"Localization transition of d-friendly walkers",
( with Nobuo Yoshida)
Probab. Th. Rel. Fields. 125(2003), 593-608.
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Friendly walkers is a stochastic model obtained from independent
one-dimensional simple random walks $\{ S^k_j \}_{j\ge 0}$, $k=1,2,\dots,
d$ by introducing ``non-crossing condition'': $S^1_j \le S^2_j \le \ldots
\le S^{d}_j, j=1,2,\dots, n$ and ``reward for collisions'' characterized
by parameters $\b_2, \ldots, \b_d \ge 0$. Here, the reward for collisions
is described as follows. If, at a given time $n$, a site in $\Z$ is occupied
by exactly $m \ge 2$ walkers, then the site increases the probabilistic weight
for the walkers by multiplicative factor $\exp (\b_m )\ge 1$. We study the
localization transition of this model in terms of the positivity of the free
energy and describe the location and the shape of the critical surface in the
$(d-1)$-dimensional space for the parameters $(\b_2, \ldots, \b_d)$.