We consider a power series $f = \lambda z + F, F \in O_2, \lambda = (f)_1 \neq 0$, with $h$ the unique series in $O_1$ such that $h \circ f = I$ and $g$ the unique series in $O_1$ such that $f \circ g = I$. Show that $g = h$.
We consider a power series $f = \lambda z + F, F \in O_2, \lambda = (f)_1 \neq 0$, with $h$ the unique series in $O_1$ such that $h \circ f = I$ and $g$ the unique series in $O_1$ such that $f \circ g = I$. Show that $g = h$.