Flocking without Alignment Interactions in Attractive Active Brownian Particles.

Within a simple model of attractive active Brownian particles, we predict flocking behavior and challenge the widespread idea that alignment interactions are necessary to observe this collective phenomenon. Here, we show that even nonaligning attractive interactions can lead to a flocking state. Monitoring the velocity polarization as the order parameter, we reveal the onset of a first-order transition from a disordered phase, characterized by several small clusters, to a flocking phase, where a single flocking cluster is emerging. The scenario is confirmed by studying the spatial connected correlation function of particle velocities, which reveals scale-free behavior in flocking states and exponential-like decay for nonflocking configurations. Our predictions can be tested in microscopic and macroscopic experiments showing flocking, such as animals, migrating cells, and active colloids.

Spontaneous Velocity Alignment in Motility-Induced Phase Separation.

We study a system of purely repulsive spherical self-propelled particles in the minimal setup inducing motility-induced phase separation (MIPS). We show that, even if explicit alignment interactions are absent, a growing order in the velocities of the clustered particles accompanies MIPS. Particles arrange into aligned or vortexlike domains whose size increases as the persistence of the self-propulsion grows, an effect that is quantified studying the spatial correlation function of the velocities. We explain the velocity alignment by unveiling a hidden alignment interaction of the Vicsek-like form, induced by the interplay between steric interactions and self-propulsion.

Entropy production and collective excitations of crystals out of equilibrium: The concept of entropons.

We study the collective vibrational excitations of crystals under out-of-equilibrium steady conditions that give rise to entropy production. Their excitation spectrum comprises equilibriumlike phonons of thermal origin and additional collective excitations called entropons because each of them represents a mode of spectral entropy production. Entropons coexist with phonons and dominate them when the system is far from equilibrium while they are negligible in near-equilibrium regimes. The concept of entropons has been recently introduced and verified in a special case of crystals formed by self-propelled particles. Here we show that entropons exist in a broader class of active crystals that are intrinsically out of equilibrium and characterized by the lack of detailed balance. After a general derivation, several explicit examples are discussed, including crystals consisting of particles with alignment interactions and frictional contact forces.

Handy fluctuation-dissipation relation to approach generic noisy systems and chaotic dynamics.

We introduce a general formulation of the fluctuation-dissipation relations (FDRs) holding also in far-from-equilibrium stochastic dynamics. A great advantage of this version of the FDR is that it does not require explicit knowledge of the stationary probability density function. Our formula applies to Markov stochastic systems with generic noise distributions: When the noise is additive and Gaussian, the relation reduces to those known in the literature; for multiplicative and non-Gaussian distributions (e.g., Cauchy noise) it provides exact results in agreement with numerical simulations. Our formula allows us to reproduce, in a suitable small-noise limit, the response functions of deterministic, strongly nonlinear dynamical models, even in the presence of chaotic behavior: This could have important practical applications in several contexts, including geophysics and climate. As a case of study, we consider the Lorenz '63 model, which is paradigmatic for the chaotic properties of deterministic dynamical systems.

[The short- and long-term results in heart surgery of the elderly]. / Risultati a breve e a lungo termine nella chirurgia cardiaca dell'anziáno.

We have studied retrospectively 106 patients aged 69 years or older (range 69 to 79) who underwent cardiac surgery between November 1986 and December 1989. The majority of patients (61%) were male. Coronary artery bypass surgery was performed in 59 patients, isolated valve replacement in 38 and combined valve replacement with coronary artery bypass surgery in 5. Two patients underwent ascending aorta replacement for aortic dissection and 2 ventricular aneurysmectomy and postinfarction ventricular septal defect repair. The mean postoperative hospital stay was 12 days. Ninety-one percent of patients underwent a primary elective operation and 9% required an emergency operation. Hospital mortality was 5% (n = 6). All hospital survivors were followed up by telephone contact (mean follow-up: 37 months) to determine presence or absence of chest pain, dyspnea, postoperative NYHA class and the overall effect of surgery on quality of life. There were 16 follow-up deaths; 5 were non cardiac. Follow-up study showed significant improvement in symptom status and quality of life (96%). We concluded that cardiac surgery in the elderly, although associated with increased operative risk, gives excellent relief of symptoms and good survival.

[The prognosis of the patient with heart failure: an analysis of the most significant clinical and instrumental parameters]. / Prognosi del paziente con insufficienza cardiaca: analisi dei parametri clinici e strumentali più significativi.

Despite the progress of the medical and surgical therapy of cardiac failure, the prognosis of this syndrome remains severe. We studied in a group of cardiac failure patients (n = 203; 18-74 years old) admitted in our division from 1982 to 1987 the most significant clinical and instrumental parameters of prognostic importance. The clinical parameters considered were: age, sex, heart rate, blood pressure, NYHA class, presence of mitral insufficiency, episodes of acute heart failure. The instrumental parameters were: presence of complete left bundle branch block (LBBB), atrial fibrillation, episodes of ventricular tachycardia, cardiothoracic index (C/T), end-diastolic and end-systolic diameters, ejection fraction (EF). Statistical analysis was performed in order to correlate single parameters with mortality. The total survival at 5 years was 50%, being higher in patients with coronary artery disease than in patients with primary dilated cardiomyopathy. The parameters worsening the prognosis were: mitral insufficiency, III-IV NYHA class, occurrence of repeated episodes of acute heart failure in the last year, complete LBBB, C/T greater than 0.55 and EF less than 20%. In conclusion, considering instrumental parameters high risk patients are detected with a precision of 80%.