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Biomedical subjects

G Caldarelli

Publications and source records attributed to G Caldarelli.

At least 19 recordsLinked to original sources
PubMed2006

Preferential attachment in the growth of social networks: the internet encyclopedia Wikipedia.

We present an analysis of the statistical properties and growth of the free on-line encyclopedia Wikipedia. By describing topics by vertices and hyperlinks between them as edges, we can represent this encyclopedia as a directed graph. The topological properties of this graph are in close analogy with those of the World Wide Web, despite the very different growth mechanism. In particular, we measure a scale-invariant distribution of the in and out degree and we are able to reproduce these features by means of a simple statistical model. As a major consequence, Wikipedia growth can be described by local rules such as the preferential attachment mechanism, though users, who are responsible of its evolution, can act globally on the network.

Journal Article↗
PubMed2006

Retroperitoneoscopic lumbar sympathectomy for nonreconstructable arterial occlusive disease.

AIM: The aim of this study was to present our experience with video-assisted lumbar sympathectomy for non-reconstructive arterial occlusive disease in a series of 23 consecutive patients whose predominant symptoms were unilateral rest pain, limited skin ulcerations or gangrene of the toes. METHODS: All the procedures were performed with retroperitoneal approach, dorsal position of the patient and simple digital dissection of the retroperitoneal space. RESULTS: The operations were successfully performed in all patients except for 2, who immediately underwent open conversion. A urinoma caused by ureteral lesion was the only severe complication in this series. The mean operative time of the procedure was 55 min and the hospital stay was 2 or 3 days. No parenteral analgesics were administered postoperatively. At 1 month from operation, 20 patients out of 23 had significant relief of rest pain and improvement of ischemic lesions. After a median follow-up of 36 months, 2 patients had died, 4 underwent some type of distal amputation, 1 had recurrent rest pain and the other 16 reported persistent improvement of pain or dystrophic changes. CONCLUSIONS: Retro-peritoneoscopic technique appears the modern and less invasive version of the lumbar surgical sympathectomy.

Aged↗
PubMed2004

Preferential exchange: strengthening connections in complex networks.

Many social, technological, and biological interactions involve network relationships whose outcome intimately depends on the structure of the network and on the strengths of the connections. Yet, although much information is now available concerning the structure of many networks, the strengths are more difficult to measure. Here we show that, for one particular social network, notably the e-mail network, a suitable measure of the strength of the connections can be available. We also propose a simple mechanism, based on positive feedback and reciprocity, that can explain the observed behavior and that hints toward specific dynamics of formation and reinforcement of network connections. Network data from contexts different from social sciences indicate that power-law, and generally broad, distributions of the connection strength are ubiquitous, and the proposed mechanism has a wide range of applicability.

Journal Article↗
PubMed2002

Scale-free networks from varying vertex intrinsic fitness.

A new mechanism leading to scale-free networks is proposed in this Letter. It is shown that, in many cases of interest, the connectivity power-law behavior is neither related to dynamical properties nor to preferential attachment. Assigning a quenched fitness value x(i) to every vertex, and drawing links among vertices with a probability depending on the fitnesses of the two involved sites, gives rise to what we call a good-get-richer mechanism, in which sites with larger fitness are more likely to become hubs (i.e., to be highly connected).

Algorithms↗
PubMed2002

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, 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 nonambiguous distinction between sandpilelike systems and diffusive systems. Numerical simulations support our analytical results.

Journal Article↗
PubMed2002

Probabilistic approach to the Bak-Sneppen model.

We study here the Bak-Sneppen model, a prototype model for the study of self-organized criticality. In this model several species interact and undergo extinction with a power-law distribution of activity bursts. Species are defined through their "fitness" whose distribution in the system is uniform above a certain threshold. Run time statistics is introduced for the analysis of the dynamics in order to explain the peculiar properties of the model. This approach based on conditional probability theory, takes into account the correlations due to memory effects. In this way, we may compute analytically the value of the fitness threshold with the desired precision. This represents a substantial improvement with respect to the traditional mean field approach.

Journal Article↗
PubMed2002

Prosthetic replacement of the inferior vena cava and the iliofemoral vein for urologically related malignancies.

OBJECTIVE: To evaluate the feasibility and results of prosthetic venous replacement, as inferior vena cava (IVC) and iliofemoral vein resection and replacement are sometimes necessary when they are extensively involved by a large and fixed tumour thrombus from renal cell carcinoma (RCC) or other urological malignancies. PATIENTS AND METHODS: Five men and two women (age range 41-75 years) were treated over a 10-year period (1991-2001) by aggressive venous surgery to achieve complete tumour resection, with prosthetic graft replacement to re-establish venous flow. The tumours included RCC of the right kidney (two), retroperitoneal liposarcoma (two), bladder cancer (one), retroperitoneal fibrosarcoma (one) and inguino-pelvic lymphoma (one). Two patients had a vena caval replacement, of whom one had a circular reinforced PTFE and one a Dacron silver graft; five patients had either an iliofemoral or an ilio-iliac circular reinforced PTFE graft. The prosthetic diameter was 18-20 mm for the IVC grafts and 8-10 mm for the iliac grafts. In all the patients, graft patency was evaluated during the follow-up by colour flow duplex imaging, and in one it was determined by angio-computed tomography scan and venography. RESULTS: One patient died 30 days after surgery; of the remaining six patients one had no evidence of regional recurrence or metastatic disease at 12 months, and five died from recurrent tumour 8-30 months after surgery. The mean time to death was 23 months. At 3 months all six prosthesis were patent; at 6 months four were patent and at 12 months three of five prostheses were patent. CONCLUSION: Resecting and replacing the IVC allows complete tumour resection and avoids renal failure, providing durable relief from the symptoms of venous obstruction. Iliofemoral prosthetic reconstruction for urological-related malignancies represents a viable option to avoid venous engorgement and lower extremity swelling, at least in the early postoperative period. The mean time to death for the present patients must be considered the limit for these aggressive operations.

Adult↗
PubMed2001

Growing dynamics of Internet providers.

In this paper we present a model for the growth and evolution of Internet providers. The model reproduces the data observed for the Internet connection as probed by tracing routes from different computers. This problem represents a paramount case of study for growth processes in general, but can also help in the understanding the properties of the Internet. Our main result is that this network can be reproduced by a self-organized interaction between users and providers that can rearrange in time. This model can then be considered as a prototype model for the class of phenomena of aggregation processes in social networks.

Journal Article↗
PubMed2001

Perturbative approach to the Bak-Sneppen model.

We study the Bak-Sneppen model in the probabilistic framework of the run time statistics (RTS). This model has attracted a large interest for its simplicity being a prototype for the whole class of models showing self-organized criticality. The dynamics is characterized by a self-organization of almost all the species fitnesses above a nontrivial threshold value, and by a lack of spatial and temporal characteristic scales. This results in avalanches of activity power law distributed. In this Letter we use the RTS approach to compute the value of x(c), the value of the avalanche exponent tau, and the asymptotic distribution of minimal fitnesses.

Journal Article↗
PubMed2001

Cold and warm swelling of hydrophobic polymers.

We introduce a polymer model where the transition from swollen to compact configurations is due to interactions between the monomers and the solvent. These interactions are the origin of the effective attractive interactions between hydrophobic amino acids in proteins. We find that in the low and high temperature phases polymers are swollen, and there is an intermediate phase where the most favorable configurations are compact. We argue that such a model captures in a single framework both the cold and the warm denaturation experimentally detected for thermosensitive polymers and for proteins.

Journal Article↗
PubMed2001

Cellular models for river networks.

A cellular model introduced for the evolution of the fluvial landscape is revisited using extensive numerical and scaling analyses. The basic network shapes and their recurrence especially in the aggregation structure are then addressed. The roles of boundary and initial conditions are carefully analyzed as well as the key effect of quenched disorder embedded in random pinning of the landscape surface. It is found that the above features strongly affect the scaling behavior of key morphological quantities. In particular, we conclude that randomly pinned regions (whose structural disorder bears much physical meaning mimicking uneven landscape-forming rainfall events, geological diversity or heterogeneity in surficial properties like vegetation, soil cover or type) play a key role for the robust emergence of aggregation patterns bearing much resemblance to real river networks.

Biophysical Phenomena↗
PubMed2000

Angular structure of lacunarity, and the renormalization group

We formulate the angular structure of lacunarity in fractals, in terms of a symmetry reduction of the three point correlation function. This provides a rich probe of universality, and first measurements yield new evidence in support of the equivalence between self-avoiding walks (SAW's) and percolation perimeters in two dimensions. We argue that the lacunarity reveals much of the renormalization group in real space. This is supported by exact calculations for random walks and measured data for percolation clusters and SAW's. Relationships follow between exponents governing inward and outward propagating perturbations, and we also find a very general test for the contribution of long-range interactions.

Journal Article↗
PubMed2000

Discretized diffusion processes

We study the properties of the "rigid Laplacian" operator; that is we consider solutions of the Laplacian equation in the presence of fixed truncation errors. The dynamics of convergence to the correct analytical solution displays the presence of a metastable set of numerical solutions, whose presence can be related to granularity. We provide some scaling analysis in order to determine the value of the exponents characterizing the process. We believe that this prototype model is also suitable to provide an explanation of the widespread presence of power law in a social and economic system where information and decision diffuse, with errors and delay from agent to agent.

Journal Article↗
PubMed2000

Invasion percolation with temperature and the nature of self-organized criticality in real systems

In this paper we present a theoretical approach that allows us to describe the transition between critical and noncritical behavior when stocastic noise is introduced in extremal models with disorder. Namely, we show that the introduction of thermal noise in invasion percolation (IP) brings the system outside the critical point. This result suggests a possible definition of self-organized criticality systems as ordinary critical systems where the critical point corresponds to set to 0 one of the parameters. We recover both the IP and Eden models for T-->0 and T-->infinity, respectively. For small T we find a dynamical second-order transition with correlation length diverging when T-->0.

Journal Article↗
PubMed2000

Putting proteins back into water

We introduce a simplified protein model where the solvent (water) degrees of freedom appear explicitly (although in an extremely simplified fashion). Using this model we are able to recover the thermodynamic phenomenology of proteins over a wide range of temperatures. In particular we describe both the warm and the cold protein denaturation within a single framework, while addressing important issues about the structure of model proteins.

Journal Article↗
PubMed1998

Modelling Coevolution in Multispecies Communities.

We introduce the Webworld model, which links together the ecological modelling of food web structure with the evolutionary modelling of speciation and extinction events. The model describes dynamics of ecological communities on an evolutionary time-scale. Species are defined as sets of characteristic features, and these features are used to determine interaction scores between species. A simple rule is used to transfer resources from the external environment through the food web to each of the species, and to determine mean population sizes. A time step in the model represents a speciation event. A new species is added with features similar to those of one of the existing species and a new food web structure is than calculated. The new species may (i) add stably to the web, (ii) become extinct immediately because it is poorly adapted, or (iii) cause one or more other species to become extinct due to competition for resources. We measure various properties of the model webs and compare these with data on real food webs. These properties include the proportions of basal, intermediate and top species, the number of links per species and the number of trophic levels. We also study the evolutionary dynamics of the model ecosystem by following the fluctuations in the total number of species in the web. Extinction avalanches occur when novel organisms arise which are significantly better adapted than existing ones. We discuss these results in relation to the observed extinction events in the fossil record, and to the theory of self-organized criticality.Copyright 1998 Academic Press

Journal Article↗