Cell motility in higher organisms (eukaryotes) is crucial to biological functions ranging from wound healing to immune response, and also implicated in diseases such as cancer. For cells crawling on hard surfaces, significant insights into motility have been gained from experiments replicating such motion in vitro. Such experiments show that crawling uses a combination of actin treadmilling (polymerization), which pushes the front of a cell forward, and myosin-induced stress (contractility), which retracts the rear. Here we present a simplified physical model of a crawling cell, consisting of a droplet of active polar fluid with contractility throughout, but treadmilling connected to a thin layer near the supporting wall. The model shows a variety of shapes and/or motility regimes, some closely resembling cases seen experimentally. Our work strongly supports the view that cellular motility exploits autonomous physical mechanisms whose operation does not need continuous regulatory effort.

Active systems, or active matter, are self-driven systems that live, or function, far from equilibrium - a paradigmatic example that we focus on here is provided by a suspension of self-motile particles. Active systems are far from equilibrium because their microscopic constituents constantly consume energy from the environment in order to do work, for instance to propel themselves. The non-equilibrium nature of active matter leads to a variety of non-trivial intriguing phenomena. An important one, which has recently been the subject of intense interest among biological and soft matter physicists, is that of the so-called "motility-induced phase separation", whereby self-propelled particles accumulate into clusters in the absence of any explicit attractive interactions between them. Here we review the physics of motility-induced phase separation, and discuss this phenomenon within the framework of the classic physics of phase separation and coarsening. We also discuss theories for bacterial colonies where coarsening may be arrested. Most of this work will focus on the case of run-and-tumble and active Brownian particles in the absence of solvent-mediated hydrodynamic interactions - we will briefly discuss at the end their role, which is not currently fully understood in this context.

We present a continuum theory of self-propelled particles, without alignment interactions, in a momentum-conserving solvent. To address phase separation, we introduce a dimensionless scalar concentration field ? with advective-diffusive dynamics. Activity creates a contribution ? to the deviatoric stress, where is odd under time reversal and d is the number of spatial dimensions; this causes an effective interfacial tension contribution that is negative for contractile swimmers. We predict that domain growth then ceases at a length scale where diffusive coarsening is balanced by active stretching of interfaces, and confirm this numerically. Thus, there is a subtle interplay of activity and hydrodynamics, even without alignment interactions.

Cosa sono le reti? Come funzionano? Lo racconta Paolo Tieri, fisico e ricercatore del CNR che da anni si dedica alla biologia dei sistemi e all'immunologia. Ogni mattina, quando andiamo a scuola, all'università, al lavoro, usiamo la rete stradale o quella dei trasporti urbani. Arrivati a destinazione ci troviamo ad interagire e a collaborare con amici e colleghi della nostra rete amicale. Ogni tanto controlliamo su Facebook cosa fanno gli amici del nostro social network preferito. E poi facciamo la spesa e prenotiamo le vacanze su Internet, usando la "rete delle reti" tecnologica mondiale. Insomma le reti sono ovunque e noi ci siamo immersi.

In remote storage services, delays in the time to retrieve data can cause economic losses to the data owners. In this paper, we address the problem of properly establishing specific clauses in the service level agreement (SLA), intended to guarantee a short and predictable retrieval time. Based on the rationale that the retrieval time mainly depends on the storage media used at the server side, we introduce the concept of Provable Storage Medium (PSM), to denote the ability of a user to efficiently verify that the provider is complying to this aspect of the SLA. We propose PSM as an extension of Provable Data Possession (PDP): embedding challenge-response PDP schemes with measurements of the response time, both properties can be enforced without any need for the user to locally store nor download her data. We describe a realistic implementation of PSM in a scenario where data should be stored both in RAM and HDD. A thorough analysis shows that, even for relatively small challenges, the total time to compute and deliver the response is sensibly affected by the remarkable difference in the access time of the two supports. An extensive simulation campaign confirms the quality and viability of our proposal.

In this paper we face the problem of maximizing the amount of time over which a set of target points, located in a given geographic region, can be monitored by means of a wireless sensor network. The problem is well known in the literature as Maximum Network Lifetime Problem (MLP). In the last few years the problem and a number of variants have been tackled with success by means of different resolution approaches, including exact approaches based on column generation techniques. In this work we propose an exact approach which combines a column generation approach with a genetic algorithm aimed at solving efficiently its separation problem. The genetic algorithm is specifically aimed at the Maximum Network ?-Lifetime Problem (?-MLP), a variant of MLP in which a given fraction of targets is allowed to be left uncovered at all times; however, since ?-MLP is a generalization of MLP, it can be used to solve the classical problem as well. The computational results, obtained on the benchmark instances, show that our approach overcomes the algorithms, available in the literature, to solve both MLP and ?-MLP.

Wireless sensor networks are generally composed of a large number of hardware devices of the same type, deployed over a region of interest in order to perform a monitoring activity on a set of target points. Nowadays, several different types of sensor devices exist, which are able to monitor different aspects of the region of interest (including sound, vibrations, proximity, chemical contaminants, among others) and may be deployed together in a heterogeneous network. In this work, we face the problem of maximizing the amount of time during which such a network can remain operational, while maintaining at all times a minimum coverage guarantee for all the different sensor types. Some global regularity conditions in order to guarantee a fair level of coverage for each sensor type to each target are also taken into account in a second variant of the proposed problem. For both problem variants we developed an exact approach, which is based on a column generation algorithm whose subproblem is either solved heuristically by means of a genetic algorithm or optimally by an appropriate ILP formulation. In our computational tests the proposed genetic algorithm is shown to be able to dramatically speed up the procedure, enabling the resolution of large-scale instances within reasonable computational times.

Overcomplete representations such as wavelets and windowed Fourier expansions have become mainstays of modern statistical data analysis. Here we derive expressions for the mean quadratic risk of shrinkage estimators in the context of general finite frames, which include any fullrank linear expansion of vector data in a finite-dimensional setting. We provide several new results and practical estimation procedures that take into account the geometric correlation structure of frame elements. These results motivate aggregation estimators and block thresholding procedures, and reinforce that the correlations induced by frame structure should be explicitly treated to yield improvements in estimation. A simulation study confirms these improvements.

SBMLMechanotransduction Map

Solutions to the n-Laplace equation with a right-hand side f are considered. We exhibit the largest rearrangement-invariant space to which f has to belong for every local weak solution to be continuous. Moreover, we find the optimal modulus of continuity of solutions when f ranges in classes of rearrangement-invariant spaces, including Lorentz, Lorentz-Zygmund and various standard Orlicz spaces.

An important topic in the numerical analysis of Volterra integral equations is the stability theory. The main results known in theliterature have been obtained on linear test equations or, at least, on nonlinear equations with convolution kernel. Here, we considerVolterra integral equations with Hammerstein nonlinearity, not necessarily of convolution type, and we study the error equation forDirect Quadrature methods with respect to bounded perturbations. For a class of Direct Quadrature methods, we obtain conditionson the stepsize h for the numerical solution to behave stably and we report numerical examples which show the robustness of thisnonlinear stability theory.

Proceedings of the International Conference (Gioia del Colle, October 21-22, 2014)

Il presente volume è compreso in una serie di attività scientifiche conclusive che rientrano in una delle principali e fondamentali azioni previste dal progetto T.He.T.A (Technological tools for the promotion of the Transadriatic Archaeological Heritage), finanziato nell' ambito dell' European Territorial Cooperation Programme Greece-Italy 2007-2013, del quale il CNR-IAC è il Leader Partner. La scelta di un tema talmente affascinante, quale il lusso delle aristocrazie peucezie, nasce dalla volontà di approfondire questo importante aspetto delle genti indigene che hanno occupato la Puglia centrale per quasi un millennio, indagando alcuni fenomeni sociologici e culturali legati alle manifestazioni del lusso e della ricchezza. Per questo motivo, sono stati spesso presi in considerazione alcuni tra i contesti più significativi provenienti dai centri più importanti dell' area, cercando di metterne in risalto gli elementi e gli atteggiamenti comuni nell' uso e nel possesso dei beni di lusso e di prestigio, con particolare riguardo ai manufatti greci, e i rapporti con l' opposta sponda adriatica e con le isole greche dello Ionio (Corfù tra tutte). Infine, nell' analisi della documentazione funeraria, una particolare attenzione è stata prestata alle informazioni inerenti l' identità sociale dei proprietari delle tombe e ai diversi rituali funerari di cui questi individui sono stati oggetto.

This paper deals with the derivation of a collective model of cell populations out of an individual-based description of the underlying physical particle system. By looking at the spatial distribution of cells in terms of time-evolving measures, rather than at individual cell paths, we obtain an ensemble representation stemming from the phenomenological behavior of the single component cells. In particular, as a key advantage of our approach, the scale of representation of the system, i.e., microscopic/discrete vs. macroscopic/continuous, can be chosen a posteriori according only to the spatial structure given to the aforesaid measures. The paper focuses in particular on the use of different scales based on the specific functions performed by cells. A two-population hybrid system is considered, where cells with a specialized/differentiated phenotype are treated as a discrete population of point masses while unspecialized/undifferentiated cell aggregates are represented by a continuous approximation. Numerical simulations and analytical investigations emphasize the role of some biologically relevant parameters in determining the specific evolution of such a hybrid cell system.

This paper presents a new approach to the modeling of vehicular traffic flows on road networks based on kinetic equations. While in the literature the problem has been extensively studied by means of macroscopic hydrodynamic models, to date there are still not, to the authors' knowledge, contributions tackling it from a genuine statistical mechanics point of view. Probably one of the reasons is the higher technical complexity of kinetic traffic models, further increased in case of several interconnected roads. Here such difficulties of the theory are overcome by taking advantage of a discrete structure of the space of microscopic states of the vehicles, which is also significant in view of including the intrinsic microscopic granularity of the system in the mesoscopic representation.

We show in this paper that the roots $x_1$ and $x_2$ of a scalar quadratic polynomial $ax^2 + bx + c = 0$ with real or complex coefficients $a, b, c$ can be computed in an element-wise mixed stable manner, measured in a relative sense. We also show that this is a stronger property than norm-wise backward stability but weaker than element-wise backward stability. We finally show that there does not exist any method that can compute the roots in an element-wise backward stable sense, which is also illustrated by some numerical experiments.

The equality constrained indefinite least squares problem involves the minimization of an indefinite quadratic form subject to a linear equality constraint. In this paper, we study this problem and present a numerical method that is proved to be backward stable in a strict sense, i.e., that the computed solution satisfies a slightly perturbed equality constrained indefinite least squares problem. We also perform a sensitivity analysis of this problem and derive bounds for the accuracy of the computed solution. We give several numerical experiments to illustrate these results.

Well-balanced schemes were introduced to numerically enforce consistency with longtime behavior of the underlying continuous PDE. When applied to linear kinetic models, like the Goldstein-Taylor system, this construction generates discretizations which are inconsistent with the hydrodynamic stiff limit (despite it captures diffusive limits quite well). A numerical hybridization, taking advantage of both time-splitting (TS) and well-balanced (WB) approaches is proposed in order to fix this defect: numerical results show that resulting composite schemes improve rendering of macroscopic fluxes while keeping a correct hydrodynamic stiff limit.