Coevolutionary dynamics of a variant of the cyclic Lotka-Volterra model with three-agent interactions

We study a variant of the cyclic Lotka-Volterra model with three-agent interactions. Inspired by a multiplayer variation of the Rock-Paper-Scissors game, the model describes an ideal ecosystem in which cyclic competition among three species develops through cooperative predation. Its rate equations in a well-mixed environment display a degenerate Hopf bifurcation, occurring as reactions involving two predators plus one prey have the same rate as reactions involving two preys plus one predator. We estimate the magnitude of the stochastic noise at the bifurcation point, where finite size effects turn neutrally stable orbits into erratically diverging trajectories. In particular, we compare analytic predictions for the extinction probability, derived in the Fokker-Planck approximation, with numerical simulations based on the Gillespie stochastic algorithm. We then extend the analysis of the phase portrait to heterogeneous rates. In a well-mixed environment, we observe a continuum of degenerate Hopf bifurcations, generalizing the above one. Neutral stability ensues from a complex equilibrium between different reactions. Remarkably, on a two-dimensional lattice, all bifurcations disappear as a consequence of the spatial locality of the interactions. In the second part of the paper, we investigate the effects of mobility in a lattice metapopulation model with patches hosting several agents. We find that strategies propagate along the arms of rotating spirals, as they usually do in models of cyclic dominance. We observe propagation instabilities in the regime of large wavelengths. We also examine three-agent interactions inducing nonlinear diffusion.Comment: 22 pages, 13 figures. v2: version accepted for publication in EPJ

Complex Magnetic Exchange Coupling between Co Nanostructures and Ni(111) across Epitaxial Graphene

We report on the magnetic coupling between isolated Co atoms as well as small Co islands and Ni(111) mediated by an epitaxial graphene layer. X-ray magnetic circular dichroism and scanning tunneling microscopy combined with density functional theory calculations reveal that Co atoms occupy two distinct adsorption sites, with different magnetic coupling to the underlying Ni(111) surface. We further report a transition from an antiferromagnetic to a ferromagnetic coupling with increasing Co cluster size. Our results highlight the extreme, sensitivity of the exchange interaction mediated by graphene to the adsorption site and to the in-plane coordination of the magnetic atoms

Schede delle opere / note sulla ricostruzione

Lucio Fontana. Ambienti/Environment

The dark side of alternative asset markets: networks, performance and risk taking

When actors invest in making strong network ties(relationships) with other actors, such ties can potentially influence behavior and subsequent financial performance, but the strength and direction of these effects is debated. Using original fine-grained data that documents the nature and extent of the relationships between Hedge Funds through their Prime Brokers (banks that provide leverage, issue credit lines and serve as bridges between Hedge Funds) we probe the social topology of Hedge Fund to Hedge Fund relationships that shapes this global alternative asset market. Contrasting much recent research that tends to stress the positive effects of network relationships, we find that investing in network relationships in this industry appears to have a “dark side” in terms of both performance and risk taking; where we probe various measures of both performance and risk in line with recent finance literature. We explore the reasons for these effects, and conclude that investing in Hedge Fund to Hedge Fund network ties can lead to inferior performance and increased risks that may not benefit the investor