Lemma 5.6.
q
:
A
→
ℤ
is a
quadratic form
if and only if
it satisfies the parallelogram law:
q
(
x
+
y
)
+
q
(
x
−
y
)
=
2
q
(
x
)
=
2
q
(
y
)
∀
x
,
y
∈
A
.