Abstract
We consider particle systems that are perturbations of the voter model and
show that when space and time are rescaled the system converges to a solution
of a reaction diffusion equation in dimensions $d 3$. Combining this result
with properties of the PDE, some methods arising from a low density
super-Brownian limit theorem, and a block construction, we give general, and
often asymptotically sharp, conditions for the existence of non-trivial
stationary distributions, and for extinction of one type. As applications, we
describe the phase diagrams of three systems when the parameters are close to
the voter model: (i) a stochastic spatial Lotka-Volterra model of Neuhauser and
Pacala, (ii) a model of the evolution of cooperation of Ohtsuki, Hauert,
Lieberman, and Nowak, and (iii) a continuous time version of the non-linear
voter model of Molofsky, Durrett, Dushoff, Griffeath, and Levin. The first
application confirms a conjecture of Cox and Perkins and the second confirms a
conjecture of Ohtsuki et al in the context of certain infinite graphs. An
important feature of our general results is that they do not require the
process to be attractive.
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