  Let X be a subset of an abelian group and a_1,...,a_h,a'_1,...,a'_h a
sequence of 2h elements of X such that a_1 + ... + a_h = a'_1 + ... + a'_h. The
set X is a Sidon set of order h if, after renumbering, a_i = a'_i for i =
1,..., h. For k \leq h, the set X is a generalized Sidon set of order (h,k),
if, after renumbering, a_i = a'_i for i = 1,..., k. It is proved that if X is a
generalized Sidon set of order (2h-1,h-1), then the maximal Sidon sets of order
h contained in X have the same cardinality. Moreover, X is a matroid where the
independent subsets of X are the Sidon sets of order h.
