  The postulation of Arithmetically Cohen-Macaulay (ACM) subschemes of the
projective space ${\mathbb P}^N_k$ is well-known in the case of codimension 2.
There are many different ways of recording this numerical information :
numerical character of Gruson/Peskine, $h$-vector, postulation character of
Martin-Deschamps/Perrin... The first aim of this paper is to show the
equivalence between these notions.
  The second, and most important aim, is to study the postulation of
codimension 3 ACM subschemes of ${\mathbb P}^N$. We use a result of Macaulay
which describes all the Hilbert functions of the quotients of a polynomial
ring. By iterating the number of variables, we obtain a new form of the growth
of these functions.
