Jun Tanimoto

Universal scaling for the dilemma strength in evolutionary games.

Why would natural selection favor the prevalence of cooperation within the groups of selfish individuals? A fruitful framework to address this question is evolutionary game theory, the essence of which is captured in the so-called social dilemmas. Such dilemmas have sparked the development of a variety of mathematical approaches to assess the conditions under which cooperation evolves. Furthermore, borrowing from statistical physics and network science, the research of the evolutionary game dynamics has been enriched with phenomena such as pattern formation, equilibrium selection, and self-organization. Numerous advances in understanding the evolution of cooperative behavior over the last few decades have recently been distilled into five reciprocity mechanisms: direct reciprocity, indirect reciprocity, kin selection, group selection, and network reciprocity. However, when social viscosity is introduced into a population via any of the reciprocity mechanisms, the existing scaling parameters for the dilemma strength do not yield a unique answer as to how the evolutionary dynamics should unfold. Motivated by this problem, we review the developments that led to the present state of affairs, highlight the accompanying pitfalls, and propose new universal scaling parameters for the dilemma strength. We prove universality by showing that the conditions for an ESS and the expressions for the internal equilibriums in an infinite, well-mixed population subjected to any of the five reciprocity mechanisms depend only on the new scaling parameters. A similar result is shown to hold for the fixation probability of the different strategies in a finite, well-mixed population. Furthermore, by means of numerical simulations, the same scaling parameters are shown to be effective even if the evolution of cooperation is considered on the spatial networks (with the exception of highly heterogeneous setups). We close the discussion by suggesting promising directions for future research including (i) how to handle the dilemma strength in the context of co-evolution and (ii) where to seek opportunities for applying the game theoretical approach with meaningful impact.

Field measurements for estimating the convective heat transfer coefficient at building surfaces

To establish a comprehensive and qualitative prediction basis for the convective heat transfer coefficient (CHTC) for various urban canopy surfaces mainly consisting of building envelopes, a series of outdoor experiments were performed. Multi-point measurements of surface heat balance lead to a distribution of the CHTC on an actual building envelope. Several turbulent statistical values acquired at two different sites enabled the development of an experimental equation depicted by non-dimensional numbers that express a relationship between CHTC and wind velocity containing a turbulent factor. An important thing is the fact that the two measuring sites, one a rooftop slab and the other the vertical wall of a test dwelling, have different scales and different surface directions facing the wind.

Simulation study on an air flow window system with an integrated roll screen

Abstract A numerical calculation procedure for an air flow window (AFW) system integrated with a roll screen is presented. Both heat and air flows within the window elements such as the outside pane of glass, the outside air space, a venetian blind, the inside air space and a roll screen, are taken into account by considering the thermal and air flow networks. Agreements between measured and calculated results of temperatures and pressure differences through a series of experiments carried out in an environmental test chamber were observed. To identify the quantitative characteristics of an AFW integrated with a roll screen, a series of numerical simulations were performed with the results from using a large resistance to the air flow from the upper half area of the roll screen. These show that this was effective in terms of thermal characteristics and suggest that a tightly-meshed roll screen, situtated in the upper half area, is suitable for both environmental and design reasons. If the heated air flow generated from the inside air space to the room were dispersed properly, the thermal efficiency of the AFW integrated with a roll screen would be equal to a conventional AFW system. The effect of a cold draft passing though the roll screen is also discussed.

An analysis of network reciprocity in Prisoner's Dilemma games using Full Factorial Designs of Experiment

Despite hundreds of studies on the Prisoners Dilemma (PD) game, understanding about network reciprocity remains a unsolved puzzle. Thus, we performed a series of Full Factorial Design of Experiments (FFDOE) to evaluate what dominates emerging cooperation in the PD game on various networks. The results qualitatively reveal the influence of each factor and show that some combinations of factors have complicated interactions. Remarkably, the choice of strategy update rule or update dynamics is much more important than the type of network imposed or, at least, the factorial effect of the average degree of the network reported by Nowak (Science 314, 5805, 1560-1563, 2006) and Ohtsuki et al. (Nature 441, 502-505, 2006). Furthermore, the decision of which PD game type to investigate (whether all PD games, PD-Chicken boundary games or Donor & Recipient games) is important for discussing network reciprocity.

Effect of Initial Fraction of Cooperators on Cooperative Behavior in Evolutionary Prisoner's Dilemma Game

We investigate the influence of initial fraction of cooperators on the evolution of cooperation in spatial prisoners dilemma games. Compared with the results of heterogeneous networks, we find that there is a relatively low initial fraction of cooperators to guarantee higher equilibrium cooperative level. While this interesting phenomenon is contrary to the commonly shared knowledge that higher initial fraction of cooperators can provide better environment for the evolution of cooperation. To support our outcome, we explore the time courses of cooperation and find that the whole course can be divided into two sequent stages: enduring (END) and expanding (EXP) periods. At the end of END period, thought there is a limited number of cooperator clusters left for the case of low initial setup, these clusters can smoothly expand to hold the whole system in the EXP period. However, for high initial fraction of cooperators, superfluous cooperator clusters hinder their effective expansion, which induces many remaining defectors surrounding the cooperator clusters. Moreover, through intensive analysis, we also demonstrate that when the tendency of three cooperation cluster characteristics (cluster size, cluster number and cluster shape) are consistent within END and EXP periods, the state that maximizes cooperation can be favored.

Validation of methodology for utility demand prediction considering actual variations in inhabitant behaviour schedules

A data set of myriad and time-varying inhabitant behaviour schedules with a 15-min time resolution, generated by the authors in a previous study, is validated through a comparison analysis. The key idea of generating a set of raw schedule data from the restricted statistical information is called the ‘generate and kill’ concept, which is commonly used in the fields of artificial intelligence and multi-agent simulation. In the present study, we show three comparisons. The first and second compare the estimated demand with a time series of measured utility demand. These comparisons indicate that the generated data and the algorithm, as described by the authors, have the required robustness. Another comparison between the estimate and the annually averaged daily water demand of a residential area, consisting of 9327 residences, also shows an acceptable consistency.

What controls network reciprocity in the Prisoner's Dilemma game?

The evolutionary Prisoners Dilemma game in structured networks has been studied extensively to understand network reciprocity. However, in some cases results of these studies cannot be compared because not only the network structures but also the network parameters, rules for updating strategies, and update dynamics differ among them. In this study, we investigated the effect of experimental conditions by conducting a series of systematic factorial experiments. We found that those experimental assumptions are significantly important in network reciprocity, although we confirmed that network reciprocity can be basically explained by the average degree of the network (Nowak, 2006; Ohtsuki et al., 2006).

How is the equilibrium of continuous strategy game different from that of discrete strategy game

Cooperation in the prisoners dilemma (PD) played on various networks has been explained by so-called network reciprocity. Most of the previous studies presumed that players can offer either cooperation (C) or defection (D). This discrete strategy seems unrealistic in the real world, since actual provisions might not be discrete, but rather continuous. This paper studies the differences between continuous and discrete strategies in two aspects under the condition that the payoff function of the former is a linear interpolation of the payoff matrix of the latter. The first part of this paper proves theoretically that for two-player games, continuous and discrete strategies have different equilibria and game dynamics in a well-mixed but finite population. The second part, conducting a series of numerical experiments, reveals that such differences become considerably large in the case of PD games on networks. Furthermore, it shows, using the Wilcoxon sign-rank test, that continuous and discrete strategy games are statistically significantly different in terms of equilibria. Intensive discussion by comparing these two kinds of games elucidates that describing a strategy as a real number blunts D strategy invasion to C clusters on a network in the early stage of evolution. Thus, network reciprocity is enhanced by the continuous strategy.

Spatial reciprocity for discrete, continuous and mixed strategy setups

The existence of cooperation in the social dilemma has been extensively studied based on spatial structure populations, namely, the so-called spatial reciprocity. However, vast majority of existing works just simply presume that agents can offer the discrete choice: either the cooperative (C) or defective (D) strategy, which, to some extent, seems unrealistic in the empirical observations since actual options might be continuous, mixed rather than discrete. Here, we propose discrete, continuous and mixed strategy setups in the social dilemma games and further explore their performance on network populations. Interestingly, it is unveiled that there is actually considerable inconsistency in terms of equilibrium among different strategy games. Furthermore, we reveal how different cooperative arrangements among these three strategy setups can be established, depending on whether the presumed dilemma subclass is a boundary game between prisoners dilemma game and Chicken game or between prisoners dilemma game and Stag-Hunt game.

ANALYSIS OF THE INFLUENCE OF LANE CHANGING ON TRAFFIC-FLOW DYNAMICS BASED ON THE CELLULAR AUTOMATON MODEL

Many cellular automaton models (CA models) have been applied to analyze traffic flow. When analyzing multilane traffic flow, it is important how we define lane-changing rules. However, conventional models have used simple lane-changing rules that are dependent only on the distance from neighboring vehicles. We propose a new lane-changing rule considering velocity differences with neighboring vehicles; in addition, we embed the rules into a variant of the Nagel–Schreckenberg (NaSch) model, called the S-NFS model, by considering an open boundary condition. Using numerical simulations, we clarify the basic characteristics resulting from different assumptions with respect to lane changing.