Suppose f , g ? L?(0, 1); let F1 , . . . , FN be ?-algebras of Borel sets in [0, 1].
Put f0 = f and fj = E(f{j ?1} |Fj ), j = 1, 2, . . . , N . The function f is transformable into g, if for any ? > 0 there exist N and such ?-algebras F1 , . . . , FN that fN ? g

I will discuss the behavior of N bosons trapped in a potential well subject
to a pairwise interaction. In the mean-field limit -- i.e., when N tends to
infinity while keeping the interaction per particle bounded -- the evolution
of a product state remains, asymptotically, a product state. The single
particle wave-function then evolves according to a non-linear Hartree
equation. Versions of this result have been proved before, e.g., by
Hepp in 1977, Spohn in 1980 or, recently, by Rodnianski and Schlein, but
the proofs are often quite technically involved. I will describe a very simple,
and ideologically correct, proof (for bounded interaction potentials) based
on exchengeability and Stormer's (aka quantum deFinetti) theorem.
Based on recent discussions with Nick Crawford.