The aim of this section is to generalize to the cubic case the well known
correspondence between binary quadratic forms and quadratic number fields.
These results are due to Davenport and Heilbronn (see [5] and [6]).
Before stating and proving the main theorem, we need a few preliminary
results. We let 
be the set of classes of primitive irreducible binary
cubic forms. This makes sense, thanks to Proposition 1.2.
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