Abstract
Aldous (2007) defined a gossip process in which space is a discrete $N \times
N$ torus, and the state of the process at time $t$ is the set of individuals
who know the information. Information spreads from a site to its nearest
neighbors at rate 1/4 each and at rate $N^-\alpha$ to a site chosen at random
from the torus. We will be interested in the case in which $< 3$, where
the long range transmission significantly accelerates the time at which
everyone knows the information. We prove three results that precisely describe
the spread of information in a slightly simplified model on the real torus. The
time until everyone knows the information is asymptotically $T=(2-2\alpha/3)
N^\alpha/3 N$. If $\rho_s$ is the fraction of the population who know
the information at time $s$ and $\ep$ is small then, for large $N$, the time
until $\rho_s$ reaches $\ep$ is $T(\ep) T + N^\alpha/3 łog(3\ep/M)$,
where $M$ is a random variable determined by the early spread of the
information. The value of $\rho_s$ at time $s = T(1/3) + t N^\alpha/3$ is
almost a deterministic function $h(t)$ which satisfies an odd looking
integro-differential equation. The last result confirms a heuristic calculation
of Aldous.
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