Local Rigidity in Sandpile Models

We address the problem of the role of the concept of local rigidity in the family of sandpile systems. We define rigidity as the ratio between the critical energy and the amplitude of the external perturbation and we show, in the framework of the Dynamically Driven Renormalization Group (DDRG), that any finite value of the rigidity in a generalized sandpile model renormalizes to an infinite value at the fixed point, i.e. on a large scale. The fixed point value of the rigidity allows then for a non ambiguous distinction between sandpile-like systems and diffusive systems. Numerical simulations support our analytical results.Comment: to be published in Phys. Rev.

The Boson peak and the phonons in glasses

Despite the presence of topological disorder, phonons seem to exist also in glasses at very high frequencies (THz) and they remarkably persist into the supercooled liquid. A universal feature of such a systems is the Boson peak, an excess of states over the standard Debye contribution at the vibrational density of states. Exploiting the euclidean random matrix theory of vibrations in amorphous systems we show that this peak is the signature of a phase transition in the space of the stationary points of the energy, from a minima-dominated phase (with phonons) at low energy to a saddle-point dominated phase (without phonons). The theoretical predictions are checked by means of numeric simulations.Comment: to appear in the proceedings of the conference "Slow dynamics in complex sistems", Sendai (Japan) 200

Anderson Localization in Euclidean Random Matrices

We study spectra and localization properties of Euclidean random matrices. The problem is approximately mapped onto that of a matrix defined on a random graph. We introduce a powerful method to find the density of states and the localization threshold. We solve numerically an exact equation for the probability distribution function of the diagonal element of the the resolvent matrix, with a population dynamics algorithm, and we show how this can be used to find the localization threshold. An application of the method in the context of the Instantaneous Normal Modes of a liquid system is given.Comment: 4 page

Extracts from microalga chlorella sorokiniana exert an anti-proliferative effect and modulate cytokines in sheep peripheral blood mononuclear cells

The objective of this experiment was to study the effects of the unsaponified fraction (UP), the acetylated unsaponified fraction (AUP), and the total lipid fraction (TL) extracted and purified from Chlorella sorokiniana (CS) on the proliferation and cytokine profile of sheep peripheral blood mononuclear cells (PBMCs). Cells were cultured with 0.4 mg/mL and 0.8 mg/mL concentrations of each extract (UP, AUP, and TL fractions) and activated with 5 μg/mL concanavalin A (ConA) and 1 μg/mL lipopolysaccharide (LPS) at 37 °C for 24 h. PBMCs cultured with ConA and LPS represented the stimulated cells (SC), and PBMCs without ConA and LPS represented the unstimulated cells (USC). Cell-free supernatants were collected to determine IL-10, IL-1β, and IL-6 secretions; on cells, measurement of proliferation was performed. All the extracts tested significantly decreased the cell proliferation; in particular, the UP fraction at 0.4 mg/mL showed the lowest proliferative response. Furthermore, at 0.8 mg/mL, the UP fraction enhanced IL-10 secretion. On the contrary, the TL fraction at 0.4 mg/mL induced an increase in IL-10, IL-6, and, to a lesser extent, IL-1β secretions by cells. The AUP fraction did not change cytokine secretion. The results demonstrated that CS extracts could be useful ingredients in animal feed in order to minimize the use of antibiotics by modulating cell proliferation and cytokine response

Regulatory networks and connected components of the neutral space

The functioning of a living cell is largely determined by the structure of its regulatory network, comprising non-linear interactions between regulatory genes. An important factor for the stability and evolvability of such regulatory systems is neutrality - typically a large number of alternative network structures give rise to the necessary dynamics. Here we study the discretized regulatory dynamics of the yeast cell cycle [Li et al., PNAS, 2004] and the set of networks capable of reproducing it, which we call functional. Among these, the empirical yeast wildtype network is close to optimal with respect to sparse wiring. Under point mutations, which establish or delete single interactions, the neutral space of functional networks is fragmented into 4.7 * 10^8 components. One of the smaller ones contains the wildtype network. On average, functional networks reachable from the wildtype by mutations are sparser, have higher noise resilience and fewer fixed point attractors as compared with networks outside of this wildtype component.Comment: 6 pages, 5 figure

Lossy data compression with random gates

We introduce a new protocol for a lossy data compression algorithm which is based on constraint satisfaction gates. We show that the theoretical capacity of algorithms built from standard parity-check gates converges exponentially fast to the Shannon's bound when the number of variables seen by each gate increases. We then generalize this approach by introducing random gates. They have theoretical performances nearly as good as parity checks, but they offer the great advantage that the encoding can be done in linear time using the Survey Inspired Decimation algorithm, a powerful algorithm for constraint satisfaction problems derived from statistical physics