Lemma 7.1.
Assuming that:
A
an abelian group
q
:
A
→
ℤ
a positive definit quadratic form
Then
|
q
(
x
+
y
)
−
q
(
x
)
−
q
(
y
)
⏟
=
⟨
x
,
y
⟩
|
≤
2
q
(
x
)
q
(
y
)
∀
x
,
y
∈
A
.