Lemma 8.4.
Assuming that:
f
(
T
)
=
a
T
+
⋯
∈
R
[
[
T
]
]
with
a
∈
R
×
Then
there exists a unique
g
(
T
)
=
a
−
1
T
+
⋯
∈
R
[
[
T
]
]
such that
f
(
g
(
T
)
)
=
g
(
f
(
T
)
)
=
T
.