Lemma 16.5.
Assuming that:
b
1
b
2
=
b
Then
b
1
(
K
∗
)
∈
Im
(
α
E
)
if and only if
w
2
=
b
1
u
4
+
a
u
2
v
2
+
b
2
v
4
(∗)
is soluble for
u
,
v
,
w
∈
K
not all zero.