  Consider a (possibly infinite) exchangeable sequence X={X_n:1\leqn<N}, where
N\in N\cup {\infty}, with values in a Borel space (A,A), and note
X_n=(X_1,...,X_n). We say that X is Hoeffding decomposable if, for each n,
every square integrable, centered and symmetric statistic based on X_n can be
written as an orthogonal sum of n U-statistics with degenerated and symmetric
kernels of increasing order. The only two examples of Hoeffding decomposable
sequences studied in the literature are i.i.d. random variables and extractions
without replacement from a finite population. In the first part of the paper we
establish a necessary and sufficient condition for an exchangeable sequence to
be Hoeffding decomposable, that is, called weak independence. We show that not
every exchangeable sequence is weakly independent, and, therefore, that not
every exchangeable sequence is Hoeffding decomposable. In the second part we
apply our results to a class of exchangeable and weakly independent random
vectors X_n^{(\alpha, c)}=(X_1^{(\alpha, c)},...,X_n^{(\alpha, c)}) whose law
is characterized by a positive and finite measure \alpha (\cdot) on A and by a
real constant c. For instance, if c=0, X_n^{(\alpha, c)} is a vector of i.i.d.
random variables with law \alpha (\cdot)/\alpha (A); if A is finite, \alpha
(\cdot) is integer valued and c=-1, X_n^{(\alpha, c)} represents the first n
extractions without replacement from a finite population; if c>0, X_n^{(\alpha,
c)} consists of the first n instants of a generalized P\'olya urn sequence.
