General non-existence theorem for phase transitions in one-dimensional systems with short range interactions, and physical examples of such transitions

We examine critically the issue of phase transitions in one-dimensional systems with short range interactions. We begin by reviewing in detail the most famous non-existence result, namely van Hove's theorem, emphasizing its hypothesis and subsequently its limited range of applicability. To further underscore this point, we present several examples of one-dimensional short ranged models that exhibit true, thermodynamic phase transitions, with increasing level of complexity and closeness to reality. Thus having made clear the necessity for a result broader than van Hove's theorem, we set out to prove such a general non-existence theorem, widening largely the class of models known to be free of phase transitions. The theorem is presented from a rigorous mathematical point of view although examples of the framework corresponding to usual physical systems are given along the way. We close the paper with a discussion in more physical terms of the implications of this non-existence theorem.Comment: Short comment on possible generalization to wider classes of systems added; accepted for publication in Journal of Statistical Physic

Altruistic behavior pays, or the importance of fluctuations in evolutionary game theory

Human behavior is one of the main problems for evolution, as it is often the case that human actions are disadvantageous for the self and advantageous for other people. Behind this puzzle are our beliefs about rational behavior, based on game theory. Here we show that by going beyond the standard game-theoretical conventions, apparently altruistic behavior can be understood as self-interested. We discuss in detail an example related to the so called Ultimatum game and illustrate the appearance of altruistic behavior induced by fluctuations. In addition, we claim that in general settings, fluctuations play a very relevant role, and we support this claim by considering a completely different example, namely the Stag-Hunt game.Comment: For the proceedings of the 8th Granada Seminar on Computational Physics (AIP Proceedeings Series

Effects of Climate Variability: A Long-term Perspective on Leaf Litter Processing in the Ogeechee River

The Intergovernmental Panel for Climate Change (IPCC) predicts a rise in temperatures and changes to precipitation patterns for the 21st century. These changes are expected to lead to a higher frequency of extreme events such as drought and floods which lead to the alteration of the hydrology, communities, and processes of freshwater ecosystems. Leaf litter decomposition in stream ecosystems is an important component of the energy and nutrient cycle representing a food source for aquatic organisms. This has made it a tool for assessing long term responses to disturbance due to changes in the assemblage of macroinvertebrates that colonize leaf packs. My objective is to use long-term data to evaluate 5 years of leaf litter decomposition data at the Ogeechee River from 2013 through 2017 after a 3-year drought period (2010 - 2012) to assess the leaf decomposition process and the macroinvertebrate community associated with it. We predicted that disturbances due to climate variability, specifically drought and flood disturbances, will have an impact in the process and that factors such as temperature, discharge and potentially the shredder functional feeding group will drive these changes. Additionally, we predicted these types of disturbances will impact macroinvertebrate abundance and richness negatively. This study reveals changes in the rate of decomposition in a post-drought in 2013 and the subsequent years that are explained by fluctuations in temperature, discharge and potentially shredders. Additionally, we detected a shift in the species composition after the post-drought year into a more stable period alluding to a lag effect in species richness. In conclusion, we can predict an increase in decomposition rates during disturbance events, especially drought as well as a decrease in both abundance and richness

Emergence and resilience of cooperation in the spatial Prisoner's Dilemma via a reward mechanism

We study the problem of the emergence of cooperation in the spatial Prisoner's Dilemma. The pioneering work by Nowak and May showed that large initial populations of cooperators can survive and sustain cooperation in a square lattice with imitate-the-best evolutionary dynamics. We revisit this problem in a cost-benefit formulation suitable for a number of biological applications. We show that if a fixed-amount reward is established for cooperators to share, a single cooperator can invade a population of defectors and form structures that are resilient to re-invasion even if the reward mechanism is turned off. We discuss analytically the case of the invasion by a single cooperator and present agent-based simulations for small initial fractions of cooperators. Large cooperation levels, in the sustainability range, are found. In the conclusions we discuss possible applications of this model as well as its connections with other mechanisms proposed to promote the emergence of cooperation

Competitive pressure and labor productivity: world iron ore markets in the 1980s

Does the extent of competitive pressure industries face influence their productivity? We study a natural experiment conducted in the iron ore industry as a result of the collapse in world steel production in the early 1980s. For iron ore producers, whose only market is the steel industry, this collapse was an exogenous shock. The drop in steel production differed dramatically by region: it fell by about a third in the Atlantic Basin but only very little in the Pacific Basin. Given that the cost of transporting iron ore is very high relative to its mine value, Atlantic iron ore producers faced a much greater increase in competitive pressure than did Pacific iron ore producers. In response to the crisis, most Atlantic iron ore producers doubled their labor productivity; Pacific iron ore producers experienced few productivity gains. ; This article originally appeared in the American Economic Review. (c) American Economic Association.Labor productivity ; Steel industry and trade