Lemma 5.8.
There exists
W
0
,
W
1
,
W
2
∈
ℤ
[
a
,
b
]
[
x
1
,
x
2
]
of degree in
x
1
and of degree
≤
2
in
x
2
such that
(
1
:
x
3
+
x
4
:
x
3
x
4
)
=
(
W
0
:
W
1
:
W
2
)
.