We consider a Markov chain that iteratively generates a sequence of random finite
words in such a way that the nth word is uniformly distributed over
the set of words of length 2n in which n letters are
a and n letters are b: at each step an
a and a b are shuffled in uniformly at random among the
letters of the current word. We obtain a concrete characterization of the Doob-Martin boundary
of this Markov chain and thereby delineate all the ways in which the Markov chain can be
conditioned to behave at large times. Writing N(u) for the number of letters
a (equivalently, b) in the finite word u,
we show that a sequence
(un)n∈ℕ of finite words
converges to a point in the boundary if, for an arbitrary word ν, there
is convergence as n tends to infinity of the probability that the selection of
N(ν) letters a and
N(ν) letters b uniformly at random
from un and maintaining their relative order results in
ν. We exhibit a bijective correspondence between the points in the
boundary and ergodic random total orders on the set {a1,
b1, a2,
b2, …} that have distributions which are separately
invariant under finite permutations of the indices of the a's and those of the
b's. We establish a further bijective correspondence between the set of such
random total orders and the set of pairs (μ, ν)
of diffuse probability measures on [0,1] such that ½(μ +
ν) is Lebesgue measure: the restriction of the random total order to
{a1, b1,…,
an, bn} is obtained by taking
X1,…, Xn (resp.
Y1,… ,Yn) i.i.d. with common
distribution μ (resp. ν), letting
(Z1,…, Z2n) be
{X1, Y1,…,
Xn, Yn} in increasing order, and
declaring that the kth smallest element in the restricted total
order is ai (resp. bj) if
Zk = Xi (resp.
Zk = Yj).