study the reaction efficiency. Starting with a little spot of product, we compute the time needed to complete the reaction in the container. We find that the reaction efficiency is not strictly related to the mixing properties of the flow. In particular, reaction acts as a "dynamical regulator."

We introduce a new class of models for aggregate or clustered point patterns, which encompasses and extends most of the classical models. We derive summary statistics and simulation procedures.

The inverse problem of constructing a symmetric Toeplitz matrix with prescribed eigenvalues has been a challenge both theoretically and computationally in the literature. It is now known in theory that symmetric Toeplitz matrices can have arbitrary real spectra. This paper addresses a similar problem--can the three largest eigenvalues of symmetric pentadiagonal Toeplitz matrices be arbitrary? Given three real numbers ? ? ?, this paper finds that the ratio ? = ?-? ?-? , including infinity if ? = ?, determines whether there is a symmetric pentadiagonal Toeplitz matrix with ?, ? and ? as its three largest eigenvalues. It is shown that such a matrix of size n × n does not exist if n is even and ? is too large or if n is odd and ? is too close to 1. When such a matrix does exist, a numerical method is proposed for the construction.

The effect of a small gravitomagnetic monopole on (accelerated) circular orbits in the equatorial plane of the Taub-NUT spacetime is compared to the corresponding (accelerated) orbits pushed slightly off the equatorial plane in the absense of the monopole (Schwarzschild spacetime).

The long standing difficulty in General Relativity of classifying the dynamics of cosmological models, e.g.\ as chaotic, is directly related to the gauge freedom intrinsic to relativistic spacetime theories: in general the invariance under diffeomorphisms makes any analysis of dynamical evolution dependent from the particular choice of time slicing one uses. We show here that the speciality index, a scalar dimensionless curvature invariant that has been mainly used in numerical relativity as an indicator of the special or non-special Petrov type character of a spacetime, is a time-independent quantity (a pure number) at each Kasner step of the Belinski-Khalatnikov-Lifshitz (BKL) map approximating the Mixmaster cosmology. Thus the BKL dynamics can be characterized in terms of the speciality index, i.e. in terms of curvature invariants directly related to observables. Possible applications for the associated Mixmaster dynamics are discussed.

The motion of spinning test particles along circular orbits in static vacuum spacetimes belonging to the Weyl class is discussed. Spin alignment and coupling with background parameters in the case of superimposed Weyl fields, corresponding to a single Schwarzschild black hole and single Chazy-Curzon particle as well as to two Schwarzschild black holes and two Chazy-Curzon particles, are studied in detail for standard choices of supplementary conditions. Applications to the gravitomagnetic "clock effect'' are also discussed.

The construction of a radar coordinate system about the world line of an observer is discussed. Radar coordinates for a hyperbolic observer as well as a uniformly rotating observer are described in detail. The utility of the notion of radar distance and the admissibility of radar coordinates are investigated. Our results provide a critical assessment of the physical significance of radar coordinates.

The periastron position advance for geodesic motion in axially symmetric solutions of the Einstein field equations belonging to the Weyl class of vacuum solutions is investigated. Explicit examples corresponding to either static solutions (single Chazy-Curzon, Schwarzschild and a pair of them), or stationary solution (single rotating Chazy-Curzon and Kerr black hole) are discussed. The results are then applied to the case of S2-SgrA$^*$ binary system of which the periastron position advance will be soon measured with a great accuracy.

The motion of a spinning test particle given by the Mathisson-Papapetrou equations is studied on an exterior vacuum C metric background spacetime describing the accelerated motion of a spherically symmetric gravitational source. We consider circular orbits of the particle around the direction of acceleration of the source. The symmetries of this configuration lead to the reduction of the differential equations of motion to algebraic relations. The spin supplementary conditions as well as the coupling between the spin of the particle and the acceleration of the source are discussed.

The behavior of charged spinning test particles moving along circular orbits in the equatorial plane of the Reissner{Nordstrom space{time is studied in the framework of the Dixon Souriau model completed with standard choices of supplementary conditions. The gravitomagnetic "clock effect," i.e. the delay in the arrival times of two oppositely circulating particles as measured by a static observer, is derived and discussed in the cases in which the particles have equal/opposite charge and spin, the latter being directed along the z-axis.

We study the behavior of nonzero rest mass spinning test particles moving along circular orbits in the Schwarzschild spacetime in the case in which the components of the spin tensor are allowed to vary along the orbit, generalizing some previous work.