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Stochastic Approximation to Understand Simple Simulation Models

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Abstract

This paper illustrates how a deterministic approximation of a stochastic process can be usefully applied to analyse the dynamics of many simple simulation models. To demonstrate the type of results that can be obtained using this approximation, we present two illustrative examples which are meant to serve as methodological references for researchers exploring this area. Finally, we prove some convergence results for simulations of a family of evolutionary games, namely, intra-population imitation models in n-player games with arbitrary payoffs.

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Notes

  1. A function f(x,γ) is O(γ) as γ→0 uniformly in xI iff ∃δ>0,∃M>0 such that |f(x,γ)|≤M⋅|γ| for |γ|<δ and for every xI, where δ,M are constants (independent of x).

  2. See for instance the Self Correcting Equilibria (SCEs) described by Macy and Flache [38] and characterised as asymptotically stable points of the mean field by Izquierdo et al. [27].

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Acknowledgements

The authors gratefully acknowledge financial support from the Spanish Ministry of Education (JC2009-00263) and MICINN (CONSOLIDER-INGENIO 2010: CSD2010-00034, and DPI2010-16920). We also thank two anonymous reviewers for their useful comments.

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Correspondence to Segismundo S. Izquierdo.

Appendix: Approximation of Difference Equations by Differential Equations

Appendix: Approximation of Difference Equations by Differential Equations

This appendix discusses the relation between a discrete-time difference equation of the form Δx n =γf(x n ), with initial point x 0, and the solution x(t,x 0) of its corresponding continuous time differential equation \(\dot{x}= f(x)\) with x(t=0)=x 0.

This relationship can be neatly formalised using Euler’s method [12]. If f(x) satisfies some conditions that guarantee convergence [23, 35], e.g. if f(x) is globally Lipschitz or if it is smooth on a forward-invariant compact domain of interest D, the difference between the solution x(t,x 0) of the differential equation at time t=T<∞ and the solution x n of the equation in differences at step \(n = \operatorname {int}(T/\gamma)\) converges to 0 as the step-size parameter γ tends to 0:

$$x_{n} = x_{0} + \sum_{i = 0}^{\operatorname {int} (\frac{T}{\gamma })} \gamma\cdot f(x_{i}) \mathop{\longrightarrow}\limits^{\gamma\to0} x(T,x_{0}) $$

and, for every \(T, \max_{n = 0,1,\ldots,N}\|x_{n} - x(n\frac{T}{N},x_{0})\| \mathop{\longrightarrow}\limits^{N\to \infty}0\).

As an example, consider a vector x=[x 1,x 2], the function f(x)=[x 2,−x 1], the differential equation \(\dot{x}= f(x)\) and its associated (deterministic) equation in differences Δx n =γf(x n ). Figure 7 shows a trajectory map of the differential equation \(\dot{x}= f(x)\), together with several values of discrete processes that follow the equation in differences Δx n =γf(x n ), for different values of γ and for the same initial value x 0=[x 1,x 2]0. It can be seen how, for decreasing values of γ and for a correspondingly increasing finite number of steps \(n = \operatorname {int}(T/\gamma)\), the discrete process gets closer and closer to the trajectory of the differential equation that goes through x 0. The reader can confirm this fact by running simulations with the interactive version of Fig. 7.

Fig. 7
figure 7

Convergence of difference and differential equations for small step-size. Let x=[x 1,x 2] be a generic point in the real plane. Figure 7 shows a trajectory map of the differential equation \(\dot{x} = f(x) =[x_{2}, -x_{1}]\), together with several values of the discrete process that follow the equation in differences x n+1x n =γf(x n ), for different values of the step-size parameter γ and for a chosen initial value x 0=[x 1,x 2]0. The background is coloured using the norm of the expected motion, rescaled to be in the interval (0,1). It can be seen how, for decreasing values of γ, the discrete process tends to follow temporarily the trajectory of the differential equation that goes through x 0. Interactive figure at http://demonstrations.wolfram.com/DifferenceEquationVersusDifferentialEquation/ (Color figure online)

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Izquierdo, S.S., Izquierdo, L.R. Stochastic Approximation to Understand Simple Simulation Models. J Stat Phys 151, 254–276 (2013). https://doi.org/10.1007/s10955-012-0654-z

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