  Let $K[x]$ be a polynomial algebra in a variable $x$ over a commutative
$\Q$-algebra $K$, and $\G'$ be the monoid of $K$-algebra monomorphisms of
$K[x]$ of the type $\s : x\mapsto x+\l_2x^2+... +\l_nx^n$, $\l_i\in K$, $\l_n$
is a unit of $K$. It is proved that for each $\s \in \G'$ there are only
finitely many distinct decompositions $\s = \s_1... \s_s$ in $\G'$. Moreover,
each such a decomposition is uniquely determined by the degrees of components:
if $\s = \s_1... \s_s= \tau_1... \tau_s$ then $\s_1=\tau_1, >..., \s_s=\tau_s$
iff $\deg (\s_1)=\deg (\tau_1), ..., \deg (\s_s)=\deg (\tau_s)$. Explicit
formulae are given for the components $\s_i$ via the coefficients $\l_j$ and
the degrees $\deg (\s_k)$ (as an application of the inversion formula for
polynomial automorphisms in {\em several} variables from \cite{Bav-inform}). In
general, for a polynomial there are no formulae (in radicals) for its divisors
(elementary Galois theory). Surprisingly, one can write such formulae where
instead of the product of polynomials one considers their composition (as
polynomial functions).
