We will now introduce a set U of cubic forms, and study some of its properties. In the next section, we will prove that U is the sought after image I of the Davenport-Heilbronn correspondence.
We first need some notation. For a prime p, we let be the set of
such that
if
or
or
if p=2. In other words, if
with
a fundamental
discriminant,
if and only if
.
In particular is the set of fundamental discriminants, i.e of
discriminants of quadratic fields.
Furthermore, let be the set of
such that either
,
or else
We will summarize the condition
for some
by saying that F has three identical roots in
, and we will
write
.
Finally, we set . The Davenport-Heilbronn theorem states
that U=I, the image of the map
that we are looking for.
Before proving this theorem, we must study in detail the set U.
Fot this, a very useful notion is that of the Hessian of a form F.
Definition 3.1 Let be a cubic form.
We define the Hessian of F by the formula
Letbe a cubic form and
its Hessian.
We are now ready to give an algorithmic description of the set U. We need two propositions.
Letbe a primitive form, and let
its Hessian. Recall that we write
if F has a triple root in
. Then
Let F be a primitive form. We can writewith
a fundamental discriminant. Then
if and only if either
, or if
Let F be a primitive cubic form and p be a prime. Thenif and only if F has at least a double root
modulo p, and
.